- Email: [email protected]
Charge Equalization Systems for Serial Valve Regulated Lead-Acid (VRLA) Connected Batteries in Hybrid Power Systems Applications
Available online at www.sciencedirect.com
ScienceDirect Energy Procedia 99 (2016) 277 – 284
10th International Renewable Energy Storage Conference, IRES 2016, 15-17 March 2016, Düsseldorf, Germany
Charge Equalization Systems for Serial Valve Regulated Lead-Acid (VRLA) Connected Batteries in Hybrid Power Systems Applications K. Belmokhtara*, H. Ibrahima,b*, Z. Fégera, M. Ghandourc a b
TechnoCentre éolien, 70 rue Bolduc, Gaspé, QC, G4X 1G2, Canada Université du Québec à Rimouski, Rimouski, QC, G5L 3A1, Canada c Faculty of engineering, Lebanese University, Beirut, Lebanon
Abstract An overview of the impact of the equalization process on performance and behavior of Valve Regulated Lead-Acid (VRLA) batteries, which are a generally used in Hybrid Power Systems (HPS) is given in this paper. In order to extend the life time and runtime of batteries, an equalization process, with a good precision is required. Indeed, as mentioned in prior works, to achieve voltage equalization, the process must have a precision around 10 mV/cell. We have focused the impact of an unbalanced cells voltage on their lifetime. Typically, there are two equalization systems: While the active cell equalization processes remove charge from higher energy cell(s) and deliver it to lower energy cell(s), the passive methods, based on resistor element, remove the excess charge until the charge matches those of the lower cells in the same pack. It shows that the performance of active systems is significantly better than passive systems. Experimental results show that even for maintenance free batteries, a periodic equalization process is needed in order to extend their lifespan. © 2016 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license © 2016 The Authors. Published by Elsevier Ltd. (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of EUROSOLAR - The European Association for Renewable Energy. Peer-review under responsibility of EUROSOLAR - The European Association for Renewable Energy Keywords: Battery degradation; unbalanced voltage; battery equalizer; state of charge (SOC); energy management.
* Corresponding author. Tel.: +1-418-368-6162 # 243; fax: +1-418-368-4315. E-mail address: [email protected], [email protected], [email protected]
1876-6102 © 2016 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of EUROSOLAR - The European Association for Renewable Energy doi:10.1016/j.egypro.2016.10.117
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K. Belmokhtar et al. / Energy Procedia 99 (2016) 277 – 284
1. Introduction A Valve Regulated Lead Acid (VRLA) battery is the most frequent battery used in Hybrid Power System (HPS) for remote communities or for backup of telecommunications systems. In fact, VRLA batteries are considered to be maintenance free, where the electrolyte does not require replacing over the life of the battery, and a valve is used to liberate any excess pressure that may build up if the battery were failing. In recent years, even for stationary or transportation applications, multiple serial cells are used and push the battery performance to the extreme. Indeed, numerous applications will be using series connected battery strings with dc-bus nominal voltage from 12 V to 300 V or more to achieve a power source. A single current is imposed to all of these cells. The individual cells within a battery string differ due to several reasons such as a manufacturing variations, temperature gradients, and aging effects. While the battery pack with some of cells connected in series being charged and discharged, the individual cells will have dissimilar voltage, and then different States of Charge (SOC). Some cells will be continually overcharged, under-charged, or over-discharged. Over many cycles, this tends toward the capacity decrease for the cell and then for battery packs [1]. This effect can ultimately lead to a rapid degradation of battery performance and even their failure. For this reason, an equalization system is necessary, mainly for both VRLA and lithium-ion batteries [1-4]. In any battery charging process, a solution to ensure a voltage balance or equalization of the charge is needed to restore balance or at least prevent it from developing [5]. A conventional equalization systems perform a charging process up to a sufficient potential or make several charge steps at high voltages in order to ensure a same overvoltage of all of the cells [5]. Several active and passive methods have been proposed to replace the technique based on the overcharge equalization (equalization voltage-based). In [6], the most common method is described in which dissipative elements (either resistance or a transistor operated in its linear regime) are activated in individual cells as they reach the maximum allowable pressure. There are many disadvantages to this approach, such as the power dissipation. Thus, this technique dissipates a substantial energy and do not scale well to large batteries. In addition, the clamp does not affect the upper end of the charge cycle. This offers no protection against excessive discharge and would have no influence on the performance of the string, and then the batteries are not fully charged. Typically, the voltage setting of the action is predetermined and fixed, and can be adjusted for