Characterising the in-vehicle vibration inputs to the high voltage battery of an electric vehicle

Journal of Power Sources 245 (2014) 510e519

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Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour

Characterising the in-vehicle vibration inputs to the high voltage battery of an electric vehicle James Michael Hooper a, James Marco b, * a b

Millbrook Proving Ground, Millbrook, Bedford MK45 2JQ, UK WMG, University of Warwick, Coventry CV4 7AL, UK

h i g h l i g h t s
A critical review into the durability of EV/HEV battery installations over a typical vehicle life.
Experimental assessment into the battery vibration loads for different road types.
Measured results from commercially available electric vehicles (Leaf, iMiEV, Smart ED).
Derivation of test profiles for assessing battery durability over a 100,000 miles vehicle life.

a r t i c l e i n f o

a b s t r a c t

Article history: Received 15 February 2013 Accepted 17 June 2013 Available online 5 July 2013

There has been little published research critically examining the mechanical integration of battery systems within either EVs or HEVs. Many of the existing Standards are designed to validate the fail safe function of the battery pack as opposed to assessing the mechanical durability of the complete system. If excessive vehicle warranty claims are to be avoided it is important that engineers tasked with the design of the battery installation properly understand the magnitude and frequency of the vibration inputs that the battery will be exposed too during the vehicle’s predicted life. The vibration characteristics of three different commercially available EVs have been experimentally evaluated over a wide range of different road surface conditions. For each vehicle, a durability profile has been sequenced to emulate the vibration energy that the battery pack may be exposed too during a representative 100,000 miles service life. The primary conclusions from the results presented are that the battery packs may well be exposed to vibration loads outside the current evaluation range of existing Standards. Ó 2013 Elsevier B.V. All rights reserved.

Keywords: Vehicle vibration Electric vehicle (EV) Hybrid electric vehicle (HEV) Battery system Battery testing

1. Introduction Within the automotive and road transport sector, one of the main drivers for technological development and innovation is the need to reduce the vehicle’s fuel consumption and the emissions of carbon dioxide (CO2). Legislative requirements are motivating manufacturers and subsystem suppliers to develop new and

Abbreviations: CAN, controller area network; DfT, Department for Transport; ECE, Economic Commission for Europe; EU, European Union; EV, electric vehicle; HEV, hybrid electric vehicle; HSC, high speed circuit; ICE, internal combustion engine; HV, high voltage; PSD, power spectral density; SAE, Society of Automotive Engineers; SOC, state of charge; UK, United Kingdom; USABC, United States Advanced Battery Consortium. * Corresponding author. Tel.: þ44 02576 573219. E-mail addresses: [email protected] (J.M. Hooper), james.marco@ warwick.ac.uk (J. Marco). 0378-7753/$ e see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jpowsour.2013.06.150

innovative electric vehicle (EV) and hybrid electric vehicle (HEV) concepts. Within this field, a key enabling technology is the design, integration and control of the vehicle’s high voltage (HV) battery system. Considerable research has been published relating to the energy management and component sizing of the HV battery with respect to both legislative drive-cycles [1,2] and real-world usage [3,4]. In contrast, there has been comparably little research published critically examining the mechanical integration of the HV battery within an EV or HEV. The effects of vibration on electrical and electronic components and subsystems are potentially a major cause of in market durability failures [5]. As discussed in Ref. [5], the dynamic loading on the electrical system may result in both a loss of electrical continuity and the structural failure within the associated housing. If excessive warranty claims are to be avoided, it is therefore important that engineers tasked with the design of the HV battery system properly understand the magnitude and

