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SECONDARY BATTERIES – LEAD– ACID SYSTEMS | Performance
Performance GJ May, FOCUS Consulting, Loughborough, UK & 2009 Elsevier B.V. All rights reserved.
Introduction The lead–acid battery is capable of being adapted for optimum performance for a variety of duty cycles. These may be broadly characterized by the rate of discharge, by the frequency of discharge and recharge, and by the time that the battery remains on charge. High discharge rates are required for starting internal combustion and gas turbine engines for cars, commercial vehicles, marine applications, aircraft, railway locomotives, military service, generators, and stationary engines. Batteries for this type of application are broadly categorized as automotive batteries. Specifications generally include high rate performance at low temperatures, and this tends to be the dominant factor in battery design. But for a number of other applications cyclic duty capability is also often considered. Regular charge and discharge cycling for materials handling, electric vehicles of all types, and other duty cycles where energy is stored for later use dictate different design features in the batteries optimized for these applications. Batteries for standby applications require that they remain on a continuous charge so that the direct current (d.c) supply is maintained without interruption. These are used for telecommunications, uninterruptible power supplies (UPSs), utilities, emergency lighting, security and fire alarms, and general standby applications. The design is then determined by the required time on charge and the discharge rate. Battery design will be discussed for high-rate/ low-temperature performance (automotive), cyclic performance (motive power), and life on floating charge (standby). Most battery applications involve a combination of these parameters, and in all cases the battery is fully charged after a discharge, except for hybrid electric vehicle (HEV) applications where the battery cannot be returned to a full state-of-charge (SoC) but is required to be regularly discharged at high rates. This important new application will be discussed separately.
Battery Design to Meet Application Requirements For automotive applications, battery design is directed at reducing the internal resistance by the design of the grids to improve conductivity, by increasing the number of grids so that they become thinner for any given low-rate capacity, by reducing the distance between the plates, and by reducing the resistance of the separator. Failure at
the end of life will result from corrosion of the positive grid, shedding of the active material, and, often, irreversible sulfation. Grid thickness and the grid pattern will be determined by the manufacturing process available, the grid alloy used, the life required, the specific performance required, and the economic cost. Other requirements will include a limited cyclic duty, but if there is a requirement for a more extended cyclic duty, the design may need to be adjusted at the expense of specific high rate performance. For cyclic applications, the design is determined by the need to maintain a stable capacity for up to 1500 deep cycles, to avoid premature failure from corrosion of the positive grids, and to avoid shedding and softening of the active materials. The selection of the grid alloy, particularly for the positive grid, is important and it is also important to have a system for retaining the positive active material intact and in compression to avoid shedding. Charging systems are another key to long cycle life. The battery needs to be brought to a full SoC and the charging terminated promptly to avoid overcharge with electrolyte loss and corrosion of the positive grid. For standby power, the battery generally remains on a continuous charge at a fixed voltage. Service life will be determined by the operational temperature, the behavior of the positive grid alloy, and the grid thickness. The required rate of discharge also has an impact on service life. At higher rates of discharge the performance will be more rapidly degraded by grid corrosion, whereas at lower rates the conductivity of the positive grid is less critical. For HEV applications, a unique duty cycle that has been designated high-rate partial-state-of-charge (HRPSoC) operation becomes important. The battery needs to be able to accept charge at high rates at all times so that energy recovered by regenerative braking can be stored in it. Discharge is also at high rates and the battery is used intensively with relatively shallow charges and discharges, but never reaching a full charge. This is damaging to the negative plate, and special design considerations are required to ensure a long service life.
Battery Design for Automotive Applications Automotive batteries (Figure 1) are required to meet specified requirements for the following parameters: 1. High rate performance (cold cranking amps, CCA) 2. Reserve capacity/20 h capacity
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Figure 1 A typical modern automotive battery in situ in a vehicle (12 V, 90 Ah).
3. 4. 5. 6. 7. 8. 9.
Cyclic performance Charge retention Corrosion endurance Water consumption Vibration resistance Electrolyte retention Cranking performance for dry-charged batteries after activation
The requirements of EN 50342-1: 2006 (‘Lead–acid starter batteries. General requirements and methods of test’) have been taken as a reference point for discussion of the design principles for automotive batteries. Design for High Rate Performance The required high rate performance is normally specified at 18 1C in amps for a short defined time (30 or 60 s) such that the on-load voltage remains above a required level. There are variations of this type of test; for example, a higher current may be used for 10 s so that the on-load voltage remains above 7.5 V, followed by a lower current (60% of the initial level) where the time to reach an on-load voltage of 6.0 V is recorded. The latter test may be a more realistic one for engine starting, but shorter-time tests lead to adjustments in battery design to meet the specification. Irrespective of the specification, automotive battery design always starts with cold cranking performance. The grid thickness and wire section of the positive grid, irrespective of the manufacturing process for any given alloy, will be as low as possible consistent with adequate corrosion resistance.
Mechanical handling during plate manufacture and cell assembly will also set practical limits to the plate thickness. The positioning of the plate lugs and the detailed design of the grid are aimed at improving conductivity. The thickness of the negative plate will be less than that of the positive plate but sufficient to provide the required active material to balance the positive active mass. The spacing between the plates will be determined by the mass of sulfuric acid required to fully discharge the active material to meet the reserve capacity or 20 h capacity required. The separator needs to have high porosity and high tortuosity between the plates and to be as thin as possible consistent with adequate separation over life in order to reduce internal resistance. The separator also needs to maintain the correct plate pitch, and so, in a flooded battery, the separator has a thin membrane (backweb) with ribs spaced apart on the surface to keep the plates at the correct distance when lightly compressed. In a valve-regulated lead–acid (VRLA) battery, the separator occupies the whole of the space between the plates and needs to be more highly compressed. This will be discussed further below. Discharge at high rates is controlled by the diffusion of electrolyte to the active material, particularly at the plate surface. Free circulation of acid at the surface will improve performance in flooded batteries. The types of separator used in VRLA batteries are such that acid is readily replenished at the plate surface. If high rate performance is more important than low-rate behavior, the plate spacing can be reduced, but then battery performance will become limited by the availability of acid.
Secondary Batteries – Lead–Acid Systems | Performance
The design of the plate straps and intercell connections need to take full account of the need to improve conductivity. Design for Low-Rate Capacity Low-rate capacity is specified either as the 20 h capacity at 25 1C at a constant current to 10.5 V or as the reserve capacity at 25 1C at a constant current of 25 A, in which case the time in minutes is measured. For certain applications, higher low-rate capacity may be required. Examples would be batteries on a boat where there is a requirement for power during engine-off periods or military applications where there are periods of silent watch. In both these cases, plate thickness could be increased to provide more active material without using a larger number of thin plates. This would be more costeffective. Cyclic Performance Failure under cyclic conditions will result from positive grid corrosion and active material shedding. The cyclic tests called up in automotive standards are shallow discharges (typically 50% depth-of-discharge (DoD)) at 25 1C with constant voltage recharges at different voltages depending on the water loss of the battery, and failure is determined by the inability of the battery to reach a specified low-temperature cold cranking performance after a given number of cyclic test units. The positive grid alloy and grid thickness are important in determining the cycle life, but for more extended cycle life, additionally the positive active material needs to be physically retained. Glass or polymer fiber mats may be attached to the separator to retain the active mass. Other parameters such as the density of the active material and the curing and drying cycle are also important. Higher densities favor better cycle life but at the expense of high rate performance. Antimonial alloys also provide better cycle life but at the expense of charge retention. For Pb–Ca alloys, the use of tin as an alloying element improves cyclic performance. Charge Retention Charge retention is measured by holding the battery at an elevated temperature for a period (e.g., 21 days at 40 1C) and then carrying out a low-temperature highrate discharge as indicated above in ‘Design for High Rate Performance’. Batteries described as ‘normal water loss’ ones have a less stringent requirement than the ones that are ‘low water loss’, ‘very low water loss’, or valveregulated. Charge retention is principally determined by the grid alloys although the electrochemical design of the battery and the purity of the materials have secondary effects. The use of Pb–Sb alloys decreases charge retention; lower levels of antimony improve charge
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retention; hybrid batteries with Pb–Sb positive grids and Pb–Ca negative grids result in further improvements; and for the highest levels of charge retention, both grids need to be made of Pb–Ca alloys, which may be further alloyed to improve other characteristics. Pure lead grids and pure lead grids with small additions of tin also provide excellent charge retention. Corrosion Endurance Corrosion endurance is measured by an accelerated life test where the battery is held at a constant voltage charge at an elevated temperature. The battery may be held at 60 1C for 13 days at 14.0 V, then on open circuit for 13 days at 60 1C, recharged and then subjected to an ambient-temperature high-rate test. A number of test units will be specified for the test. As for charge retention, the selection of grid alloy is important for corrosion resistance: Pb–Ca alloys may be alloyed with silver as well as tin to improve corrosion resistance. This has been used extensively in markets where ambient temperatures are high, to improve battery life. Water Consumption Water consumption is determined by holding the battery at an elevated temperature for a defined period and on charge at a constant voltage. The temperature is typically 40 1C, the voltage 14.4 V, and the time 500 h. Water loss is measured as a weight loss and expressed in g Ah 1. For low water loss batteries the weight loss should not exceed 4 g Ah 1 and for very low water loss batteries it should not exceed 1 g Ah 1 in each case referenced to nominal capacity. Water consumption depends on the grid alloys, with Pb–Ca alloys having lower water losses than Pb–Sb alloys, as the water loss is directly related to the hydrogen overpotential of the alloy. For VRLA batteries, the time period for measurement is extended, but a period of overcharge is used prior to measurement in order to stabilize the battery. Vibration Resistance Vibration resistance is important for automotive service and is assessed by clamping the battery to a vibration tester and applying vibration, generally in a vertical plane, at, say, 30 Hz with a specified maximum level of acceleration, for a defined period. The integrity of the battery is verified by carrying out a room-temperature high-rate test to a defined end voltage. Vibration specifications vary depending on application: for normal automotive service, they are at a normal level, but for commercial vehicles, or for military service, much higher levels of vibration resistance may be required. Damage under vibration tends to occur when the various components move relative to each other and the usual failure mode is for the plate lugs to become detached from the group bar. For a normal level of
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vibration resistance, provided the cell element is tightly fitted in the battery container and the group bar is secured at the intercell partition or the terminals in a standard construction, the battery will meet the required levels. However, if higher levels of vibration resistance are required, the various components need to be locked in position so that there is no relative movement. Plate locks are used to keep the elements in a fixed position. These may be small mechanical clamps, or the element may be fixed in position using an adhesive. A well-compressed element is also needed and a glass or synthetic mat may be used to reduce shedding of the active material that will be exacerbated under vibration. The design of the group bar is important and adhesive may be used to reduce relative movement. Vibration resistance may be further improved by designing the battery case to be more mechanically robust. Electrolyte Retention Electrolyte retention may be assessed for flooded batteries by tilting the battery such that the vents are immersed in the electrolyte for a short period. No visible leakage should occur. The vent needs to have some form of labyrinth so that the electrolyte will take time to leak out when tilted, and can drain back into the cell in upright position. Alternatively, the vent may incorporate a flame retardant device that will take the form of a porous ceramic disk or a porous polymer disk. This will prevent any externally ignited hydrogen flame from entering the battery and causing an explosion, but it will also provide an effective means of retaining the electrolyte. Some vehicle manufacturers specify that flooded batteries should not leak even when fully inverted. In these cases, more elaborate battery lids and vents may be used to retain the electrolyte, or devices that close the cell when inverted may be incorporated. For VRLA batteries, the tests are more stringent and the battery should not show any sign of leakage even when fully inverted for several hours. Cranking Performance for Dry-Charged Batteries after Activation Dry-charged batteries are processed so that the negative plate contains an oxidation inhibitor, and when filled with electrolyte, they will provide sufficient energy at a high rate to meet a specified requirement without additional charging. The processes used to achieve the required storage life before activation are proprietary but may use an additive in the paste mix such as mineral oil or stearic acid or may be a spray or a dip during the drying process. Boric acid may also be used. The plate drying process needs to be under nonoxidizing conditions; a vacuum dryer may be used, but more often a gas-fired dryer burning under substoichiometric conditions is used so that the atmosphere is rendered reducing. There
are no special processing requirements for the other components. Other Requirements Batteries require external features for secure fitment in a vehicle. These are molded into the container in the form of a ledge at the base of the battery so that they may be clamped securely. Alternatively, the battery may be clamped at the top. Batteries are often provided with handles for convenience. The lid on modern polypropylene batteries is securely welded to the container and molded or rope handles attached to the lid are widely in use. Valve-Regulated Automotive Batteries The majority of automotive batteries use a flooded type construction, but VRLA batteries are used in some vehicles and for certain applications such as aircraft batteries and in military service. VRLA automotive batteries use Pb–Ca–Sn or, in some cases, pure lead grids to reduce hydrogen emissions, an absorptive glass mat separator, and a limited acid volume in order that oxygen recombination is facilitated. The separator needs to be under a moderately high compression to ensure effective operation and avoid active material shedding. There are important differences in the behavior of VRLA and flooded batteries, but many of the design principles are similar. Some of the important differences are highlighted above, but the behavior of VRLA batteries under cyclic service is discussed below.