temperature compensation or adapted easily to different chemistries. The main advantage of a dissipative technique is its simplicity [1]. A Direct clamping method, which is equivalent to placing a Zener diode through a cell or monobloc, is also frequent. This technique has the same characteristics as the dissipative methods. The advantage is that this technique is passive, and achieves its function without requiring any external sensing system [8]. As an alternative to dissipative techniques, active methods which use power switching techniques have been introduced. The most active equalization methods are built around standard or modified dc-dc converters [1]. Some of these methods are isolated, using forward [7-9] or a Flyback converter as reported in [10]. Isolated techniques tend to be voltage controlled. Using an input side tension control and relies on precision transformers with many windings to balance individual piece are listed in [11]. Others authors use a buck-boost converters [12-13] or boostbuck converters [14], or distributed Cuk converter [15-17]. A recent alternative is to use a buck-boost converter where inductor is replaced by a resonant tank in place of the inductor for a quasi-resonant zero current switching converters [1, 18]. This is essentially equivalent to the quasiresonant version reported in [19]. Non-isolated dc-dc converters methods tend to focus on controlling the current by sensing the voltage difference between cells or batteries-driving positive or negative current to drive the difference to zero. They all share required for magnetic components, whether inductors, transformers, or coupled inductors [1]. The configuration of dc-dc converters for charge equalization can be configured in many ways. Indeed, in a modular approach, each converter equalizes two adjacent batteries. In this case, the entire system requires fewer modules than the number of batteries. Otherwise, the full charge equalization system can be built to permit an appropriate circulate power [1]. Each of these systems has its advantages and disadvantages in terms of equalization speed, circuit complexity, number of parts needed in the application (in terms of nominal current for switching elements and the breakdown voltage for diodes), but they all need the information on the SOC to make the appropriate control [20]. Even though several works dealing with respect to equalization problem have considered long series strings (100 or more cells in series), issues of charge equalization have extended down to short strings of just a few cells [5]. A
K. Belmokhtar et al. / Energy Procedia 99 (2016) 277 – 284
conventional block of six lead-acid cells was tested with a voltage-limited charge process, in which the maximum charge potential is kept below 2.35 V for each cell [21]. It has been shown that when the load is extended from 12 to 16 hours, the charging process has sufficient time to recover. To achieve properly a charging equalization, there are many keys must be addressed such as [5]: x The benefit of equalization process; x The impact of the equalization process on extension of a lifetime or reduce cost; x The accuracy of the process for each cell to achieve successfully a charge equalization; In this paper, we have achieved an overview study of charge equalization, with the focus on the impact on batteries lifetime and runtime. Experimental analysis has been accomplished, the show batteries degradation and the advantage to add an appropriate and effective equalization solution. This paper is organized as follows. Factors which contribute to accelerated aging mechanisms of batteries are addressed in section 2. Section 3 describes the impact of unbalanced voltage on the lifespan of batteries, where experimental results are presented. Finally, a conclusion about the effectiveness and the performance of the proposed equalizing system are outlined. 2. Main factors affecting batteries life The VRLA batteries which are widely used for standby service due to their reliability are destined for earlier degradation even in ideal conditions. Despite their maintenance free characteristics, VRLA batteries will fail sooner when used in certain conditions such as [22]: x Corrosion of the positive grid structure due to oxidation of the grid and plate materials. This factor, which is unavoidable, is the most common natural failure for lead-acid batteries. x Loss of active material from the positive plate. x Physical changes in the active material of the positive plate leading a loss of capacity. x Elevated temperatures reduce battery life. Indeed, an increase of 8.3 C can reduce lead-acid battery life around 50% or more. x A frequent deep-discharge cycle contributes to reduce the lifetime of the batteries. x Overcharging can cause an excessive gassing, and a high float voltage causes a higher positive plate corrosion rate. To overcome a potential damage, after discharge, recharging process must be controlled to 10% of the discharge current. x The most consequence of the undercharging is the sulfation, which can damage the plates. Moreover, a low voltage reduces capacity due to self-discharge phenomena. x VRLA batteries are considered to be very sensible to ripple dc-current. Indeed, this can lead to cell heating and will accelerate the degradation of cells. Some manufacturers provide an acceptable minimum frequency of voltage ripple to prevent an additional battery heating.