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frequency of the vibration inputs that the system will be exposed too during the vehicle’s predicted life. The research presented here aims to assess the structural durability requirements for the HV battery installation of an EV. Experimental results are presented for three commercially available EVs; the Nissan Leaf, the Smart ED and the Mitsubishi iMiEV. Vibration profiles are recorded from the vehicle’s respective battery packs as each vehicle traverses a wide range of different road surfaces. Using this measured data, it is possible to formulate a test profile that emulates the total vibration energy that the battery system will be exposed to during a typical 100,000 miles vehicle life. This paper is structured as follows; Section 2 presents the pertinent academic literature, in addition to the different legislative and duty of care Standards that vehicle manufacturers and system integrators must consider. Section 3 discusses the experimental methodology and highlights the key characteristics of the different EVs and road surfaces employed. Section 4 presents how the measured data may be sequenced to form an accelerated life assessment of the structural durability of the EV battery installation. Conclusions are presented in Section 5. 2. Related research and test Standards 2.1. Published research There has been little peer-reviewed, published research relating to the durability of the battery system with an EV or HEV. The most significant study is that reported by Martin et al. [6]. The research describes the derivation of a vibration profile for the testing of an EV derivative of a small conventional vehicle. Tri-axle accelerometers were placed on the B-pillars of the conventional vehicle. The vehicle was then driven over a range of different road surfaces, each relating to a particular road-type, such as motorways, urban roads and cobble stone pavements. The study also recorded the excitation associated with specific driving events, such as the shock loading from the vehicle traversing a series of speed bumps. The vibration test profile for the EV was derived by sequencing the vibration measurements from the different road surfaces to emulate a representative (but undisclosed) vehicle life. The resultant Power Spectral Density (PSD) plot highlights that the durability of the EV was evaluated over a frequency range of 7 Hze150 Hz. Peak vibration energy in the Z-axis was found to be in the order of 0.48 g2 Hz1 and occur between 10 Hz and 20 Hz. Conversely, peak vibration energy for both the X and Y axes was stated as being 0.063 g2 Hz1 at 15 Hz. The subject of sequencing vibration data to ascertain an appropriate durability test profile for a predicted vehicle life is discussed further in Section 4. While the research presented in Ref. [6] represents the most comprehensive study reported within the literature, there are a number of limitations associated with the methodology employed,

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the wider applicability of the conclusions reported and the level of dissemination presented. For example:
The data was measured from a conventional vehicle rather than from the EV derivative. As a result, only generic vibration measurements were possible since it was not possible to mount the instrumentation at key locations on the EV, such as directly onto the HV battery pack.
While the study presents the PSD plot for the resultant vibration test profile, there is no discussion as to the derivation of the test profile. In particular, the relative exposure of the EV to different road surfaces and how such measurements were sequenced to form a representative vehicle life from which an accelerated life durability assessment of the design could be made. 2.2. Vibration test Standards for the evaluation of EV battery systems Table 1 summarises both the legislative and duty of care test Standards that have evolved to evaluate the battery installation within an EV or HEV. Duty of care Standards are generally developed by appropriately recognised quality bodies (such as the British Standards) and can be used by vehicle manufacturers and systems integrators to assess the reliability, robustness and quality of their designs. Conversely, legislative test Standards represent a legal requirement that the manufacturer must meet. Detailed information as to the respective test requirements is well documented within each of the Standards and will therefore not be duplicated here. Instead, the aim of this section is to critically evaluate each test strategy and to highlight their respective strengths and weaknesses within the context of assessing the durability of an EV battery installation over the expected life of the vehicle. 2.2.1. Regulation UN38.3 Regulation UN38.3 is a legal requirement relating to the transportation of lithium ion battery systems [7]. The vibration test element of the Standard (Test 3) is designed to emulate the potential vibration conditions that a battery system may be exposed to during air transport. Discussed within [7,8], is a comprehensive review of the test process; denoting both the frequency range and force levels that the battery must be exposed too. In order to meet the requirements of Regulation UN38.3, the individual cells (or complete battery system) must not exhibit any loss off mass, leakage, venting, disassembly, rupture or fire. In addition, the open circuit voltage of the battery after the vibration profile has been applied must not be less than 90% of the initial conditions [7,8]. It is noteworthy that a number of vehicle manufacturers and system suppliers have raised concerns regarding the applicability of Regulation UN38.3 for the vibration assessment of EV and HEV battery systems [7]. In particular they note the high acceleration

Table 1 A summary of legislative and duty of care vibration test Standards applicable to EV and HEV battery systems. Standard

Type of vibration test

Frequency range (Hz)

Peak loading (swept sine wave)

Peak loading (random PSD wave)

Scope of the Standard

Vibration profile created from EV data

Test correspond to a desired vehicle life

UN38.3

7e200

8 g 50 Hze200 Hz

N/A

Legislative

No

No

ECE R100 Vibration J2380/USABC Procedure 10

Shaped swept sine wave (X, Y and Z axes) Shaped swept sine wave (Z axis) Random PSD/shaped swept sine (X, Y and Z axes)

7e50 10e190

1.02 g 7 Hze18 Hz 3 g 10 Hze20 Hz

Legislative Duty of care

No No

No Yes 100,000 miles

BS62660

Random PSD (X, Y, and Z axes)