Batteries for Deep Cycling Batteries for deep cycling (Figure 2) are required to meet specified requirements for the following parameters: 1. 2. 3. 4.
capacity, charge retention, high-rate discharge performance, and cyclic endurance.
There are a number of standards for lead–acid batteries for cyclic applications and EN 60254-1:2005 (‘Lead– acid traction batteries. General requirements and methods of test’) has been taken as representative of the requirements. Capacity Cell capacity for motive power cells is measured at 30 1C by discharging the battery at a constant current at the 5 h rate to 1.70 V per cell. Cell capacity is required to be at least 85% of the rated capacity on the first cycle and 100% within 10 cycles. Cell or monobloc design for capacity is simply a matter of ensuring that the correct
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Vent plug
Pillar and grommet Lid
Negative grid
Positive plate spine Positive plate active material Positive plate gauntlet Positive plate bottom bar
Cell box
Plate support Negative active material Positive plate sleeve (separator)
Figure 2 Cross section of a tubular motive power cell. Courtesy of Exide Technologies.
quantity of active materials and acid is available for the battery design adopted. The number of cycles needed to achieve 100% of rated capacity is dependent on the manufacturing process; more extensive formation regimes will reduce the number of cycles required.
Charge Retention Charge retention is measured by storing fully charged cells for a period (28 days) at a constant temperature (20 1C) and then carrying out a normal capacity test as indicated in the section ‘Capacity’ above. A residual capacity of at least 85% of the nominal value is required. The values are specified such that all conventionally constructed cells or monoblocs should be compliant. Batteries using Pb–Sb alloys will, however, have lower levels of charge retention than batteries using either very low levels of antimony or Pb–Ca alloys.
High-Rate Discharge Performance For motive power cells, high rate performance is regarded as the 1 h rate at 30 1C to 1.60 V. This is not an onerous requirement and gives the user data about battery performance at higher rates rather than presenting the battery designer with a challenge. If substantially higher rates are required, thinner plates may be specified and cycle life may be compromised. Cyclic Endurance When cyclic endurance is the principal requirement for a lead–acid battery, the battery design is driven by that need. Cycle life tests require that the battery retain 80% of nominal capacity for 1500 cycles or as specified. For flooded cells, tests are carried out at 40 1C with a discharge for 3 h at the 4 h rate and a recharge for 9 h. For motive power cells, recharge is important and an excess of charge over the ampere-hour discharged of 15% or
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25% is required to bring the cells back to a fully charged condition unless the cell has an electrolyte agitation system in which case a lower charge factor specified by the manufacturer may be used. For VRLA cells, the discharge is at a slightly lower rate, and a more extended recharge is used with a constant current until the voltage reaches 2.40 V, followed by a low constant current for the final 2 h of charge. Cell design for motive power cells is radically different for cells with pasted or tubular positive plates, and these will be discussed separately below. VRLA cells – further subdivided into cells with gelled electrolyte and absorptive glass mat (AGM) separators – will also be discussed separately. Pasted Plate Cells The majority of traction cells supplied in North America use pasted positive plates, and although they tend to be less efficient than other designs in terms of material utilization and energy density, they are fully capable of meeting and exceeding cycle life requirements. Pasted plate cells use a thick conventionally cast positive plate made of a Pb–Sb alloy. The antimony level varies from B2% to B6%. Antimony stabilizes the active material and improves the cycle life. It also produces strong and durable grids. The disadvantage of a high antimony content, however, is an increase in water loss with service time. Antimony is released from the positive grid as it corrodes and is deposited on the negative electrode where it facilitates hydrogen evolution, leading to increased water loss. The separator can have an important influence on antimony transfer and reduce this effect. Lower antimony contents reduce water loss and the effects of corrosion, but this needs to be balanced against the beneficial effects of including the element. The lower hardness and strength of low-antimony alloys can be compensated by the use of selenium as a grainrefining additive. Copper and sulfur also act in the same way as low-level additives. Most batteries use antimony levels of B4% as a trade-off between the various effects and the cost of antimony. The active mass is conventionally pasted and a high paste density is used for durability on cycling. Curing and drying is generally carefully controlled and some suppliers use a curing cycle designed to increase the proportion of tetrabasic lead sulfate in order to improve cycle life. The key to long life with a pasted plate construction is the method of retaining the active material. Two layers of glass mat are used: an inner wrap of a looser mat around the base of the plate and an outer wrap of a finer mat around the side of the plate. These are held in place with a perforated poly(vinyl chloride) (PVC) sheet wrapped around the plate that is welded together. A plastic boot is fitted to the base of the plate.
Various types of separator may be used, but the majority of batteries use microporous polyethylene. A thick backweb (compared to that for automotive designs) is used with deep ribs to maintain plate separation. The separator may be either a sleeve around the negative plate or an envelope sealed on three sides. The negative plates are also of pasted plate construction. The grids are cast in low-antimony alloys. The cell element needs to be kept in reasonably strong compression in order to keep the retainer mat fully effective. The acid gravity needs to be as high as possible consistent with the corrosion life of the positive grids. Cell cases are molded in polypropylene with heat-sealed polypropylene lids. Batteries for materials handling applications are assembled from individual cells in steel trays. Packing pieces are used to ensure the assembly is robust and the cell compression is maintained. Better active material utilization and higher energy densities are achieved with thinner plates but at the expense of cycle life. This also allows a lower initial cost. Pasted plate batteries for traction applications – where less intense use is expected such as in golf carts, people movers, access platforms, and similar equipment – use thinner plates and are constructed as monoblocs.
Tubular Plate Cells Tubular plate traction cells are used in Europe, Japan, and other markets. The positive grid takes the form of an array of lead alloy spines with a diameter of B3 mm joined at the top and with the plate lug at the top. A fabric multitube gauntlet is placed over the spines so that there is an annular gap between the gauntlet and the spines. The gauntlet is usually a polyester fabric, in either woven or nonwoven form, stitched to define the tubes, or it can be cross-woven and is lightly impregnated with a resin to form a rigid array of small tubes. An alternative, but now a less used construction, is a resin-impregnated woven glass tube. This is placed on the spines that have a wider part at the top that fits closely to seal the tubes. The spines have protrusions or flights at intervals alternately at right angles to ensure the spines be in the center of the tubes. The active material is then filled into the annulus and the bottom of the spine is closed with a plastic boot. The active material may be pure red lead or a mixture of red lead and gray battery oxide in dry form or the same materials in slurry form. Dry filling is difficult to manage from an environmental point of view and wet filling is preferred. In the latter case, the fabric gauntlet retains the active material but permits excess water to be removed. The filled plates are dried, or they may be presulfated prior to drying. If they are dried after filling, the period after the acid filling and prior to the formation is chosen for sulfation.