Fig. 1. A typical cycling performance of VRLA batteries
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Fig. 2. Impact of active and passive equalization on capacity of pack of batteries [23]
A typical cycling performance of VRLA batteries is illustrated in Figure 1, where the impact deep of discharge (DOD) on battery lifetime is addressed. There are other factors which are considered as aging mechanisms such as an improper storage and misapplications where batteries are used in not designed applications. A reduction to 80% of the rated capacity is commonly defined as the end of life for VRLA batteries. In fact, under 80% of rated capacity, the degradation of batteries will be accelerated, and a sudden failure is expected due to several factors such as a high discharge rate. Even they are considered maintenance free, VRLA batteries require a periodic maintenance program must be planned. Indeed, even under recommended conditions, batteries can observe a sudden failure. Figure 2 illustrates the impact of using an active equalization system for a pack of batteries. Indeed, with an active equalization system, a pack of batteries accomplishes at least 450 charging/discharging cycles, where the pack of batteries without active equalization reaches only 140 driving cycles. 3. Impact of unbalanced voltage on lifespan of VRLA batteries The unbalance between cells or battery voltages due the difference in their capacities, damage, and operating temperature will be accelerated by the charge and discharge cycles. This will cause over-charging, deep discharging, decrease time storage capacity and lifespan.
Fig. 3. Battery pack's current
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Fig. 4. Imbalanced voltage of VRLA batteries connected in series
Figure 3 shows the measured current for four VRLA installed in TechnoCentre Eolien (TCE) microgrid (MG). As we can see, the unbalance voltage of connected batteries strings is up to 3 V. This means that these batteries/cells have not had the same SOC. Fig. 4 illustrates the evolution of the voltage of the same four VRLA batteries, which were installed at the same time. As we can see, these batteries have not the same voltage, which mean that their capacities are not the same. The charge unbalance contributes to reduce battery lifetime [24, 25]. In practice, the recharge of cells connected in series must be stopped when the cell that has the highest voltage reaches its critical level. Thus, other cells are not fully charged. On the other hand, when cell that has the lower voltage reaches its critical value, the discharging process must be turned off. Thus, the battery's capacity is limited by the cell with a low voltage. In order to extend the battery life cycle and prevent failure to the cells as described above, it is recommended that all the serial cells have the same SOC during battery operations. Consequently, the use of an equalization process is necessary. 4. VRLA batteries maintenance requirement for buckup applications In order to increase battery lifetimes, reduce replacement costs, and improve reliability, VRLA batteries must be equipped with a monitoring system and undergo a proper maintenance and testing program. Indeed, replacement costs significantly increase the cost of a Battery Management System (BMS). Moreover, a number of systems allow for proper assessment of battery SOH by estimating the internal resistance of each cell/battery. This method is easier and more cost-effective compared to a capacity testing approach. The most simple way to track battery SOH is to know the baseline value of its internal resistance at 100% capacity (acceptance test), and then to periodically (quarterly for VRLA batteries) compare internal/intercell resistances to baseline values. A number of recommendations from industry experts exist, such as the IEEE 1188 standard for VRLA batteries which can be applied in maintenance inspections. Note that visual inspections should be performed at least once a year. Continuous connection of the BMS can also significantly increase overall reliability of the system. On the other hand, a BMS can help to avoid loss of data, and to determine the optimal time for replacement. 5. Experimental set-up 5.1. Description Figure 5 shows the block diagram of TCE's test bench, with a 7.5 kW Wind Turbine Generator (WTG) based on Direct-Drive (DD) Permanent Magnet Synchronous Generator (PMSG) to supply the VRLA battery bank via a controlled rectifier, which allows the optimization of the wind power generation. In order to validate the effectiveness of the active equalization process, a device illustrated in Figure 6 from HDM was used with a four 12 V Deka
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monobloc VRLA battery connected in series. The full charge of these batteries cannot exceed 14.6 V, and can be discharged up to 10.5 V. The active equalization system used in this paper is illustrated in Figure 6 balances voltage among four connected batteries strings during charge, discharge and even idle phases. In fact, this system contributes to prevent a severe undercharge and overcharge voltage which can lead to the rapid degradation of the performance, the reliability and life of battery. The active equalization ensures a balancing voltage between batteries within ± 0.4 V.
Figure 5. Block diagram of a TCE's test bench
Fig. 6. A bidirectional battery active equalization circuit from HDM
5.2. Results Figure 7 illustrates the dc-current from rectifier, a dc bus voltage and the estimated power from WTG respectively. Figure 8 shows the evolution of the stack’s voltage with a battery equalizer system installed on the battery bank at TCE’s facility.