10e2000

N/A

N/A 0.1 (g)2/Hz at 10 Hze20 Hz or 3 g from 10 to 20 Hz 2.05 (g)2/Hz at 10 Hz

Duty of care

No

No

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levels associated within the test. As seen in Table 1, acceleration levels on the battery in the order of 8 g (78 ms2) are required to be generated. Given that many HV battery packs within an EV or HEV may weigh in excess of 200 kge400 kg [9], this places a considerable constraint on the specification and structural integrity of both the battery system and the test equipment that is required to generate the excitation. While Regulation UN38.3 is a legal requirement, the Standard is primarily focussed on emulating the worst case shock loads and vibrations that may be experienced by the battery. The load profiles are unrepresentative of those that would be experienced by the battery during road vehicle operation. As a result, the authors deem that this specification is unsuitable for assessing the durability and in-vehicle service life of the battery pack. 2.3. SAE J2380 and USABC Procedure 10 Both the SAE J2380 and USABC Procedure 10 share the same vibration test profile and broadly the same test methodology. The SAE Standard is comprised of three separate profiles that can be executed sequentially to form the complete test. The aim of both Standards is to derive a test procedure for characterising the long term impact of road-induced vibration and shock loading on the performance and service life of EV batteries [10]. A summary of the respective vibration profiles is provided in Table 1, with a complete review in Refs. [10,11]. It is noteworthy that the vibration profiles defined have been synthesised from actual rough road measurement data and sequenced to emulate a 100,000 miles of vehicle usage. However, no clarification is provided within either Standard as to whether the data was measured from an EV/HEV or a conventional vehicle using a traditional internal combustion engine (ICE) and driveline. Within both Standards, the battery system undergoes electrical performance testing at different charge and discharge rates and different states of charge (SOC). In general, the battery system is deemed to have failed if there is a loss of electrical performance after the vibration test profile has been applied. In addition, further attributes of the HV battery are recorded and include [10]:
The loss of electrical isolation between the cell terminals and the pack structure.
The measurement of an abnormal pack voltage indicating the presence of either an open circuit or short circuit condition.
The measurement of an unexpected mechanical resonance within the battery indicating the structural failure of a mount or support and finally,
the measurement of excessive temperatures with the battery pack potentially indicating the mechanical damage of a cell or a failure of the thermal management system. Both the SAE J2380 and USABC Standards employ significantly time-compressed vibration profiles. This has the advantage that the tests can be completed within a comparatively short time period (between 13.6 h and 92.6 h respectively). Three further points should be noted in regard to both the SAE J2380 and USABC Standards. Firstly, both Standards treat the XeY vibration axis the same. While this reduces the complexity of the durability assessment, no evidence is provided that supports this assumption. Secondly, both Standards excite the HV battery over a frequency of between 10 Hz and 190 Hz. This range is similar to that discussed in Ref. [6]. However, no evidence is provided that supports the assertion that loads less than 10 Hz or above 190 Hz do not contribute to the performance degradation or mechanical failure of the HV battery. The final point to note is that the USABC Procedure

advises that battery designers should avoid or suppress any resonant frequencies within the battery pack that are in the range of possible vehicle resonant frequencies [11]. This implies that the USABC may have identified that the durability and life of EV batteries can be affected by the resonant frequencies generated within the host vehicle. 2.4. ECE Regulation 100 The purpose of Annex 8 from ECE Regulation 100 (ECE R100) is to verify the safety and performance of the energy storage system within a vibration environment that the battery will likely experience during normal operation of the vehicle [12]. It is noteworthy that ECE R100 is still in its draft form and has yet to be ratified by the European Commission. However, approval is envisaged during 2013 at which point conformance to the Standard will become a legal requirement for EV type approval within both the UK and the wider European Union (EU) [13]. The frequency range of the vibration profile applied to the battery system is between 7 Hz and 50 Hz. The load cycle is traversed within 15 min and must be repeated 12 times for a total of 3 h in the vertical mounting axis of the battery system [12]. Similar to the J2380 Standard and USABC Procedure 10, open circuit voltage measurements are conducted before and after the vibration profile is applied. For the battery to meet the requirements of the Standard, the device must not inhibit the Standard charging and discharging cycle described within ECE R100 [12]. The draft release of the Standard does not formally define any pass or fail criteria other than noting that the battery must be observed for 1 h at ambient temperature after the vibration profile and load current has been applied. Annex 8 of ECE R100 provides limited scope for manufacturers to determine the durability of their EV battery systems. There is little evidence presented within the Standard that it correlates to any expected vehicle life, other than stating that it replicates a representative vibration environment for the EV. It is argued therefore that this Standard is primarily focussed on short-term abuse testing of the battery system. Another limitation of the draft Standard is that it only evaluates the battery within the vertical axis, meaning that the component is not subjected to any vibration loads along either the X or Y mounting axis of the battery within the vehicle. 2.5. BS EN 62660-2:2011 BS EN 62660-2:2011 (Secondary Lithium-ion Cells for the Propulsion of Electric Road Vehicles e Part 2 e Section 6.1.1) is not primarily aimed at vibration testing of a complete battery pack, but rather individual cells. For completeness, it is included within this discussion since depending on the pack design, individual cells may experience similar levels of excitation. Within Section 6.1.1 of BS EN 62660-2:2011 the cell is energised to either 100% SOC or 80% SOC depending on if the cell is designed for use within an EV or a HEV [14]. The cell is then subjected to a defined vibration profile for 8 h within each axis. At the end of the test the item is checked. The cell meets the test requirement if it displays no loss of mechanical integrity or loss of electrical continuity [14]. It is noteworthy that BS EN 62660-2:2011 represents the only Standard that defines different SOC values depending on whether the cell is to be employed within an EV or a HEV. Similarly, since the vibration range of the test is between 10 Hz and 2000 Hz, it is also the only specification to consider vibration levels above 200 Hz. Similar to ECE R100, it is argued that this specification is designed to be a short term integrity test and is not aligned to any given vehicle life.