Secondary Batteries – Lead–Acid Systems | Performance
The function of the gauntlet is the same as that of the retainer mat. It retains the positive active material intact and is in close contact with the spine. The other design features are similar to those of pasted plate cells although the need for a compressed cell element is lower. The considerations for grid alloy are the same from an electrochemical point of view although some casting techniques for the spines necessitate the use of Pb–11%Sb alloy, which is the eutectic composition in order to achieve the highest melt fluidity and congruent solidification, but more modern equipment permits lower antimony contents to be handled with ease. As with pasted plate batteries, thinner spines can be used to improve energy density, but at the expense of cycle life, and are also used for tubular monoblocs for lighter duties. Acid stratification can occur in all types of cells that are regularly cycled, with lower gravity developing near the top of the cell and higher gravity at the base. The problem is more acute with taller cells. It will lead to capacity decline that may be irreversible. Normally, there is sufficient overcharge for the cell to be freely gassing in the final stages of charging, and this serves to mix the electrolyte. Mechanical agitation may also be used to avoid stratification. Air may be bubbled into the electrolyte to mix it or to form an airlift pump. Alternatively, a higher voltage pulse may be introduced into the charging cycle at intervals to produce gassing and mixing. Mechanical or electrical agitation is used with lower levels of overcharge to reduce water loss and increase the period between maintenance watering. Valve-Regulated Gel Cells VRLA cells using gelled electrolyte are used for traction applications. Smaller monoblocs use pasted plates, and larger single cells use tubular plates. The principal design changes are the use of Pb–Ca grids, which in the case of the positive grid need to have a significant tin content, and the use of a separator with a pore size larger than that of polyethylene separators. Charging needs to be carefully controlled with limited overcharge. The cycle life will be shorter than that of a flooded cell with antimonial alloys, but for users who prefer zero maintenance watering and have lighter duty cycles, VRLA traction cells are recommended. In the early stages of life, gel cells do not recombine as efficiently as when there has been some water loss and behave more like flooded cells. This will tend to have a favorable influence on cycle life. Valve-Regulated Cells with Absorptive Glass Mat Separators VRLA cells and monoblocs with AGM separators are used for cyclic applications, and in order to achieve a reasonable cycle life, the selection of the positive grid
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alloy and the separator compression are important factors. Three distinct failure modes for VRLA batteries in deep cycling have been designated premature capacity loss (PCL) -1, -2, and -3. PCL-1 is a grid/positive active material effect where a passivation layer is formed at the grid/active material interface. It may be overcome by the use of Pb–Ca alloys or pure lead with additions of tin, which avoids the formation of insulating layers at the interface as for VRLA gel cells. PCL-2 is an active material effect where connective lead dioxide particles in the positive active mass become partially disconnected through the formation of areas of lead sulfate that are not recharged. It may be avoided by high compression of the separator so as to keep the active material under compression. PCL-3 is an effect where the negative plate is not sufficiently charged and becomes sulfated resulting in permanent capacity loss. It will be described further for batteries subjected to HRPSoC cycling. The charging profile is, however, important; for all types of VRLA batteries, a regime with constant current up to a voltage limit and then a final stage with a low constant current is normally used. A charge factor of 1.05–1.10 is used; higher charge factors cause excessive water loss, whereas lower charge factors lead to capacity walk-down. As the battery tends to dry out, a larger proportion of the current in the final stages of charging is used for oxygen recombination and the charge factor will tend to decrease. It is possible to increase the finishing current, but this has to be balanced against the heating effects from increased recombination. Accurate electronic control of charging is essential. One manufacturer uses a Pb–Sb–Cd alloy for the positive grids of traction cells. Antimony and cadmium form an intermetallic compound SbCd. The alloy does not passivate on cycling, and although antimony and cadmium are released by corrosion, the cadmium compensates for the effects of antimony on the negative plate. Reasonable cycle lives are achieved. Recycling of batteries is a problem as they need to be segregated and treated in facilities equipped to prevent escapes of cadmium from the waste streams. The design principles for VRLA batteries for cyclic service are applicable to VRLA batteries for automotive and standby applications where there is a limited requirement for cyclic endurance.
Batteries for Standby Applications Valve-Regulated Standby Batteries VRLA standby batteries (Figure 3) are required to meet specified requirements for the following parameters: 1. gas emission, 2. high current tolerance, 3. short-circuit current and DC internal resistance,
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requirement, as it will be within normal limits if the battery meets service life requirements. High current tolerance
This is specified as a requirement that no internal component in the battery show signs of melting or loss of electrical continuity within 30 s of a current of three times the 5 m rate (36 1C) to 1.80 V. From a design perspective, the group bars, terminals, and, for monoblocs, the intercell connectors need to have adequate sections to meet this requirement. Short-circuit current and direct current resistance
Figure 3 Value-regulated lead–acid (VRLA) 12 V, 100 Ah standby battery. Courtesy of NorthStar Battery.
This is a requirement to report a figure for the shortcircuit current and DC resistance of the battery. It is not a design requirement. Internal ignition from external spark sources
4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18.
internal ignition from external spark sources, protection against ground shorts, valve operation, flammability of materials, intercell connector performance, discharge capacity, charge retention during storage, float service with daily discharges, recharge behavior, service life at an operating temperature of 40 1C, effect of a stress temperature of 55 or 60 1C, abusive overdischarge, thermal runaway stability, low temperature sensitivity, dimensional stability at elevated internal pressure and temperature, and 19. resistance to mechanical abuse.
This defines that for specified volumes of hydrogen emission from the battery an external spark will not cause an explosion within the cell. To comply with this requirement, the final venting from the battery needs to have a flame retardant device incorporated in a manner similar to automotive batteries. The battery may vent into a plenum chamber that has a flame retardant device before it vents to the atmosphere.
These requirements are taken from EN 60896-21:2004 (‘Stationary lead-acid batteries. Valve regulated types. Methods of test’). This standard specifies the tests, and it is for the user to define the parameters required depending on the application, duty cycle, and expected service life. Standby batteries cover a wide range of applications: security and fire alarms use small batteries with a relatively short life; UPS applications require higher discharge rates and moderate lives; and telecommunications applications require lower discharge rates and longer lives. The design of batteries to meet these requirements is outlined below with reference to the parameters listed above.
Valve operation
Protection against ground shorts
This stipulates that the battery will not suffer ground shorts, burning, or charring of the container when 500 V DC is applied to case-to-cover joints over an extended period. The requirement will be met by ensuring that there are no leaks in the case-to-cover seal or the pillar seal and that the vent does not release acid spray to provide a conductive path on the container or cover.
The valves are required to open and close at the pressures specified by the supplier before and after the hightemperature stress test. Compliance requires that valve components be thermally stable and do not distort at higher temperatures. It is also important that the components do not stick together on exposure to elevated temperatures. Flammability of materials
The user may specify a flammability rating for the container and cover. The battery designer needs to specify a polymer grade with the required rating. Intercell connector performance
Gas emission
Gas emission is measured at the normal float voltage and at 2.40 V as milliliters per hour or milliliters per amperehour. It does not need to be considered as a design
The requirement is for the maximum temperature of the intercell connector to be reported when the cell or monobloc is discharged at the 15 m rate (4 1C). It should be below the softening point of the case and cover
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material. This is more specific than the safe limit for high currents as the intercell connector has a smaller section than the group bars. From a design point of view, it is simply necessary to ensure that the sections are adequate at all points. Discharge capacity
The standard stipulates that the battery will have on dispatch 95% of the rated capacity at the rate or rates required by the user. Compliance will be determined by battery processing during formation. More importantly, the rated capacity will be achieved by the correct electrochemical design in terms of active material and electrolyte volume and density. For lower rate performance, plate thickness is not critical, but for higher rate performance, thinner plates will deliver better performance but at the expense of service life. Active material density also needs to be considered. Lower densities will favor higher rate performance but may reduce integrity for longer service lives. Cycle life is, however, not critical and active material densities lower than those specified for motive power cells may be used. Charge retention during storage
The requirement for capacity retention during storage is for 70% of the 3 h capacity to remain after 180 days of storage at 25 1C. This is not an onerous requirement and all correctly designed VRLA standby batteries should be compliant. It does, however, impose a requirement for the battery to be delivered with the correct minimum capacity as indicated above and for batteries to be regularly recharged during storage.
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recommended float voltage for 24 and 168 h, respectively. The design features for float service with daily discharges will also influence this behavior. Service life at an operating temperature of 40 1C
The test requires that the battery have a residual capacity at the 3 h rate of >80% of the nominal value after exposure for specified duration to a 40 1C environment at the ambient temperature float voltage. Four levels of compliance are indicated: >500 days, >750 days, >1100 days, and >1700 days. These correspond approximately to 25 1C float lives of B3, 5–6, 8–10, and 12–14 years, respectively. A temperature of 40 1C represents the highest service temperature for normal use. Design for long service life on float is generally based on an assumption that service life will be determined by the thickness of the positive grid and the grid alloy. Other failure modes, such as dry out, corrosion of the group bars, and positive plate growth, should be such that they will take place over a longer timescale. The case and cover should not suffer any distortion at these temperatures. Dry out should not be a failure mode in any correctly designed normal VRLA standby battery. Corrosion of the group bars will be determined by alloy selection and the process used, and positive plate growth will be related to the grid thickness and grid alloy. Recent developments in Pb–Ca–Sn grid alloys with lower calcium levels and higher tin levels have improved corrosion resistance. The use of pure lead with small additions of tin also leads to long corrosion life. Silver as an alloying element has also been investigated. The thickness of plates used will be largely determined by balancing performance and economic requirements.
Float service with daily discharges
This test applies a partial discharge at twice the 10 h rate to remove 40% of the rated capacity at daily intervals at 25 1C with recharges at the recommended float voltage. Three levels of compliance are required: for normal utility reliability of more than 50 cycles; for unreliable mains power of more than 100 cycles; and for very unreliable mains power of more than 150 cycles. Features that improve cyclability, including the use of tin in the grid alloys and cell compression, are important in achieving compliance. Charge acceptance on float is also important and so thinner plate batteries will perform better. The higher levels of compliance fall short of regular deep discharges, and so factors such as active material density and plate processing are relatively less important. Recharge behavior
The requirement is for the batteries to have a 10 h capacity to 1.80 V per cell at 25 1C of >90% of the 10 h capacity and >98% of the 10 h capacity after a discharge at the 10 h rate to 1.80 V per cell when recharged at the
Effect of a stress temperature of 55 or 60 1C
These tests are used as accelerated life tests and the battery is exposed to 55 or 60 1C for varying periods at the ambient temperature float voltage, and discharges are carried out at intervals at the 3 h rate at 25 1C. The failure point is when the battery reaches 80% of the nominal capacity. Different levels of requirement are set at >150 days, >250 days, >350 days, and >500 days for 55 1C exposure or somewhat shorter time periods for 60 1C exposure. A variant of the test designed to assess durability for UPS applications uses a discharge at the 15 m rate, and the time periods in days for compliance are halved. This recognizes that loss of grid metal through corrosion will affect high rate performance more rapidly than low rate performance. The exposure times in days for compliance at the 3 h rate are broadly in line with the 40 1C requirement. For the purposes of testing, the battery may be clamped in a fixture to prevent any thermal distortion during the test. The design features for 40 1C service will have the same effect for a 55 or 60 1C accelerated test.