Fig. 7. Evolution of the stack’s voltage with a battery equalizer system installed on battery bank (connected in series) at TCE’s facility
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It can be clearly seen on these figures that battery voltage imbalances are widely removed during the test cycle, when wind power is available. However, battery n°3 is being undercharged during the discharge cycle and overcharged (gassing) in the charging and equalizing cycles, while the rest of the remaining batteries have the same voltage due to the battery equalizer system. This means that battery n°3 have a greater internal resistance compared to the rest of the batteries. This means that the capacity of battery 3 is seriously diminished, and should be replaced.
Fig. 8. Evolution of the stack’s voltage with an active battery equalizer system installed on battery bank (connected in series) at TCE’s facility
6. Conclusion This paper has discussed the degradation phenomena of VRLA batteries used in a hybrid power system for remote applications. We have addressed some factors that are held responsible of the rapid degradation of batteries. Moreover, we have demonstrated the importance to pay attention to the maintenance even for VRLA batteries, which are considered as maintenance free. Indeed, using a battery management system can help assess the state of health of batteries by periodically conducting suitable tests with respect to recommendations of industry experts. Using an appropriate battery management system which provides sufficient information to users can help properly track the capacity of batteries and determine the optimal moment for their replacement. Indeed, this approach can help to prevent some phenomena believed to accelerate the aging mechanisms of batteries such as irreversible sulphation, corrosion, stratification and softening. Moreover, connecting batteries in a series in backup applications involves an imbalance in battery voltage which can significantly reduce their lifespan. Experimental tests have demonstrated the effectiveness of the active equalization system in order to avoid an unbalance voltage of connected battery strings. Furthermore, experimental tests have shown that the active charging equalization system can help to detect damaged batteries for its appropriate replacing. The future works will address the impact of the active equalization methods on the efficiency of the VRLA batteries, and the aging phenomena by tracking the evolution of their capacities.
Acknowledgements The authors gratefully acknowledge the funding received from Natural Resources Canada and the Natural Sciences and Engineering Research Council of Canada.
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References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25]
J. W. Kimball, B. T. Kuhn, and P. T. Krein, "Increased performance of battery packs by active equalization," in Vehicle Power and Propulsion Conference, 2007. VPPC 2007. IEEE, 2007, pp. 323-327. P. Som and J. Szymborski, "Operational impacts on VRLA life," in Battery Conference on Applications and Advances, 1998., The Thirteenth Annual, 1998, pp. 285-290. S. T. Hung, D. C. Hopkins, and C. R. Mosling, "Extension of battery life via charge equalization control," Industrial Electronics, IEEE Transactions on, vol. 40, pp. 96-104, 1993. D. C. Hopkins, C. R. Mosling, and S. T. Hung, "Dynamic equalization during charging of serial energy storage elements," Industry Applications, IEEE Transactions on, vol. 29, pp. 363-368, 1993. P. T. Krein and R. S. Balog, "Life extension through charge equalization of lead-acid batteries," in Telecommunications Energy Conference, 2002. INTELEC. 24th Annual International, 2002, pp. 516-523. W. H. DeLuca, F. Hornstra Jr, G. H. Gelb, B. Berman, and L. W. Moede, "System and method for charging electrochemical cells in series," ed: Google Patents, 1980. J. A. Cox, "Charging system and method for multicell storage batteries," ed: Google Patents, 1978. Kimball, Jonathan W., Brian T. Kuhn, and Philip T. Krein. "Increased performance of battery packs by active equalization." Vehicle Power and Propulsion Conference, 2007. VPPC 2007. IEEE. IEEE, 2007. N. H. Kutkut, D. M. Divan, and D. W. Novotny, "Charge equalization for series connected battery strings," Industry Applications, IEEE Transactions on, vol. 31, pp. 562-568, 1995. N. H. Kutkut, H. L. Wiegman, D. M. Divan, and D. W. Novotny, "Charge equalization for an electric vehicle battery system," Aerospace and Electronic Systems, IEEE Transactions on, vol. 34, pp. 235-246, 1998. J. B. Davis Jr, "Battery charger for charging a plurality of batteries," ed: Google Patents, 1991. G. L. Brainard, "Non-dissipative battery charger equalizer," ed: Google Patents, 1995. C. S. Moo, Y. C. Hsieh, and I. Tsai, "Charge equalization for series-connected batteries," Aerospace and Electronic Systems, IEEE Transactions on, vol. 39, pp. 704-710, 2003. M. Tang and T. Stuart, "Selective buck-boost equalizer for series battery packs," Aerospace and Electronic Systems, IEEE Transactions on, vol. 36, pp. 201-211, 2000. Y.