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2.6. Discussion It is clear from Table 1 and the discussion presented thus far that most Standards have not been derived to represent a given vehicle life (such as 10 years in-vehicle service life or a 100,000 miles). Many of the Standards represent short term abuse tests designed to validate the fail safe function of the battery pack or cells as opposed to the mechanical durability of the complete system. In addition, there is evidence to suggest that these Standards have been created from data generated from vehicles employing a conventional powertrain or derived from existing test Standards from the consumer electronics sector. The author’s would argue that if the baseline data is not representative, it may in turn result in the battery system being over-engineered to compensate for the arduous test requirements. This may in turn, result in a final design of battery installation in which the weight and cost of the solution is prohibitive for successful vehicle integration. Another noticeable limitation is that some specifications only assess the integrity of the battery assembly in the vertical axis. As a result, defects within the pack design along the XeY mounting axis may not be identified. Based on the numerical data provided in Table 1, Fig. 1 presents the range of vibration levels that may be used to validate the EV battery system. Fig. 1(a) defines the vibration levels associated with UN38.3, USABC and the ECE R100 Standards. Conversely, Fig. 1(b) presents a comparison of the PSD test profiles defined within the SAE J2380 and the BS EN 62660-2:2011 Standards.

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3. Experimental assessment of the vibration inputs to an EV battery system Given the uncertainty as to the source of the baseline data used to derive the different vibration Standards, the aim of this section is to obtain a set of experimental measurements of the in-vehicle vibration loads within three commercially available EVs. Vibration profiles are recorded directly from the vehicle’s respective battery packs as each vehicle traverses a range of different road surfaces. 3.1. Experimental set-up The vibration characteristics of three different commercially available EVs were evaluated over the durability surfaces available at the Millbrook Proving Ground. A control vehicle was also employed in the form of a Standard C-segment passenger vehicle. The motivation for including a conventional vehicle was to better understand if the trends in vibration response were unique to the EVs or if a correlation existed between the EVs and the conventional ICE powered derivative. The four vehicles evaluated as part of this study are:





SMART Electric Drive (ED) Nissan Leaf Mitsubishi iMiEV Vauxhall Astra

Table 2 provides a detailed breakdown of the pertinent mechanical characteristics of each vehicle. Each EV was instrumented with 6 tri-axial accelerometers mounted at the following locations:
Two tri-axial accelerometers on the A-post pillars
Two tri-axial accelerometers on the chassis next to the battery system
Two tri-axial accelerometers directly mounted on the battery system

Fig. 1. A comparison of vibration profiles from each of the battery test Standards.