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Abusive overdischarge
A requirement for recovery after abusive overdischarges may be specified. This would involve a series of discharges to 1.25 V per cell followed by float recharges and a capacity test. Design for good cyclic performance will improve behavior under these conditions. Thermal runaway sensitivity
This test requires that a battery in a 25 1C environment will not reach 60 1C after 168 h at 2.45 V per cell or 24 h at 2.60 V per cell. A correctly designed VRLA standby battery will have no difficulty in complying with this requirement, but it should be noted that an aged battery may have a greater sensitivity to thermal runaway. Low temperature sensitivity
This requires that a battery exposed to an environment that is sufficiently cold to freeze the electrolyte retain 95% of the nominal 3 h capacity and not suffer any failure of the container. This is an extreme requirement, and if very low temperatures are expected, the battery should be heated or the level of discharge limited such that the electrolyte will not freeze. Nonetheless, VRLA batteries will perform satisfactorily even if the electrolyte has been frozen and thawed. The container material needs to have an adequate level of mechanical strength and fracture toughness. Dimensional stability at elevated internal pressure and temperature
This is a test to confirm that the container will not distort at higher temperatures over time such that compression would be lost. The test is carried out by subjecting the container to the maximum vent operating pressure, exposing the container to 50 1C for 24 h, and then measuring for any distortion. The correct grade of polymer for the container needs to be specified, and if extended periods at higher temperature are required, more thermally stable materials should be recommended. Resistance to mechanical abuse
Resistance to mechanical abuse may be evaluated by dropping the battery on a corner or an edge from a defined height. It should not split, fracture, or leak. The test is intended to show that the battery will not suffer damage in transit prior to installation. The strength and fracture toughness of the container are the key parameters. General Design Features VRLA standby batteries are, as for motive power batteries, of two main types: those with AGM separators and flat pasted plates and those with gelled electrolyte and either tubular or flat plates. The design principles outlined above are principally for AGM types but are also
applicable to gelled electrolyte types where there is less influence from the separator. Tubular gel cells
Tubular gel cells generally comply with the DIN (Deutsches Institut fur Normung/German Institute for Standardization) OPzV (Ortsfeste verschlossene Batterien mit positiven Panzerplatten und festgelegtem Electrolyt/sealed standby battery with tubular plates and gelled electrolyte) specifications and dimensions. Both grids use Pb–Ca–Sn alloys as described, with tubular positive plates and flat pasted negative grids. The separator is similar to those used for cyclic applications and the electrolyte is gelled with finely divided silica. Cell cases and lids are molded in flame retardant acrylonitrile butadiene styrene (ABS) copolymer. Pillar seals are proprietary and the valves are normally of a simple Bunsen valve type. The life in floating service is B12 years and this type of cell has a reasonably good cyclic performance. The rate capability is modest. Pasted plate gel cells
Pasted plate gel cells differ only in the construction of the positive plate that is in the form of a flat pasted plate. Life in floating service will depend principally on grid profile and alloy selection. Value-regulated lead–acid absorptive glass mat batteries with Pb–Ca–Sn grids
The majority of VRLA batteries produced, either as cells or as monoblocs, use pasted plates with Pb–Ca–Sn grids and AGM separators. Cell cases may be polypropylene, ABS, polycarbonate (PC)/ABS, or PVC with or without flame retardant additives. Grid alloys and thickness and the other details of cell design are as described. Pb–Sb–Cd alloys have been used for standby batteries by one manufacturer. Pillar seals use a variety of rubber sealing rings, mechanical compression seals, and thermosetting resins. Valves may be of a Bunsen valve type or a more complex arrangement and normally incorporate a flame filter. Value-regulated lead–acid absorptive glass mat batteries with pure lead grids
Pure lead or pure lead–tin grids may be used instead of Pb–Ca–Sn grids. These are fabricated from a continuously cast or wrought strip by punching the grid pattern into the strip, which is then processed conventionally. Thinner grids may be used because the corrosion rate is much lower and the active material utilization is higher. Other details of construction are similar to those for other types of cell. Thicker plate cells with pure lead plates are also manufactured. The corrosion life is extended, but the rate performance is unaltered. Service lives in excess of 25 years at 25 1C and up to 10 years at 40 1C are claimed.
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Flooded Standby Batteries Flooded standby batteries are important for some telecommunications applications, for utilities including nuclear power and large security applications, and for large critical UPS applications. Two technologies dominate the market: pasted plate Pb–Ca batteries are mainly used in North America and tubular plate batteries in European and other markets. Special round cells and Plante´ cells have limited applications. Pasted plate cells
Pasted plate cells use Pb–Ca–Sn grids of substantial thickness, and a life of up to 25 years is achieved in a well-controlled environment. The electrolyte density may be reduced in order to achieve a longer life. The positive plates are designed in such a way that plate growth can occur in a benign manner with the plates suspended from the top of the container. The positive plates are wrapped in a glass fiber retainer mat to ensure that shedding of active material cannot occur. Microporous plastic separators are used. Cell containers are molded in transparent PVC or styrene acrylonitrile (SAN), allowing visual inspection of the condition of the cells. Pillar seals are proprietary and are designed to have a high degree of integrity in order to avoid stressing the seal over the life of the battery. Tubular plate cells
Tubular plate cells for standby service follow the OPzS (Ortsfeste gerschlossene Batterien mit positven Panzerplatten/open standby battery with tubular plates) specification to DIN requirements. The positive plates have lowantimony alloy spines with a fabric gauntlet, as for cells for cyclic applications. Some suppliers offer Pb–Ca–Sn grids as an alternative. The principal difference for the positive plate is that Pb–Sb grids have very low antimony contents in order to reduce water loss. The negative plates are a pasted plate type and generally microporous polyethylene separators are used. Cell containers are molded in SAN so that the electrolyte level and visual condition can be readily seen. Pillar seals offer freedom from leakage and corrosion for the service life of the battery but vary in important details between suppliers. The specific gravity of the acid is higher than that for pasted plate cells. Sufficient excess of electrolyte above the group bar can be provided to permit a watering interval of up to 3 years, and the overall service life is up to 15 years. Cells with Pb–Sb grids provide good cyclic performance, and for applications where the public utility is unreliable or for supporting solar photovoltaic power systems, they provide good performance. Round cells
The round cells designed by AT&T Bell Laboratories have a number of unique features incorporated in order
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to provide a life of 40 years or more. These have a circular pure lead positive grid shaped as a shallow cone and stacked horizontally with pure lead negative plates of similar construction. The positives are welded together externally and the negatives welded to a central conductive core with an insulator at the outside edge. Microporous polyethylene separators are used with a glass retainer mat. The cell containers and lids are molded in PVC. The positive active material is a mixture of chemically produced tetrabasic lead sulfate and red lead before formation. The use of pure lead reduces plate growth and corrosion. The special shape is designed to counter the effects of plate growth and to ensure that the grid and active material remain in good contact throughout the life of the battery. Grid growth is caused by the formation of lead dioxide at the surface of the grid that distorts the grid, but in this case the shape change causes no adverse effects. The conversion of grid material into lead dioxide causes a small but measurable capacity increase over time. The specific gravity of the acid is low and the float voltage is also low. This could cause a problem with the recharge of the positive plate, which may be resolved by adding a small amount of platinum as a depolarizer to the negative expander rather than by increasing the float voltage. The terminal design is also unique. A Pb–Sn alloy is used to prevent nodular corrosion, and an epoxy sheath is formed around the pillar, which then has a rubber boot to seal to the cover. It is important for seal integrity that the area coated with epoxy is not below the electrolyte level. This type of cell is deployed in large telecommunications switching centers and long service lives are being achieved. Plante´ cells
Plante´ cells were used in Europe and elsewhere for telecommunications and utility standby applications, but their use is now confined to replacement. These cells use a positive grid cast in pure lead with an extended surface such that the active material is formed directly by corrosion of the grid material. The grids are hung from the top of the cell such that plate growth can occur harmlessly downward. Pasted plates with Pb–Sn alloys are used for the negative plates and microporous plastic separators. Cell cases and lids are molded in SAN, and pillar seals vary in design from simple rubber bushings to arrangements similar to the round Bell-designed cells. This type of battery has a service life of up to 25 years.
Batteries for Hybrid Electric Vehicle Applications Batteries for HEV applications are a new category with an HRPSoC duty cycle. As the battery cannot be returned to a full SOC, different failure modes occur that
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require new approaches to provide solutions. The duty cycle involves repeated discharge at very high rates for vehicle acceleration followed by charging also at high rates to recover energy by regeneration on braking. In order to be able to charge at high rates, the battery cannot be fully charged at any time in normal service, as the charge acceptance and internal resistance of the battery increase rapidly at top of charge. This has been designated PCL-3 as discussed above. This causes the negative plate to deteriorate by irreversibly sulfating and progressively losing capacity. The problem may be resolved, at least in part, by adding special carbons to the negative plate in larger quantities than for a conventional battery. These improve the recharge behavior and have a beneficial effect on cycle life. The need for discharge as well as recharge at high rates also results in designs that are further optimized for high rates as compared to automotive. Grid designs have been developed that provide better active material utilization at high rates. The batteries used in this area are invariably VRLA types, and the design features to improve cycle life also need to be applied. Strategies for equalization charging and then discharging prior to service have received attention. The use of lead–acid batteries for HEV applications is receiving considerable attention as a research topic, and new developments may certainly be expected.
Nomenclature Abbreviations and Acronyms ABS AGM CCA d.c. DIN
acrylonitrile butadiene styrene absorptive glass mat cold cranking amps direct current Deutsches Institut fur Normung/ German Institute for Standardization
DoD HEV HRPSoC OPzS
OPzV
PC PCL PVC SAN SoC UPS VRLA
depth of discharge hybrid electric vehicle high-rate partial state of charge Ortsfeste gerschlossene Batterien mit positven Panzerplatten/open standby battery with tubular plates Ortsfeste verschlossene Batterien mit positiven Panzerplatten und festgelegtem Electrolyt/sealed standby battery with tubular plates and gelled electrolyte polycarbonate premature capacity loss polyvinyl chloride styrene acrylonitrile state-of-charge uninterruptible power supply valve-regulated lead–acid
See also: .Secondary Batteries – Lead–Acid Systems: Overview.
Further Reading Berndt D (1993) Maintenance-Free Batteries: Lead–Acid, Nickel/ Cadmium, Nickel/Metal Hydride: A Handbook of Battery Technology. Taunton: Research Studies Press, John Wiley & Sons. Bode H (1977) Lead–Acid Batteries. New York: John Wiley & Sons. May GJ (2006) Standby battery requirements for telecommunications power. Journal of Power Sources 158: 1117--1123. Rand DAJ, Moseley PT, Garche J, and Parker CD (2004) ValveRegulated Lead–Acid Batteries. Amsterdam: Elsevier. Vinal GW (1955) Storage Batteries: A General Treatise on the Physics and Chemistry of Secondary Batteries and Their Engineering Applications. New York: John Wiley & Sons.