-S. Lee and M.-W. Cheng, "Intelligent control battery equalization for series connected lithium-ion battery strings," Industrial Electronics, IEEE Transactions on, vol. 52, pp. 1297-1307, 2005. L. Yuang-Shung, M.-W. Cheng, Y. Shun-Ching, and H. Co-Lin, "Individual cell equalization for series connected lithium-ion batteries," IEICE transactions on communications, vol. 89, pp. 2596-2607, 2006. M. Chen, Z. Zhang, Z. Feng, J. Chen, and Z. Qian, "An improved control strategy for the charge equalization of lithium ion battery," in Applied Power Electronics Conference and Exposition, 2009. APEC 2009. Twenty-Fourth Annual IEEE, 2009, pp. 186-189. H.-S. Park, C.-E. Kim, C.-H. Kim, G.-W. Moon, and J.-H. Lee, "A modularized charge equalizer for an HEV lithium-ion battery string," Industrial Electronics, IEEE Transactions on, vol. 56, pp. 1464-1476, 2009. Y.-S. Lee and G.-T. Cheng, "Quasi-resonant zero-current-switching bidirectional converter for battery equalization applications," Power Electronics, IEEE Transactions on, vol. 21, pp. 1213-1224, 2006. C. Pascual and P. T. Krein, "Switched capacitor system for automatic battery equalization," ed: Google Patents, 1998. C. Speltino, A. Stefanopoulou, and G. Fiengo, "Cell equalization in battery stacks through state of charge estimation polling," in American Control Conference (ACC), 2010, 2010, pp. 5050-5055. A. Lohner, E. Karden, and R. W. De Doncker, "Charge equalizing and lifetime increasing with a new charging method for VRLA batteries," in Telecommunications Energy Conference, 1997. INTELEC 97., 19th International, 1997, pp. 407-411. P. Thru, "Lead acid battery working - Lifetme Study." N. H. Kutkut, "Life cycle testing of series battery strings with individual battery equalizers," white paper, Power Designers, Inc., Madison, WI, 2000. L. Zhong, C. Zhang, Y. He, and Z. Chen, "A method for the estimation of the battery pack state of charge based on in-pack cells uniformity analysis," Applied Energy, vol. 113, pp. 558-564, 2014.
ScienceDirect Energy Procedia 99 (2016) 277 – 284
10th International Renewable Energy Storage Conference, IRES 2016, 15-17 March 2016, Düsseldorf, Germany
Charge Equalization Systems for Serial Valve Regulated Lead-Acid (VRLA) Connected Batteries in Hybrid Power Systems Applications K. Belmokhtara*, H. Ibrahima,b*, Z. Fégera, M. Ghandourc a b
TechnoCentre éolien, 70 rue Bolduc, Gaspé, QC, G4X 1G2, Canada Université du Québec à Rimouski, Rimouski, QC, G5L 3A1, Canada c Faculty of engineering, Lebanese University, Beirut, Lebanon
Abstract An overview of the impact of the equalization process on performance and behavior of Valve Regulated Lead-Acid (VRLA) batteries, which are a generally used in Hybrid Power Systems (HPS) is given in this paper. In order to extend the life time and runtime of batteries, an equalization process, with a good precision is required. Indeed, as mentioned in prior works, to achieve voltage equalization, the process must have a precision around 10 mV/cell. We have focused the impact of an unbalanced cells voltage on their lifetime. Typically, there are two equalization systems: While the active cell equalization processes remove charge from higher energy cell(s) and deliver it to lower energy cell(s), the passive methods, based on resistor element, remove the excess charge until the charge matches those of the lower cells in the same pack. It shows that the performance of active systems is significantly better than passive systems. Experimental results show that even for maintenance free batteries, a periodic equalization process is needed in order to extend their lifespan. © 2016 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license © 2016 The Authors. Published by Elsevier Ltd. (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of EUROSOLAR - The European Association for Renewable Energy. Peer-review under responsibility of EUROSOLAR - The European Association for Renewable Energy Keywords: Battery degradation; unbalanced voltage; battery equalizer; state of charge (SOC); energy management.