The A-post pillars were selected as a measurement position because they represent an industry standard location for the measurement of vehicle vibration. Measuring the vibration directly on the battery system and on a solid member of the chassis in close proximity to the battery installation, highlights the potential transfer of vibration energy from the base vehicle to the energy storage device. For the conventional vehicle, accelerometers were mounted in the closest equivalent locations to those mounted on the battery installations. A vibration measurement at each of the test locations on the battery pack and chassis were recorded by driving the vehicles over the durability surfaces at Millbrook Proving Ground. For reference, a short description of the different road surfaces is provided in Table 3. All vehicles were assessed with the same driver and passenger to ensure consistency in the measurements and payloads. The vehicles were also assessed with their tyres inflated to the manufacturers recommended pressures. The data was recorded using the traditional vehicle axis convention shown in Fig. 2. The raw vibration data was post-processed using the commercially available nCodeÓ software to produce the required PSD plots for each of the X, Y and Z axes. One limitation associated with this study is that intrusive measurements within the EVs were not possible. As a result, no vibration measurements could be made from inside the respective battery packs. Furthermore, logging of the vehicle’s control strategies and modes of operation via the on-board controller area network or CAN bus was also not possible. A better understanding

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Table 2 The mechanical characteristics of the different test vehicles [21e23]. Vehicle

Smart ED

Mitsubishi iMiEV

Nissan Leaf

Vauxhall Astra

Kerb weight Length Width Height Wheel base Track width front Track width rear Ground clearance Battery length Battery width Location of battery Tyre specifications

820 kg 2695 mm 1560 mm 1542 mm 1867 mm 1283 mm 1384 mm 140 mm 460 mm 1065 mm 750 mm from front of wheel centre 155/60/R15 e front 175/55/R15 e rear

1110 kg 3475 mm 1475 mm 1610 mm 2250 mm 1310 mm 1270 mm 150 mm 1500 mm 980 mm 720 mm from front of wheel centre 145/65R15

1521 kg 4445 mm 1770 mm 1550 mm 2700 mm 1539 mm 1534 mm 160 mm 1540 mm 1150 mm 800 mm from front of wheel centre 205/55R16

1885 kg 4419 mm 1814 mm 1510 mm 2685 mm 1550 mm 1600 mm 110 mm N/A N/A N/A 235/50/R18

therefore of the in-pack or cell level vibration characteristics and also the correlation between the vehicle control strategies and the induced vibration on the battery system represent two interesting areas of further study. 3.2. Results As part of the experimental programme of work, each of the four test vehicles was driven over each of the 13 road surfaces. In total, 52 individual data-sets were recorded representing over 25 km of driving. It is beyond the scope of this paper to present the complete set of measured data from each of the four vehicles under evaluation. Instead a representative subset is presented here that highlights noteworthy conclusions. Fig. 3 presents the PSD plot recorded from the accelerometer mounted on the battery pack of the Nissan Leaf, Mitsubishi iMiEV and the Smart ED. For the conventional vehicle, the results shown were measured from the accelerometer mounted in the closest equivalent location. Measurements were taken as each vehicle was driven over the city course defined in Table 3. From Fig. 3, it can be seen that within each of the EVs, the battery pack is exposed to significant vibration energy within the frequency range of 0e7 Hz. The measurements shown relate to the vertical mounting axis of the battery packs within the respective vehicles. However, similar results were also noted for both the X and Y axes. The results highlight that even though the recognised Standards do not assess

the battery within this low frequency range, development engineers should consider frequencies below 7 Hz for rig based durability testing of EV battery systems. Fig. 3 also shows that the conventional vehicle is subject to similar levels of low frequency vibration, indicating that this behaviour is potentially linked to the motion of the vehicle’s suspension and sprung mass [15,16]. From reviewing the PSD plots obtained it is noteworthy that significant peaks in vibration energy can be observed when the EVs are either accelerated to a high speed or when they are being driven at a comparatively high cruise speed. Fig. 4 shows the vibration energy recorded from the accelerometer mounted directly on the vehicle’s respective battery packs when the vehicles are driven on the mile straight at wide open throttle (WOT). It can be seen that the battery systems for both the Mitsubishi iMiEV and the Nissan Leaf are exposed to significant vibration inputs outside the evaluation ranges of current specifications such as SAE J2380 and ECE R100 that only evaluate up to 190 Hz and 50 Hz respectively. What is also evident from this graph is the difference in vibration behaviour between the Mitsubishi iMiEV and Nissan Leaf at frequencies above 300 Hz. As discussed in Section 3.1, given the limitations of this study, it is not possible to provide a definitive reason for the higher frequency modes of vibration measured. However, several possible reasons for this difference may be proposed. Firstly, the Nissan Leaf and Mitsubishi iMiEV utilise two different methods of battery pack construction. Furthermore, both EVs employ different thermal management strategies; the iMiEV employs a liquid cooling

Table 3 Definition of the road surface types employed for durability assessment. Road surface

Road surface classification

Road surface description

Belgian pave

Urban

Cats eyes (30 mph) Cats eyes (50 mph) City course

Rural Rural Urban

Handling circuit Hill route (loop 1)