Introduction The lead–acid battery is capable of being adapted for optimum performance for a variety of duty cycles. These may be broadly characterized by the rate of discharge, by the frequency of discharge and recharge, and by the time that the battery remains on charge. High discharge rates are required for starting internal combustion and gas turbine engines for cars, commercial vehicles, marine applications, aircraft, railway locomotives, military service, generators, and stationary engines. Batteries for this type of application are broadly categorized as automotive batteries. Specifications generally include high rate performance at low temperatures, and this tends to be the dominant factor in battery design. But for a number of other applications cyclic duty capability is also often considered. Regular charge and discharge cycling for materials handling, electric vehicles of all types, and other duty cycles where energy is stored for later use dictate different design features in the batteries optimized for these applications. Batteries for standby applications require that they remain on a continuous charge so that the direct current (d.c) supply is maintained without interruption. These are used for telecommunications, uninterruptible power supplies (UPSs), utilities, emergency lighting, security and fire alarms, and general standby applications. The design is then determined by the required time on charge and the discharge rate. Battery design will be discussed for high-rate/ low-temperature performance (automotive), cyclic performance (motive power), and life on floating charge (standby). Most battery applications involve a combination of these parameters, and in all cases the battery is fully charged after a discharge, except for hybrid electric vehicle (HEV) applications where the battery cannot be returned to a full state-of-charge (SoC) but is required to be regularly discharged at high rates. This important new application will be discussed separately.
Battery Design to Meet Application Requirements For automotive applications, battery design is directed at reducing the internal resistance by the design of the grids to improve conductivity, by increasing the number of grids so that they become thinner for any given low-rate capacity, by reducing the distance between the plates, and by reducing the resistance of the separator. Failure at
the end of life will result from corrosion of the positive grid, shedding of the active material, and, often, irreversible sulfation. Grid thickness and the grid pattern will be determined by the manufacturing process available, the grid alloy used, the life required, the specific performance required, and the economic cost. Other requirements will include a limited cyclic duty, but if there is a requirement for a more extended cyclic duty, the design may need to be adjusted at the expense of specific high rate performance. For cyclic applications, the design is determined by the need to maintain a stable capacity for up to 1500 deep cycles, to avoid premature failure from corrosion of the positive grids, and to avoid shedding and softening of the active materials. The selection of the grid alloy, particularly for the positive grid, is important and it is also important to have a system for retaining the positive active material intact and in compression to avoid shedding. Charging systems are another key to long cycle life. The battery needs to be brought to a full SoC and the charging terminated promptly to avoid overcharge with electrolyte loss and corrosion of the positive grid. For standby power, the battery generally remains on a continuous charge at a fixed voltage. Service life will be determined by the operational temperature, the behavior of the positive grid alloy, and the grid thickness. The required rate of discharge also has an impact on service life. At higher rates of discharge the performance will be more rapidly degraded by grid corrosion, whereas at lower rates the conductivity of the positive grid is less critical. For HEV applications, a unique duty cycle that has been designated high-rate partial-state-of-charge (HRPSoC) operation becomes important. The battery needs to be able to accept charge at high rates at all times so that energy recovered by regenerative braking can be stored in it. Discharge is also at high rates and the battery is used intensively with relatively shallow charges and discharges, but never reaching a full charge. This is damaging to the negative plate, and special design considerations are required to ensure a long service life.
Battery Design for Automotive Applications Automotive batteries (Figure 1) are required to meet specified requirements for the following parameters: 1. High rate performance (cold cranking amps, CCA) 2. Reserve capacity/20 h capacity
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Figure 1 A typical modern automotive battery in situ in a vehicle (12 V, 90 Ah).
3. 4. 5. 6. 7. 8. 9.
Cyclic performance Charge retention Corrosion endurance Water consumption Vibration resistance Electrolyte retention Cranking performance for dry-charged batteries after activation
The requirements of EN 50342-1: 2006 (‘Lead–acid starter batteries. General requirements and methods of test’) have been taken as a reference point for discussion of the design principles for automotive batteries. Design for High Rate Performance The required high rate performance is normally specified at 18 1C in amps for a short defined time (30 or 60 s) such that the on-load voltage remains above a required level. There are variations of this type of test; for example, a higher current may be used for 10 s so that the on-load voltage remains above 7.5 V, followed by a lower current (60% of the initial level) where the time to reach an on-load voltage of 6.0 V is recorded. The latter test may be a more realistic one for engine starting, but shorter-time tests lead to adjustments in battery design to meet the specification. Irrespective of the specification, automotive battery design always starts with cold cranking performance. The grid thickness and wire section of the positive grid, irrespective of the manufacturing process for any given alloy, will be as low as possible consistent with adequate corrosion resistance.
Mechanical handling during plate manufacture and cell assembly will also set practical limits to the plate thickness. The positioning of the plate lugs and the detailed design of the grid are aimed at improving conductivity. The thickness of the negative plate will be less than that of the positive plate but sufficient to provide the required active material to balance the positive active mass. The spacing between the plates will be determined by the mass of sulfuric acid required to fully discharge the active material to meet the reserve capacity or 20 h capacity required. The separator needs to have high porosity and high tortuosity between the plates and to be as thin as possible consistent with adequate separation over life in order to reduce internal resistance. The separator also needs to maintain the correct plate pitch, and so, in a flooded battery, the separator has a thin membrane (backweb) with ribs spaced apart on the surface to keep the plates at the correct distance when lightly compressed. In a valve-regulated lead–acid (VRLA) battery, the separator occupies the whole of the space between the plates and needs to be more highly compressed. This will be discussed further below. Discharge at high rates is controlled by the diffusion of electrolyte to the active material, particularly at the plate surface. Free circulation of acid at the surface will improve performance in flooded batteries. The types of separator used in VRLA batteries are such that acid is readily replenished at the plate surface. If high rate performance is more important than low-rate behavior, the plate spacing can be reduced, but then battery performance will become limited by the availability of acid.
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The design of the plate straps and intercell connections need to take full account of the need to improve conductivity. Design for Low-Rate Capacity Low-rate capacity is specified either as the 20 h capacity at 25 1C at a constant current to 10.5 V or as the reserve capacity at 25 1C at a constant current of 25 A, in which case the time in minutes is measured. For certain applications, higher low-rate capacity may be required. Examples would be batteries on a boat where there is a requirement for power during engine-off periods or military applications where there are periods of silent watch. In both these cases, plate thickness could be increased to provide more active material without using a larger number of thin plates. This would be more costeffective. Cyclic Performance Failure under cyclic conditions will result from positive grid corrosion and active material shedding. The cyclic tests called up in automotive standards are shallow discharges (typically 50% depth-of-discharge (DoD)) at 25 1C with constant voltage recharges at different voltages depending on the water loss of the battery, and failure is determined by the inability of the battery to reach a specified low-temperature cold cranking performance after a given number of cyclic test units. The positive grid alloy and grid thickness are important in determining the cycle life, but for more extended cycle life, additionally the positive active material needs to be physically retained. Glass or polymer fiber mats may be attached to the separator to retain the active mass. Other parameters such as the density of the active material and the curing and drying cycle are also important. Higher densities favor better cycle life but at the expense of high rate performance. Antimonial alloys also provide better cycle life but at the expense of charge retention. For Pb–Ca alloys, the use of tin as an alloying element improves cyclic performance. Charge Retention Charge retention is measured by holding the battery at an elevated temperature for a period (e.g., 21 days at 40 1C) and then carrying out a low-temperature highrate discharge as indicated above in ‘Design for High Rate Performance’. Batteries described as ‘normal water loss’ ones have a less stringent requirement than the ones that are ‘low water loss’, ‘very low water loss’, or valveregulated. Charge retention is principally determined by the grid alloys although the electrochemical design of the battery and the purity of the materials have secondary effects. The use of Pb–Sb alloys decreases charge retention; lower levels of antimony improve charge
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retention; hybrid batteries with Pb–Sb positive grids and Pb–Ca negative grids result in further improvements; and for the highest levels of charge retention, both grids need to be made of Pb–Ca alloys, which may be further alloyed to improve other characteristics. Pure lead grids and pure lead grids with small additions of tin also provide excellent charge retention. Corrosion Endurance Corrosion endurance is measured by an accelerated life test where the battery is held at a constant voltage charge at an elevated temperature. The battery may be held at 60 1C for 13 days at 14.0 V, then on open circuit for 13 days at 60 1C, recharged and then subjected to an ambient-temperature high-rate test. A number of test units will be specified for the test. As for charge retention, the selection of grid alloy is important for corrosion resistance: Pb–Ca alloys may be alloyed with silver as well as tin to improve corrosion resistance. This has been used extensively in markets where ambient temperatures are high, to improve battery life. Water Consumption Water consumption is determined by holding the battery at an elevated temperature for a defined period and on charge at a constant voltage. The temperature is typically 40 1C, the voltage 14.4 V, and the time 500 h. Water loss is measured as a weight loss and expressed in g Ah 1. For low water loss batteries the weight loss should not exceed 4 g Ah 1 and for very low water loss batteries it should not exceed 1 g Ah 1 in each case referenced to nominal capacity. Water consumption depends on the grid alloys, with Pb–Ca alloys having lower water losses than Pb–Sb alloys, as the water loss is directly related to the hydrogen overpotential of the alloy. For VRLA batteries, the time period for measurement is extended, but a period of overcharge is used prior to measurement in order to stabilize the battery. Vibration Resistance Vibration resistance is important for automotive service and is assessed by clamping the battery to a vibration tester and applying vibration, generally in a vertical plane, at, say, 30 Hz with a specified maximum level of acceleration, for a defined period. The integrity of the battery is verified by carrying out a room-temperature high-rate test to a defined end voltage. Vibration specifications vary depending on application: for normal automotive service, they are at a normal level, but for commercial vehicles, or for military service, much higher levels of vibration resistance may be required. Damage under vibration tends to occur when the various components move relative to each other and the usual failure mode is for the plate lugs to become detached from the group bar. For a normal level of
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vibration resistance, provided the cell element is tightly fitted in the battery container and the group bar is secured at the intercell partition or the terminals in a standard construction, the battery will meet the required levels. However, if higher levels of vibration resistance are required, the various components need to be locked in position so that there is no relative movement. Plate locks are used to keep the elements in a fixed position. These may be small mechanical clamps, or the element may be fixed in position using an adhesive. A well-compressed element is also needed and a glass or synthetic mat may be used to reduce shedding of the active material that will be exacerbated under vibration. The design of the group bar is important and adhesive may be used to reduce relative movement. Vibration resistance may be further improved by designing the battery case to be more mechanically robust. Electrolyte Retention Electrolyte retention may be assessed for flooded batteries by tilting the battery such that the vents are immersed in the electrolyte for a short period. No visible leakage should occur. The vent needs to have some form of labyrinth so that the electrolyte will take time to leak out when tilted, and can drain back into the cell in upright position. Alternatively, the vent may incorporate a flame retardant device that will take the form of a porous ceramic disk or a porous polymer disk. This will prevent any externally ignited hydrogen flame from entering the battery and causing an explosion, but it will also provide an effective means of retaining the electrolyte. Some vehicle manufacturers specify that flooded batteries should not leak even when fully inverted. In these cases, more elaborate battery lids and vents may be used to retain the electrolyte, or devices that close the cell when inverted may be incorporated. For VRLA batteries, the tests are more stringent and the battery should not show any sign of leakage even when fully inverted for several hours. Cranking Performance for Dry-Charged Batteries after Activation Dry-charged batteries are processed so that the negative plate contains an oxidation inhibitor, and when filled with electrolyte, they will provide sufficient energy at a high rate to meet a specified requirement without additional charging. The processes used to achieve the required storage life before activation are proprietary but may use an additive in the paste mix such as mineral oil or stearic acid or may be a spray or a dip during the drying process. Boric acid may also be used. The plate drying process needs to be under nonoxidizing conditions; a vacuum dryer may be used, but more often a gas-fired dryer burning under substoichiometric conditions is used so that the atmosphere is rendered reducing. There
are no special processing requirements for the other components. Other Requirements Batteries require external features for secure fitment in a vehicle. These are molded into the container in the form of a ledge at the base of the battery so that they may be clamped securely. Alternatively, the battery may be clamped at the top. Batteries are often provided with handles for convenience. The lid on modern polypropylene batteries is securely welded to the container and molded or rope handles attached to the lid are widely in use. Valve-Regulated Automotive Batteries The majority of automotive batteries use a flooded type construction, but VRLA batteries are used in some vehicles and for certain applications such as aircraft batteries and in military service. VRLA automotive batteries use Pb–Ca–Sn or, in some cases, pure lead grids to reduce hydrogen emissions, an absorptive glass mat separator, and a limited acid volume in order that oxygen recombination is facilitated. The separator needs to be under a moderately high compression to ensure effective operation and avoid active material shedding. There are important differences in the behavior of VRLA and flooded batteries, but many of the design principles are similar. Some of the important differences are highlighted above, but the behavior of VRLA batteries under cyclic service is discussed below.