* Corresponding author. Tel.: +1-418-368-6162 # 243; fax: +1-418-368-4315. E-mail address: [email protected], [email protected], [email protected]
1876-6102 © 2016 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of EUROSOLAR - The European Association for Renewable Energy doi:10.1016/j.egypro.2016.10.117
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K. Belmokhtar et al. / Energy Procedia 99 (2016) 277 – 284
1. Introduction A Valve Regulated Lead Acid (VRLA) battery is the most frequent battery used in Hybrid Power System (HPS) for remote communities or for backup of telecommunications systems. In fact, VRLA batteries are considered to be maintenance free, where the electrolyte does not require replacing over the life of the battery, and a valve is used to liberate any excess pressure that may build up if the battery were failing. In recent years, even for stationary or transportation applications, multiple serial cells are used and push the battery performance to the extreme. Indeed, numerous applications will be using series connected battery strings with dc-bus nominal voltage from 12 V to 300 V or more to achieve a power source. A single current is imposed to all of these cells. The individual cells within a battery string differ due to several reasons such as a manufacturing variations, temperature gradients, and aging effects. While the battery pack with some of cells connected in series being charged and discharged, the individual cells will have dissimilar voltage, and then different States of Charge (SOC). Some cells will be continually overcharged, under-charged, or over-discharged. Over many cycles, this tends toward the capacity decrease for the cell and then for battery packs [1]. This effect can ultimately lead to a rapid degradation of battery performance and even their failure. For this reason, an equalization system is necessary, mainly for both VRLA and lithium-ion batteries [1-4]. In any battery charging process, a solution to ensure a voltage balance or equalization of the charge is needed to restore balance or at least prevent it from developing [5]. A conventional equalization systems perform a charging process up to a sufficient potential or make several charge steps at high voltages in order to ensure a same overvoltage of all of the cells [5]. Several active and passive methods have been proposed to replace the technique based on the overcharge equalization (equalization voltage-based). In [6], the most common method is described in which dissipative elements (either resistance or a transistor operated in its linear regime) are activated in individual cells as they reach the maximum allowable pressure. There are many disadvantages to this approach, such as the power dissipation. Thus, this technique dissipates a substantial energy and do not scale well to large batteries. In addition, the clamp does not affect the upper end of the charge cycle. This offers no protection against excessive discharge and would have no influence on the performance of the string, and then the batteries are not fully charged. Typically, the voltage setting of the action is predetermined and fixed, and can be adjusted for temperature compensation or adapted easily to different chemistries. The main advantage of a dissipative technique is its simplicity [1]. A Direct clamping method, which is equivalent to placing a Zener diode through a cell or monobloc, is also frequent. This technique has the same characteristics as the dissipative methods. The advantage is that this technique is passive, and achieves its function without requiring any external sensing system [8]. As an alternative to dissipative techniques, active methods which use power switching techniques have been introduced. The most active equalization methods are built around standard or modified dc-dc converters [1]. Some of these methods are isolated, using forward [7-9] or a Flyback converter as reported in [10]. Isolated techniques tend to be voltage controlled. Using an input side tension control and relies on precision transformers with many windings to balance individual piece are listed in [11]. Others authors use a buck-boost converters [12-13] or boostbuck converters [14], or distributed Cuk converter [15-17]. A recent alternative is to use a buck-boost converter where inductor is replaced by a resonant tank in place of the inductor for a quasi-resonant zero current switching converters [1, 18]. This is essentially equivalent to the quasiresonant version reported in [19]. Non-isolated dc-dc converters methods tend to focus on controlling the current by sensing the voltage difference between cells or batteries-driving positive or negative current to drive the difference to zero. They all share required for magnetic components, whether inductors, transformers, or coupled inductors [1]. The configuration of dc-dc converters for charge equalization can be configured in many ways. Indeed, in a modular approach, each converter equalizes two adjacent batteries. In this case, the entire system requires fewer modules than the number of batteries. Otherwise, the full charge equalization system can be built to permit an appropriate circulate power [1]. Each of these systems has its advantages and disadvantages in terms of equalization speed, circuit complexity, number of parts needed in the application (in terms of nominal current for switching elements and the breakdown voltage for diodes), but they all need the information on the SOC to make the appropriate control [20]. Even though several works dealing with respect to equalization problem have considered long series strings (100 or more cells in series), issues of charge equalization have extended down to short strings of just a few cells [5]. A