Rural Rural

HSC

Motorway

Mile straight (part throttle) Mile straight (wide open throttle) Pot holes

Motorway Motorway Urban

Random waves

Urban

Sine waves Twist humps

Urban Urban

Industry-standard surface for evaluating vehicle’s noise vibration and harshness and durability. 1.45 km of block granite paving. 44 cats eyes along a 90 m length of track 44 cats eyes along a 90 m length of track Level asphalt paved surface with multiple tight turns, speed hump and posted speed limits typical of an urban driving environment. A concrete paved 6 m wide track with varying camber. Typical of rural roads. A simulated alpine road which has numerous assents, descents and bends with changing camber. A circular constant radius banked concrete paved track constructed to simulate motorway driving conditions. Long, precisely levelled surface with fast approach and departure lanes. Long, precisely levelled surface with fast approach and departure lanes. Two large simulated pot holes, made of cast iron laid into the concrete surface of the road. Undulating surface out of phase, inducing maximum suspension travel and high amplitude low frequency input to vehicle structure. Sine waves out of phase, for high frequency input to the vehicle interior and structure. Series of 10 handed angled humps of tarmac construction that has been developed to apply torsional chassis inputs.

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Z Axis (Up and Down)

X Axis (Fwd. and Aft) Y Axis (Left to Right)

Fig. 2. Axis convention for battery pack instrumentation.

strategy, whereas the Nissan Leaf utilises an air cooled system to regulate the temperature of the battery. During periods of rapid vehicle acceleration or when the vehicle is travelling at a high speed, the control systems may activate the respective cooling ancillaries thereby transferring higher frequency vibration energy to the battery packs. Another credible reason to explain the vibration profiles between the two EVs, is that vibration energy is being transmitted to the battery pack from the conventional drivetrain components, such as the gearbox, final drive and tyres. Both the Nissan leave and the Mitsubishi iMiEV employ a single ratio stepdown transmission between the electrical machine and the road wheels. As discussed within [15,16] and as noted on the vibration response measured from the conventional vehicle, powertrain resonances are known to occur within this frequency band. 4. Assessing the durability of an EV battery system The aim of this Section is to formulate a test profile for each EV that emulates the total vibration energy that the battery system will be exposed to during a typical 100,000 miles vehicle life. This vibration profile could typically be employed during laboratory based testing in which the battery assembly is mounted on a multi-axis shaker table. In order for the PSD plot to emulate the in-vehicle service life of the battery system, two parameters must be defined by the development engineer. Firstly, the test duration and secondly the total number of times that each road surface is to be repeated. There are obvious commercial and technical motivations for developing accelerated-life durability tests for engineering

Fig. 4. PSD plot for battery pack vibration over the mile straight WOT road-surface (Zaxis).

components and systems. However, if the test duration is compressed too far, it may no longer be representative of real-world conditions. As discussed in Ref. [9], the shorter the duration of the test profile, the greater the severity of the vibration loads applied to the system. For example, the 24 kWh battery system employed within the Nissan Leaf weighs in the order of 300 kg. Evaluating the 100,000 miles durability of this battery system over the SAE J2380 vibration profile would involve exposing the battery to a peak load in the Z axis of approximately 3.1 kN or 4 kN depending on whether the test programme was designed to run for either 14 h or 92 h respectively. The nCodeÓ software environment was employed to derive the durability test profiles. The software allows the user to evaluate different levels of time compression with respect to the peak loading applied to the system, thereby ensuring that an upper threshold, or shock vibration level is not exceeded. The Millbrook Structural Durability Procedure was employed to define the number of repeated road surfaces that would be sequenced together to form the complete test profile. This durability schedule is representative of the typical surfaces and vibration inputs that a European passenger car is exposed to during a 100,000 miles vehicle life. While this procedure represents an internal organisational Standard, it has evolved over 20 years of experience and is currently employed by a number of leading vehicle manufacturers to assess the service life of their products. Table 4 presents the relative percentage of each road surface types that comprises the durability procedure. 4.1. Results

Fig. 3. PSD plot for battery pack vibration over the city course road-surface (Z-axis).