Batteries for Deep Cycling Batteries for deep cycling (Figure 2) are required to meet specified requirements for the following parameters: 1. 2. 3. 4.
capacity, charge retention, high-rate discharge performance, and cyclic endurance.
There are a number of standards for lead–acid batteries for cyclic applications and EN 60254-1:2005 (‘Lead– acid traction batteries. General requirements and methods of test’) has been taken as representative of the requirements. Capacity Cell capacity for motive power cells is measured at 30 1C by discharging the battery at a constant current at the 5 h rate to 1.70 V per cell. Cell capacity is required to be at least 85% of the rated capacity on the first cycle and 100% within 10 cycles. Cell or monobloc design for capacity is simply a matter of ensuring that the correct
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Vent plug
Pillar and grommet Lid
Negative grid
Positive plate spine Positive plate active material Positive plate gauntlet Positive plate bottom bar
Cell box
Plate support Negative active material Positive plate sleeve (separator)
Figure 2 Cross section of a tubular motive power cell. Courtesy of Exide Technologies.
quantity of active materials and acid is available for the battery design adopted. The number of cycles needed to achieve 100% of rated capacity is dependent on the manufacturing process; more extensive formation regimes will reduce the number of cycles required.
Charge Retention Charge retention is measured by storing fully charged cells for a period (28 days) at a constant temperature (20 1C) and then carrying out a normal capacity test as indicated in the section ‘Capacity’ above. A residual capacity of at least 85% of the nominal value is required. The values are specified such that all conventionally constructed cells or monoblocs should be compliant. Batteries using Pb–Sb alloys will, however, have lower levels of charge retention than batteries using either very low levels of antimony or Pb–Ca alloys.
High-Rate Discharge Performance For motive power cells, high rate performance is regarded as the 1 h rate at 30 1C to 1.60 V. This is not an onerous requirement and gives the user data about battery performance at higher rates rather than presenting the battery designer with a challenge. If substantially higher rates are required, thinner plates may be specified and cycle life may be compromised. Cyclic Endurance When cyclic endurance is the principal requirement for a lead–acid battery, the battery design is driven by that need. Cycle life tests require that the battery retain 80% of nominal capacity for 1500 cycles or as specified. For flooded cells, tests are carried out at 40 1C with a discharge for 3 h at the 4 h rate and a recharge for 9 h. For motive power cells, recharge is important and an excess of charge over the ampere-hour discharged of 15% or
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25% is required to bring the cells back to a fully charged condition unless the cell has an electrolyte agitation system in which case a lower charge factor specified by the manufacturer may be used. For VRLA cells, the discharge is at a slightly lower rate, and a more extended recharge is used with a constant current until the voltage reaches 2.40 V, followed by a low constant current for the final 2 h of charge. Cell design for motive power cells is radically different for cells with pasted or tubular positive plates, and these will be discussed separately below. VRLA cells – further subdivided into cells with gelled electrolyte and absorptive glass mat (AGM) separators – will also be discussed separately. Pasted Plate Cells The majority of traction cells supplied in North America use pasted positive plates, and although they tend to be less efficient than other designs in terms of material utilization and energy density, they are fully capable of meeting and exceeding cycle life requirements. Pasted plate cells use a thick conventionally cast positive plate made of a Pb–Sb alloy. The antimony level varies from B2% to B6%. Antimony stabilizes the active material and improves the cycle life. It also produces strong and durable grids. The disadvantage of a high antimony content, however, is an increase in water loss with service time. Antimony is released from the positive grid as it corrodes and is deposited on the negative electrode where it facilitates hydrogen evolution, leading to increased water loss. The separator can have an important influence on antimony transfer and reduce this effect. Lower antimony contents reduce water loss and the effects of corrosion, but this needs to be balanced against the beneficial effects of including the element. The lower hardness and strength of low-antimony alloys can be compensated by the use of selenium as a grainrefining additive. Copper and sulfur also act in the same way as low-level additives. Most batteries use antimony levels of B4% as a trade-off between the various effects and the cost of antimony. The active mass is conventionally pasted and a high paste density is used for durability on cycling. Curing and drying is generally carefully controlled and some suppliers use a curing cycle designed to increase the proportion of tetrabasic lead sulfate in order to improve cycle life. The key to long life with a pasted plate construction is the method of retaining the active material. Two layers of glass mat are used: an inner wrap of a looser mat around the base of the plate and an outer wrap of a finer mat around the side of the plate. These are held in place with a perforated poly(vinyl chloride) (PVC) sheet wrapped around the plate that is welded together. A plastic boot is fitted to the base of the plate.
Various types of separator may be used, but the majority of batteries use microporous polyethylene. A thick backweb (compared to that for automotive designs) is used with deep ribs to maintain plate separation. The separator may be either a sleeve around the negative plate or an envelope sealed on three sides. The negative plates are also of pasted plate construction. The grids are cast in low-antimony alloys. The cell element needs to be kept in reasonably strong compression in order to keep the retainer mat fully effective. The acid gravity needs to be as high as possible consistent with the corrosion life of the positive grids. Cell cases are molded in polypropylene with heat-sealed polypropylene lids. Batteries for materials handling applications are assembled from individual cells in steel trays. Packing pieces are used to ensure the assembly is robust and the cell compression is maintained. Better active material utilization and higher energy densities are achieved with thinner plates but at the expense of cycle life. This also allows a lower initial cost. Pasted plate batteries for traction applications – where less intense use is expected such as in golf carts, people movers, access platforms, and similar equipment – use thinner plates and are constructed as monoblocs.
Tubular Plate Cells Tubular plate traction cells are used in Europe, Japan, and other markets. The positive grid takes the form of an array of lead alloy spines with a diameter of B3 mm joined at the top and with the plate lug at the top. A fabric multitube gauntlet is placed over the spines so that there is an annular gap between the gauntlet and the spines. The gauntlet is usually a polyester fabric, in either woven or nonwoven form, stitched to define the tubes, or it can be cross-woven and is lightly impregnated with a resin to form a rigid array of small tubes. An alternative, but now a less used construction, is a resin-impregnated woven glass tube. This is placed on the spines that have a wider part at the top that fits closely to seal the tubes. The spines have protrusions or flights at intervals alternately at right angles to ensure the spines be in the center of the tubes. The active material is then filled into the annulus and the bottom of the spine is closed with a plastic boot. The active material may be pure red lead or a mixture of red lead and gray battery oxide in dry form or the same materials in slurry form. Dry filling is difficult to manage from an environmental point of view and wet filling is preferred. In the latter case, the fabric gauntlet retains the active material but permits excess water to be removed. The filled plates are dried, or they may be presulfated prior to drying. If they are dried after filling, the period after the acid filling and prior to the formation is chosen for sulfation.