K. Belmokhtar et al. / Energy Procedia 99 (2016) 277 – 284
conventional block of six lead-acid cells was tested with a voltage-limited charge process, in which the maximum charge potential is kept below 2.35 V for each cell [21]. It has been shown that when the load is extended from 12 to 16 hours, the charging process has sufficient time to recover. To achieve properly a charging equalization, there are many keys must be addressed such as [5]: x The benefit of equalization process; x The impact of the equalization process on extension of a lifetime or reduce cost; x The accuracy of the process for each cell to achieve successfully a charge equalization; In this paper, we have achieved an overview study of charge equalization, with the focus on the impact on batteries lifetime and runtime. Experimental analysis has been accomplished, the show batteries degradation and the advantage to add an appropriate and effective equalization solution. This paper is organized as follows. Factors which contribute to accelerated aging mechanisms of batteries are addressed in section 2. Section 3 describes the impact of unbalanced voltage on the lifespan of batteries, where experimental results are presented. Finally, a conclusion about the effectiveness and the performance of the proposed equalizing system are outlined. 2. Main factors affecting batteries life The VRLA batteries which are widely used for standby service due to their reliability are destined for earlier degradation even in ideal conditions. Despite their maintenance free characteristics, VRLA batteries will fail sooner when used in certain conditions such as [22]: x Corrosion of the positive grid structure due to oxidation of the grid and plate materials. This factor, which is unavoidable, is the most common natural failure for lead-acid batteries. x Loss of active material from the positive plate. x Physical changes in the active material of the positive plate leading a loss of capacity. x Elevated temperatures reduce battery life. Indeed, an increase of 8.3 C can reduce lead-acid battery life around 50% or more. x A frequent deep-discharge cycle contributes to reduce the lifetime of the batteries. x Overcharging can cause an excessive gassing, and a high float voltage causes a higher positive plate corrosion rate. To overcome a potential damage, after discharge, recharging process must be controlled to 10% of the discharge current. x The most consequence of the undercharging is the sulfation, which can damage the plates. Moreover, a low voltage reduces capacity due to self-discharge phenomena. x VRLA batteries are considered to be very sensible to ripple dc-current. Indeed, this can lead to cell heating and will accelerate the degradation of cells. Some manufacturers provide an acceptable minimum frequency of voltage ripple to prevent an additional battery heating.
Fig. 1. A typical cycling performance of VRLA batteries
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Fig. 2. Impact of active and passive equalization on capacity of pack of batteries [23]
A typical cycling performance of VRLA batteries is illustrated in Figure 1, where the impact deep of discharge (DOD) on battery lifetime is addressed. There are other factors which are considered as aging mechanisms such as an improper storage and misapplications where batteries are used in not designed applications. A reduction to 80% of the rated capacity is commonly defined as the end of life for VRLA batteries. In fact, under 80% of rated capacity, the degradation of batteries will be accelerated, and a sudden failure is expected due to several factors such as a high discharge rate. Even they are considered maintenance free, VRLA batteries require a periodic maintenance program must be planned. Indeed, even under recommended conditions, batteries can observe a sudden failure. Figure 2 illustrates the impact of using an active equalization system for a pack of batteries. Indeed, with an active equalization system, a pack of batteries accomplishes at least 450 charging/discharging cycles, where the pack of batteries without active equalization reaches only 140 driving cycles. 3. Impact of unbalanced voltage on lifespan of VRLA batteries The unbalance between cells or battery voltages due the difference in their capacities, damage, and operating temperature will be accelerated by the charge and discharge cycles. This will cause over-charging, deep discharging, decrease time storage capacity and lifespan.