The measured vibration data for each EV under evaluation was sequenced to the structural durability procedure described in Table 4. Through experimentation with the nCodeÓ software a nominal test duration of 50 h for the X, Y and Z axes was deemed to provide a suitable trade-off between the contradictory requirements of time compression and test accuracy. Fig. 5 presents the vibration recorded in the Z axis from the accelerometers mounted onto the EV battery packs. In the case of the conventional vehicle, the closest comparable accelerometer readings are shown. It is noteworthy that all the EVs have an initial peak in vibration energy around 1 Hz. This initial peak is likely to relate the natural

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Table 4 Relative weighting of each road surface type for the structural durability assessment of in-vehicle life for an EU passenger vehicle. Road surface

Road surface classification

Road surface distance (km)

Percentage composition within sequenced data

Belgian pave Cats eyes 30 mph Cats eyes 50 mph City course Handling circuit Hill route (loop 1) HSC Mile straight (PT) Mile straight (WOT) Pot holes Random waves Sine wave Twist humps

Urban Rural Rural Urban Rural Rural Motorway Motorway Motorway Urban Urban Urban Urban

870 96 96 8475 988 5956 2500 1548 1548 9 768 193 288

3.7% 0.4% 0.4% 36.3% 4.2% 25.5% 10.7% 6.6% 6.6% 0.03% 3.3% 0.8% 1.2%

frequency of the sprung vehicle mass and suspension systems [15e17]. The Figure also shows that the EVs have a second peak in vibration energy in the Z axis between 7 Hz and 20 Hz, with the peak occurring at 11 Hz for the Smart ED and 13 Hz for the iMiEV and Nissan Leaf. Resonances within this frequency band traditionally relate to the powertrain modes, such as the dynamics of the driveline and gearbox. These results would tend to contradict the common perception that EV’s have a greater degree of powertrain refinement than a conventional vehicle due to there being fewer reciprocating parts within the powertrain [9]. Another interesting observation from Fig. 5, is the resonant peak associated with both the Nissan Leaf and the iMiEV between 20 Hz and 40 Hz. Within a conventional vehicle, vibration modes within this frequency range typically relate to the torsional dynamics of the chassis system [15,16]. Given that within both vehicles, the design and integration of the battery pack constitutes a significant structural element of the vehicle, it is likely that these loads are being transmitted through to the battery installation during the service life of the vehicle. Figs. 6 and 7 present the vibration profiles sequenced from the measured data along both the X and Y mounting axes respectively. As it can be seen, similarities exist between the two sets of result. As with the results presented in Fig. 5 credible reasons can be postulated as to the impact of vehicle mass distribution and the method of battery pack e chassis integration on the final vibration profile.

Fig. 5. Durability vibration profile for a 100,000 vehicle life (Z-axis).

Fig. 6. Durability vibration profile for a 100,000 vehicle life (X-axis).

However, simply from a visual inspection of both Figs. 6 and 7 it is clear that the battery installation experiences significant loading along both the X and Y mounting axes. Contrary to the recommendations within the ECE R100 Standard, this implies that the durability of the battery pack should therefore be evaluated along these axes. Furthermore, it is clear from Figs. 6 and 7 that they are not the same. This fact contradicts the implicit assumption within the USABC Procedure 10 and SAE J2380 Standard that assume a common vibration test profile for both the X and Y axes. A clear recommendation therefore is that these axes should be tested separately and bespoke vibration profiles derived as part of a laboratory based durability assessment of future EV battery packs. 4.2. A comparison of the structural durability procedure with SAE J2380 For the vibration inputs recorded directly on the battery packs, three PSD plots were generated for each of the X, Y and Z axes. To facilitate a comparison with the SAE Standard, the test duration for each axes was compressed to the same test duration, namely 16 h

Fig. 7. Durability vibration profile for a 10,100,000 vehicle life (Y-axis).

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for the Z axis and 38 h for both the X and Y axes respectively. This resulted in an overall durability test time of 92 h. Fig. 8 presents the vibration profile for the vertical mounting axis of the battery pack within each EV. In addition, the three vibration profiles denoting the vibration test requirements from the SAE J2380 Standard are shown for comparison. It is clear from the Figure that there is a significant difference in the respective vibration energy levels. The large discrepancy indicates that the SAE J2380 Standard defines a durability test that is unrepresentative of the typical life of an EV within the European market. As discussed in Section 4, it is possible to increase the vibration levels within the test profile by further time compressing the sequenced data. However, through experimentation it was found that it was not possible to further reduce the time compression without inducing excessive peaks in the test profile, thereby compromising the validity of the durability

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test. As discussed in Section 2.3, no information is provided within the SAE J2380 Standard as to the nature and relative durations of the different road surfaces that have been employed to generate the test profile. As a result, it is not possible to provide a definitive argument as to why such differences exist. However, it is reasonable to assume that in addition to the type of road surfaces presented in Table 3, the SAE Standard also includes an element of abuse testing, such as the shock loading associated with the vehicle striking a large pot hole or a kerb on the side of the road. 4.3. Understanding the impact of different road surfaces on battery durability requirements The structural durability schedule introduced in Table 4 is known to accurately emulate the vibration levels that would be applied to a European passenger vehicle over a 100,000 miles

Fig. 8. Comparison of durability profile (Z axis) with the SAE-J238 Standard.