Secondary Batteries – Lead–Acid Systems | Performance
The function of the gauntlet is the same as that of the retainer mat. It retains the positive active material intact and is in close contact with the spine. The other design features are similar to those of pasted plate cells although the need for a compressed cell element is lower. The considerations for grid alloy are the same from an electrochemical point of view although some casting techniques for the spines necessitate the use of Pb–11%Sb alloy, which is the eutectic composition in order to achieve the highest melt fluidity and congruent solidification, but more modern equipment permits lower antimony contents to be handled with ease. As with pasted plate batteries, thinner spines can be used to improve energy density, but at the expense of cycle life, and are also used for tubular monoblocs for lighter duties. Acid stratification can occur in all types of cells that are regularly cycled, with lower gravity developing near the top of the cell and higher gravity at the base. The problem is more acute with taller cells. It will lead to capacity decline that may be irreversible. Normally, there is sufficient overcharge for the cell to be freely gassing in the final stages of charging, and this serves to mix the electrolyte. Mechanical agitation may also be used to avoid stratification. Air may be bubbled into the electrolyte to mix it or to form an airlift pump. Alternatively, a higher voltage pulse may be introduced into the charging cycle at intervals to produce gassing and mixing. Mechanical or electrical agitation is used with lower levels of overcharge to reduce water loss and increase the period between maintenance watering. Valve-Regulated Gel Cells VRLA cells using gelled electrolyte are used for traction applications. Smaller monoblocs use pasted plates, and larger single cells use tubular plates. The principal design changes are the use of Pb–Ca grids, which in the case of the positive grid need to have a significant tin content, and the use of a separator with a pore size larger than that of polyethylene separators. Charging needs to be carefully controlled with limited overcharge. The cycle life will be shorter than that of a flooded cell with antimonial alloys, but for users who prefer zero maintenance watering and have lighter duty cycles, VRLA traction cells are recommended. In the early stages of life, gel cells do not recombine as efficiently as when there has been some water loss and behave more like flooded cells. This will tend to have a favorable influence on cycle life. Valve-Regulated Cells with Absorptive Glass Mat Separators VRLA cells and monoblocs with AGM separators are used for cyclic applications, and in order to achieve a reasonable cycle life, the selection of the positive grid
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alloy and the separator compression are important factors. Three distinct failure modes for VRLA batteries in deep cycling have been designated premature capacity loss (PCL) -1, -2, and -3. PCL-1 is a grid/positive active material effect where a passivation layer is formed at the grid/active material interface. It may be overcome by the use of Pb–Ca alloys or pure lead with additions of tin, which avoids the formation of insulating layers at the interface as for VRLA gel cells. PCL-2 is an active material effect where connective lead dioxide particles in the positive active mass become partially disconnected through the formation of areas of lead sulfate that are not recharged. It may be avoided by high compression of the separator so as to keep the active material under compression. PCL-3 is an effect where the negative plate is not sufficiently charged and becomes sulfated resulting in permanent capacity loss. It will be described further for batteries subjected to HRPSoC cycling. The charging profile is, however, important; for all types of VRLA batteries, a regime with constant current up to a voltage limit and then a final stage with a low constant current is normally used. A charge factor of 1.05–1.10 is used; higher charge factors cause excessive water loss, whereas lower charge factors lead to capacity walk-down. As the battery tends to dry out, a larger proportion of the current in the final stages of charging is used for oxygen recombination and the charge factor will tend to decrease. It is possible to increase the finishing current, but this has to be balanced against the heating effects from increased recombination. Accurate electronic control of charging is essential. One manufacturer uses a Pb–Sb–Cd alloy for the positive grids of traction cells. Antimony and cadmium form an intermetallic compound SbCd. The alloy does not passivate on cycling, and although antimony and cadmium are released by corrosion, the cadmium compensates for the effects of antimony on the negative plate. Reasonable cycle lives are achieved. Recycling of batteries is a problem as they need to be segregated and treated in facilities equipped to prevent escapes of cadmium from the waste streams. The design principles for VRLA batteries for cyclic service are applicable to VRLA batteries for automotive and standby applications where there is a limited requirement for cyclic endurance.
Batteries for Standby Applications Valve-Regulated Standby Batteries VRLA standby batteries (Figure 3) are required to meet specified requirements for the following parameters: 1. gas emission, 2. high current tolerance, 3. short-circuit current and DC internal resistance,
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requirement, as it will be within normal limits if the battery meets service life requirements. High current tolerance
This is specified as a requirement that no internal component in the battery show signs of melting or loss of electrical continuity within 30 s of a current of three times the 5 m rate (36 1C) to 1.80 V. From a design perspective, the group bars, terminals, and, for monoblocs, the intercell connectors need to have adequate sections to meet this requirement. Short-circuit current and direct current resistance
Figure 3 Value-regulated lead–acid (VRLA) 12 V, 100 Ah standby battery. Courtesy of NorthStar Battery.
This is a requirement to report a figure for the shortcircuit current and DC resistance of the battery. It is not a design requirement. Internal ignition from external spark sources
4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18.
internal ignition from external spark sources, protection against ground shorts, valve operation, flammability of materials, intercell connector performance, discharge capacity, charge retention during storage, float service with daily discharges, recharge behavior, service life at an operating temperature of 40 1C, effect of a stress temperature of 55 or 60 1C, abusive overdischarge, thermal runaway stability, low temperature sensitivity, dimensional stability at elevated internal pressure and temperature, and 19. resistance to mechanical abuse.
This defines that for specified volumes of hydrogen emission from the battery an external spark will not cause an explosion within the cell. To comply with this requirement, the final venting from the battery needs to have a flame retardant device incorporated in a manner similar to automotive batteries. The battery may vent into a plenum chamber that has a flame retardant device before it vents to the atmosphere.
These requirements are taken from EN 60896-21:2004 (‘Stationary lead-acid batteries. Valve regulated types. Methods of test’). This standard specifies the tests, and it is for the user to define the parameters required depending on the application, duty cycle, and expected service life. Standby batteries cover a wide range of applications: security and fire alarms use small batteries with a relatively short life; UPS applications require higher discharge rates and moderate lives; and telecommunications applications require lower discharge rates and longer lives. The design of batteries to meet these requirements is outlined below with reference to the parameters listed above.
Valve operation
Protection against ground shorts
This stipulates that the battery will not suffer ground shorts, burning, or charring of the container when 500 V DC is applied to case-to-cover joints over an extended period. The requirement will be met by ensuring that there are no leaks in the case-to-cover seal or the pillar seal and that the vent does not release acid spray to provide a conductive path on the container or cover.
The valves are required to open and close at the pressures specified by the supplier before and after the hightemperature stress test. Compliance requires that valve components be thermally stable and do not distort at higher temperatures. It is also important that the components do not stick together on exposure to elevated temperatures. Flammability of materials
The user may specify a flammability rating for the container and cover. The battery designer needs to specify a polymer grade with the required rating. Intercell connector performance
Gas emission
Gas emission is measured at the normal float voltage and at 2.40 V as milliliters per hour or milliliters per amperehour. It does not need to be considered as a design
The requirement is for the maximum temperature of the intercell connector to be reported when the cell or monobloc is discharged at the 15 m rate (4 1C). It should be below the softening point of the case and cover
Secondary Batteries – Lead–Acid Systems | Performance
material. This is more specific than the safe limit for high currents as the intercell connector has a smaller section than the group bars. From a design point of view, it is simply necessary to ensure that the sections are adequate at all points. Discharge capacity
The standard stipulates that the battery will have on dispatch 95% of the rated capacity at the rate or rates required by the user. Compliance will be determined by battery processing during formation. More importantly, the rated capacity will be achieved by the correct electrochemical design in terms of active material and electrolyte volume and density. For lower rate performance, plate thickness is not critical, but for higher rate performance, thinner plates will deliver better performance but at the expense of service life. Active material density also needs to be considered. Lower densities will favor higher rate performance but may reduce integrity for longer service lives. Cycle life is, however, not critical and active material densities lower than those specified for motive power cells may be used. Charge retention during storage
The requirement for capacity retention during storage is for 70% of the 3 h capacity to remain after 180 days of storage at 25 1C. This is not an onerous requirement and all correctly designed VRLA standby batteries should be compliant. It does, however, impose a requirement for the battery to be delivered with the correct minimum capacity as indicated above and for batteries to be regularly recharged during storage.
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recommended float voltage for 24 and 168 h, respectively. The design features for float service with daily discharges will also influence this behavior. Service life at an operating temperature of 40 1C
The test requires that the battery have a residual capacity at the 3 h rate of >80% of the nominal value after exposure for specified duration to a 40 1C environment at the ambient temperature float voltage. Four levels of compliance are indicated: >500 days, >750 days, >1100 days, and >1700 days. These correspond approximately to 25 1C float lives of B3, 5–6, 8–10, and 12–14 years, respectively. A temperature of 40 1C represents the highest service temperature for normal use. Design for long service life on float is generally based on an assumption that service life will be determined by the thickness of the positive grid and the grid alloy. Other failure modes, such as dry out, corrosion of the group bars, and positive plate growth, should be such that they will take place over a longer timescale. The case and cover should not suffer any distortion at these temperatures. Dry out should not be a failure mode in any correctly designed normal VRLA standby battery. Corrosion of the group bars will be determined by alloy selection and the process used, and positive plate growth will be related to the grid thickness and grid alloy. Recent developments in Pb–Ca–Sn grid alloys with lower calcium levels and higher tin levels have improved corrosion resistance. The use of pure lead with small additions of tin also leads to long corrosion life. Silver as an alloying element has also been investigated. The thickness of plates used will be largely determined by balancing performance and economic requirements.
Float service with daily discharges
This test applies a partial discharge at twice the 10 h rate to remove 40% of the rated capacity at daily intervals at 25 1C with recharges at the recommended float voltage. Three levels of compliance are required: for normal utility reliability of more than 50 cycles; for unreliable mains power of more than 100 cycles; and for very unreliable mains power of more than 150 cycles. Features that improve cyclability, including the use of tin in the grid alloys and cell compression, are important in achieving compliance. Charge acceptance on float is also important and so thinner plate batteries will perform better. The higher levels of compliance fall short of regular deep discharges, and so factors such as active material density and plate processing are relatively less important. Recharge behavior
The requirement is for the batteries to have a 10 h capacity to 1.80 V per cell at 25 1C of >90% of the 10 h capacity and >98% of the 10 h capacity after a discharge at the 10 h rate to 1.80 V per cell when recharged at the
Effect of a stress temperature of 55 or 60 1C
These tests are used as accelerated life tests and the battery is exposed to 55 or 60 1C for varying periods at the ambient temperature float voltage, and discharges are carried out at intervals at the 3 h rate at 25 1C. The failure point is when the battery reaches 80% of the nominal capacity. Different levels of requirement are set at >150 days, >250 days, >350 days, and >500 days for 55 1C exposure or somewhat shorter time periods for 60 1C exposure. A variant of the test designed to assess durability for UPS applications uses a discharge at the 15 m rate, and the time periods in days for compliance are halved. This recognizes that loss of grid metal through corrosion will affect high rate performance more rapidly than low rate performance. The exposure times in days for compliance at the 3 h rate are broadly in line with the 40 1C requirement. For the purposes of testing, the battery may be clamped in a fixture to prevent any thermal distortion during the test. The design features for 40 1C service will have the same effect for a 55 or 60 1C accelerated test.