Fig. 3. Battery pack's current
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Fig. 4. Imbalanced voltage of VRLA batteries connected in series
Figure 3 shows the measured current for four VRLA installed in TechnoCentre Eolien (TCE) microgrid (MG). As we can see, the unbalance voltage of connected batteries strings is up to 3 V. This means that these batteries/cells have not had the same SOC. Fig. 4 illustrates the evolution of the voltage of the same four VRLA batteries, which were installed at the same time. As we can see, these batteries have not the same voltage, which mean that their capacities are not the same. The charge unbalance contributes to reduce battery lifetime [24, 25]. In practice, the recharge of cells connected in series must be stopped when the cell that has the highest voltage reaches its critical level. Thus, other cells are not fully charged. On the other hand, when cell that has the lower voltage reaches its critical value, the discharging process must be turned off. Thus, the battery's capacity is limited by the cell with a low voltage. In order to extend the battery life cycle and prevent failure to the cells as described above, it is recommended that all the serial cells have the same SOC during battery operations. Consequently, the use of an equalization process is necessary. 4. VRLA batteries maintenance requirement for buckup applications In order to increase battery lifetimes, reduce replacement costs, and improve reliability, VRLA batteries must be equipped with a monitoring system and undergo a proper maintenance and testing program. Indeed, replacement costs significantly increase the cost of a Battery Management System (BMS). Moreover, a number of systems allow for proper assessment of battery SOH by estimating the internal resistance of each cell/battery. This method is easier and more cost-effective compared to a capacity testing approach. The most simple way to track battery SOH is to know the baseline value of its internal resistance at 100% capacity (acceptance test), and then to periodically (quarterly for VRLA batteries) compare internal/intercell resistances to baseline values. A number of recommendations from industry experts exist, such as the IEEE 1188 standard for VRLA batteries which can be applied in maintenance inspections. Note that visual inspections should be performed at least once a year. Continuous connection of the BMS can also significantly increase overall reliability of the system. On the other hand, a BMS can help to avoid loss of data, and to determine the optimal time for replacement. 5. Experimental set-up 5.1. Description Figure 5 shows the block diagram of TCE's test bench, with a 7.5 kW Wind Turbine Generator (WTG) based on Direct-Drive (DD) Permanent Magnet Synchronous Generator (PMSG) to supply the VRLA battery bank via a controlled rectifier, which allows the optimization of the wind power generation. In order to validate the effectiveness of the active equalization process, a device illustrated in Figure 6 from HDM was used with a four 12 V Deka
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monobloc VRLA battery connected in series. The full charge of these batteries cannot exceed 14.6 V, and can be discharged up to 10.5 V. The active equalization system used in this paper is illustrated in Figure 6 balances voltage among four connected batteries strings during charge, discharge and even idle phases. In fact, this system contributes to prevent a severe undercharge and overcharge voltage which can lead to the rapid degradation of the performance, the reliability and life of battery. The active equalization ensures a balancing voltage between batteries within ± 0.4 V.
Figure 5. Block diagram of a TCE's test bench
Fig. 6. A bidirectional battery active equalization circuit from HDM
5.2. Results Figure 7 illustrates the dc-current from rectifier, a dc bus voltage and the estimated power from WTG respectively. Figure 8 shows the evolution of the stack’s voltage with a battery equalizer system installed on the battery bank at TCE’s facility.
Fig. 7. Evolution of the stack’s voltage with a battery equalizer system installed on battery bank (connected in series) at TCE’s facility
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It can be clearly seen on these figures that battery voltage imbalances are widely removed during the test cycle, when wind power is available. However, battery n°3 is being undercharged during the discharge cycle and overcharged (gassing) in the charging and equalizing cycles, while the rest of the remaining batteries have the same voltage due to the battery equalizer system. This means that battery n°3 have a greater internal resistance compared to the rest of the batteries. This means that the capacity of battery 3 is seriously diminished, and should be replaced.
Fig. 8. Evolution of the stack’s voltage with an active battery equalizer system installed on battery bank (connected in series) at TCE’s facility
6. Conclusion This paper has discussed the degradation phenomena of VRLA batteries used in a hybrid power system for remote applications. We have addressed some factors that are held responsible of the rapid degradation of batteries. Moreover, we have demonstrated the importance to pay attention to the maintenance even for VRLA batteries, which are considered as maintenance free. Indeed, using a battery management system can help assess the state of health of batteries by periodically conducting suitable tests with respect to recommendations of industry experts. Using an appropriate battery management system which provides sufficient information to users can help properly track the capacity of batteries and determine the optimal moment for their replacement. Indeed, this approach can help to prevent some phenomena believed to accelerate the aging mechanisms of batteries such as irreversible sulphation, corrosion, stratification and softening. Moreover, connecting batteries in a series in backup applications involves an imbalance in battery voltage which can significantly reduce their lifespan. Experimental tests have demonstrated the effectiveness of the active equalization system in order to avoid an unbalance voltage of connected battery strings. Furthermore, experimental tests have shown that the active charging equalization system can help to detect damaged batteries for its appropriate replacing. The future works will address the impact of the active equalization methods on the efficiency of the VRLA batteries, and the aging phenomena by tracking the evolution of their capacities.
Acknowledgements The authors gratefully acknowledge the funding received from Natural Resources Canada and the Natural Sciences and Engineering Research Council of Canada.
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