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Table 5 Road surface types within the structural durability assessment of the in-vehicle life of a passenger vehicle (weighted to urban and rural roads). Road surface

Road surface classification

Road surface distance (km)

Percentage composition within sequenced data

4 Belgian pave Cats eyes 30 mph Cats eyes 50 mph City course Handling circuit Hill route (loop 1) HSC Mile straight (PT) Mile straight (WOT) Pot holes Random waves Sine wave Twist humps

Urban Rural Rural Urban Rural Rural Motorway Motorway Motorway Urban Urban Urban Urban

7263 758 758 70,753 7799 47,013 7190 4452 4452 72 6412 1608 2404

4.5% 0.5% 0.5% 44% 4.8% 29.2% 4.5% 2.8% 2.8% 0.04% 4.0% 1.0% 1.5%

vehicle life. However, a number of recent studies have highlighted the differences between conventional vehicles and EVs when being driven in the real-world [18,19]. Research has highlighted that EVs tend to operate more within urban and rural environments with comparatively less operation within high speed motorway conditions. In order to assess the impact that this may have on the required structural durability of the battery installation, the vibration measurements taken from the vertical axis of the accelerometer mounted directly on the battery pack of the Nissan Leaf were resequenced to include a greater percentage of both urban and rural driving. Table 5 shows how the required breakdown of urban road surfaces (55%), rural road surfaces (35%) and motorway (10%) was achieved using the available road types. Derivation of the relative percentages was taken from recommendations made by the UK Department for Transport (DfT) [20]. Using the revised composition of road surface data presented in Table 5, Fig. 9 shows the vibration durability test profile that emulates a 100,000 miles vehicle life. In order to facilitate a comparison with previous results, the test profile has been time compressed to a duration of 50 h. From Fig. 9 it can be seen that the general trend in vibration profiles as a function of frequency are largely similar. However, the durability test profile that has been weighted to emulate more

urban and rural driving includes greater peaks in vibration. This is particularly true within the frequency range of 1 Hze25 Hz, where vibration energy levels increase by as much as 10%. While the results presented in Fig. 9 are primarily for illustration, the graph does highlight that an EV operated within a mainly urban and rural environment may well be subjected to a more arduous life than a conventional vehicle that was driven over a more traditional composition of road surface types.

5. Conclusions There has been little published research critically examining the mechanical integration of the HV battery system within an EV or HEV. However, if excessive vehicle warranty claims are to be avoided it is important that engineers tasked with the design of the battery installation properly understand the magnitude and frequency of the vibration inputs that the system will be exposed too during the vehicle’s predicted life. From a critical evaluation of existing legislative and duty of care Standards, it is clear that most test Standards have not been derived to emulate a given vehicle life. Instead, many of the Standards represent short term abuse tests designed to validate the fail safe function of the battery pack as opposed to assessing the mechanical durability of the complete system. In addition, there is evidence to suggest that these Standards have been created from data generated from vehicles employing a conventional powertrain or derived from existing test Standards from the consumer electronics sector. As a result, a battery pack designed specially to meet such Standards may well be over-engineered with a weight and cost that is prohibitive for successful vehicle integration. The vibration characteristics of three different commercially available EVs were evaluated over the durability surfaces available at the Millbrook Proving Ground. For each vehicle a durability profile was sequenced to emulate the vibration energy that the battery pack would be exposed too during a representative 100,000 miles service life within Europe. From these results, it can be concluded that the battery packs may well be exposed to vibration loads outside the current evaluation range of existing Standards. In addition, whereas most specifications propose a single test specification for both the X and Y mounting axes of the battery pack, measured data from all three EVs highlight that these axes experience different loads and therefore should have bespoke test profiles defined for them. Finally, if the assertion is correct that EVs will operate more within urban and rural environments, the results presented here suggest that the battery installation will be exposed to greater peak loads in vibration than a comparable vehicle driven over a traditional composition of European road surfaces.

References

Fig. 9. Durability vibration profile for a 100,000 vehicle life over urban and rural road surfaces (Z-axis).

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