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Abusive overdischarge
A requirement for recovery after abusive overdischarges may be specified. This would involve a series of discharges to 1.25 V per cell followed by float recharges and a capacity test. Design for good cyclic performance will improve behavior under these conditions. Thermal runaway sensitivity
This test requires that a battery in a 25 1C environment will not reach 60 1C after 168 h at 2.45 V per cell or 24 h at 2.60 V per cell. A correctly designed VRLA standby battery will have no difficulty in complying with this requirement, but it should be noted that an aged battery may have a greater sensitivity to thermal runaway. Low temperature sensitivity
This requires that a battery exposed to an environment that is sufficiently cold to freeze the electrolyte retain 95% of the nominal 3 h capacity and not suffer any failure of the container. This is an extreme requirement, and if very low temperatures are expected, the battery should be heated or the level of discharge limited such that the electrolyte will not freeze. Nonetheless, VRLA batteries will perform satisfactorily even if the electrolyte has been frozen and thawed. The container material needs to have an adequate level of mechanical strength and fracture toughness. Dimensional stability at elevated internal pressure and temperature
This is a test to confirm that the container will not distort at higher temperatures over time such that compression would be lost. The test is carried out by subjecting the container to the maximum vent operating pressure, exposing the container to 50 1C for 24 h, and then measuring for any distortion. The correct grade of polymer for the container needs to be specified, and if extended periods at higher temperature are required, more thermally stable materials should be recommended. Resistance to mechanical abuse
Resistance to mechanical abuse may be evaluated by dropping the battery on a corner or an edge from a defined height. It should not split, fracture, or leak. The test is intended to show that the battery will not suffer damage in transit prior to installation. The strength and fracture toughness of the container are the key parameters. General Design Features VRLA standby batteries are, as for motive power batteries, of two main types: those with AGM separators and flat pasted plates and those with gelled electrolyte and either tubular or flat plates. The design principles outlined above are principally for AGM types but are also
applicable to gelled electrolyte types where there is less influence from the separator. Tubular gel cells
Tubular gel cells generally comply with the DIN (Deutsches Institut fur Normung/German Institute for Standardization) OPzV (Ortsfeste verschlossene Batterien mit positiven Panzerplatten und festgelegtem Electrolyt/sealed standby battery with tubular plates and gelled electrolyte) specifications and dimensions. Both grids use Pb–Ca–Sn alloys as described, with tubular positive plates and flat pasted negative grids. The separator is similar to those used for cyclic applications and the electrolyte is gelled with finely divided silica. Cell cases and lids are molded in flame retardant acrylonitrile butadiene styrene (ABS) copolymer. Pillar seals are proprietary and the valves are normally of a simple Bunsen valve type. The life in floating service is B12 years and this type of cell has a reasonably good cyclic performance. The rate capability is modest. Pasted plate gel cells
Pasted plate gel cells differ only in the construction of the positive plate that is in the form of a flat pasted plate. Life in floating service will depend principally on grid profile and alloy selection. Value-regulated lead–acid absorptive glass mat batteries with Pb–Ca–Sn grids
The majority of VRLA batteries produced, either as cells or as monoblocs, use pasted plates with Pb–Ca–Sn grids and AGM separators. Cell cases may be polypropylene, ABS, polycarbonate (PC)/ABS, or PVC with or without flame retardant additives. Grid alloys and thickness and the other details of cell design are as described. Pb–Sb–Cd alloys have been used for standby batteries by one manufacturer. Pillar seals use a variety of rubber sealing rings, mechanical compression seals, and thermosetting resins. Valves may be of a Bunsen valve type or a more complex arrangement and normally incorporate a flame filter. Value-regulated lead–acid absorptive glass mat batteries with pure lead grids
Pure lead or pure lead–tin grids may be used instead of Pb–Ca–Sn grids. These are fabricated from a continuously cast or wrought strip by punching the grid pattern into the strip, which is then processed conventionally. Thinner grids may be used because the corrosion rate is much lower and the active material utilization is higher. Other details of construction are similar to those for other types of cell. Thicker plate cells with pure lead plates are also manufactured. The corrosion life is extended, but the rate performance is unaltered. Service lives in excess of 25 years at 25 1C and up to 10 years at 40 1C are claimed.
Secondary Batteries – Lead–Acid Systems | Performance
Flooded Standby Batteries Flooded standby batteries are important for some telecommunications applications, for utilities including nuclear power and large security applications, and for large critical UPS applications. Two technologies dominate the market: pasted plate Pb–Ca batteries are mainly used in North America and tubular plate batteries in European and other markets. Special round cells and Plante´ cells have limited applications. Pasted plate cells
Pasted plate cells use Pb–Ca–Sn grids of substantial thickness, and a life of up to 25 years is achieved in a well-controlled environment. The electrolyte density may be reduced in order to achieve a longer life. The positive plates are designed in such a way that plate growth can occur in a benign manner with the plates suspended from the top of the container. The positive plates are wrapped in a glass fiber retainer mat to ensure that shedding of active material cannot occur. Microporous plastic separators are used. Cell containers are molded in transparent PVC or styrene acrylonitrile (SAN), allowing visual inspection of the condition of the cells. Pillar seals are proprietary and are designed to have a high degree of integrity in order to avoid stressing the seal over the life of the battery. Tubular plate cells
Tubular plate cells for standby service follow the OPzS (Ortsfeste gerschlossene Batterien mit positven Panzerplatten/open standby battery with tubular plates) specification to DIN requirements. The positive plates have lowantimony alloy spines with a fabric gauntlet, as for cells for cyclic applications. Some suppliers offer Pb–Ca–Sn grids as an alternative. The principal difference for the positive plate is that Pb–Sb grids have very low antimony contents in order to reduce water loss. The negative plates are a pasted plate type and generally microporous polyethylene separators are used. Cell containers are molded in SAN so that the electrolyte level and visual condition can be readily seen. Pillar seals offer freedom from leakage and corrosion for the service life of the battery but vary in important details between suppliers. The specific gravity of the acid is higher than that for pasted plate cells. Sufficient excess of electrolyte above the group bar can be provided to permit a watering interval of up to 3 years, and the overall service life is up to 15 years. Cells with Pb–Sb grids provide good cyclic performance, and for applications where the public utility is unreliable or for supporting solar photovoltaic power systems, they provide good performance. Round cells
The round cells designed by AT&T Bell Laboratories have a number of unique features incorporated in order
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to provide a life of 40 years or more. These have a circular pure lead positive grid shaped as a shallow cone and stacked horizontally with pure lead negative plates of similar construction. The positives are welded together externally and the negatives welded to a central conductive core with an insulator at the outside edge. Microporous polyethylene separators are used with a glass retainer mat. The cell containers and lids are molded in PVC. The positive active material is a mixture of chemically produced tetrabasic lead sulfate and red lead before formation. The use of pure lead reduces plate growth and corrosion. The special shape is designed to counter the effects of plate growth and to ensure that the grid and active material remain in good contact throughout the life of the battery. Grid growth is caused by the formation of lead dioxide at the surface of the grid that distorts the grid, but in this case the shape change causes no adverse effects. The conversion of grid material into lead dioxide causes a small but measurable capacity increase over time. The specific gravity of the acid is low and the float voltage is also low. This could cause a problem with the recharge of the positive plate, which may be resolved by adding a small amount of platinum as a depolarizer to the negative expander rather than by increasing the float voltage. The terminal design is also unique. A Pb–Sn alloy is used to prevent nodular corrosion, and an epoxy sheath is formed around the pillar, which then has a rubber boot to seal to the cover. It is important for seal integrity that the area coated with epoxy is not below the electrolyte level. This type of cell is deployed in large telecommunications switching centers and long service lives are being achieved. Plante´ cells
Plante´ cells were used in Europe and elsewhere for telecommunications and utility standby applications, but their use is now confined to replacement. These cells use a positive grid cast in pure lead with an extended surface such that the active material is formed directly by corrosion of the grid material. The grids are hung from the top of the cell such that plate growth can occur harmlessly downward. Pasted plates with Pb–Sn alloys are used for the negative plates and microporous plastic separators. Cell cases and lids are molded in SAN, and pillar seals vary in design from simple rubber bushings to arrangements similar to the round Bell-designed cells. This type of battery has a service life of up to 25 years.
Batteries for Hybrid Electric Vehicle Applications Batteries for HEV applications are a new category with an HRPSoC duty cycle. As the battery cannot be returned to a full SOC, different failure modes occur that
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require new approaches to provide solutions. The duty cycle involves repeated discharge at very high rates for vehicle acceleration followed by charging also at high rates to recover energy by regeneration on braking. In order to be able to charge at high rates, the battery cannot be fully charged at any time in normal service, as the charge acceptance and internal resistance of the battery increase rapidly at top of charge. This has been designated PCL-3 as discussed above. This causes the negative plate to deteriorate by irreversibly sulfating and progressively losing capacity. The problem may be resolved, at least in part, by adding special carbons to the negative plate in larger quantities than for a conventional battery. These improve the recharge behavior and have a beneficial effect on cycle life. The need for discharge as well as recharge at high rates also results in designs that are further optimized for high rates as compared to automotive. Grid designs have been developed that provide better active material utilization at high rates. The batteries used in this area are invariably VRLA types, and the design features to improve cycle life also need to be applied. Strategies for equalization charging and then discharging prior to service have received attention. The use of lead–acid batteries for HEV applications is receiving considerable attention as a research topic, and new developments may certainly be expected.
Nomenclature Abbreviations and Acronyms ABS AGM CCA d.c. DIN
acrylonitrile butadiene styrene absorptive glass mat cold cranking amps direct current Deutsches Institut fur Normung/ German Institute for Standardization
DoD HEV HRPSoC OPzS
OPzV
PC PCL PVC SAN SoC UPS VRLA
depth of discharge hybrid electric vehicle high-rate partial state of charge Ortsfeste gerschlossene Batterien mit positven Panzerplatten/open standby battery with tubular plates Ortsfeste verschlossene Batterien mit positiven Panzerplatten und festgelegtem Electrolyt/sealed standby battery with tubular plates and gelled electrolyte polycarbonate premature capacity loss polyvinyl chloride styrene acrylonitrile state-of-charge uninterruptible power supply valve-regulated lead–acid
See also: .Secondary Batteries – Lead–Acid Systems: Overview.
Further Reading Berndt D (1993) Maintenance-Free Batteries: Lead–Acid, Nickel/ Cadmium, Nickel/Metal Hydride: A Handbook of Battery Technology. Taunton: Research Studies Press, John Wiley & Sons. Bode H (1977) Lead–Acid Batteries. New York: John Wiley & Sons. May GJ (2006) Standby battery requirements for telecommunications power. Journal of Power Sources 158: 1117--1123. Rand DAJ, Moseley PT, Garche J, and Parker CD (2004) ValveRegulated Lead–Acid Batteries. Amsterdam: Elsevier. Vinal GW (1955) Storage Batteries: A General Treatise on the Physics and Chemistry of Secondary Batteries and Their Engineering Applications. New York: John Wiley & Sons.