The results presented suggest that as part of the technology selection process, OEM’s should study the susceptibility of their chosen cell technology to mechanically induced vibration profiles at different “vehicle to cell” orientations to mitigate their effects through improved system design. Within these two studies there has been a considerable difference of behaviour with the two cell chemistries. Whilst typically the NCA cells evaluated are typically unaffected by a representative 100,000 mile road vibration excitation, there were some specific aging behaviour (such as an observed increase in DC resistance, derived from pulse power testing, in Z:X oriented samples) identified. Any aging behaviour as a function of vibration would have to be characterized to ensure effective battery management system (BMS) development and to maximise useful service life. A study investigating the effects of NMC vibration aged cells from the investigation presented in Section 5.4 on a BMS strategy is discussed in [39].
The results from this study also show that both the electrical performance and the mechanical properties of lithium-ion cells can be affected by exposing the cell to vibration energy that is representative of a typical vehicle life. Whilst this is evident from the data presented within Section 5.4, the underlying causality is not yet clear. As a result, it is not possible to quantify the relationship that defines cell ageing caused by vibration excitation. Irrespective of this limitation, both the electrical and mechanical data show that NMC cells subject to vibration have a much greater
spread in the internal resistance, energy capacity and natural frequency. Managing this diversity may potentially drive further complexity in the systems engineering functions required to scale individual cells into a complete RESS. A number of articles discuss the need to minimise cell-to-cell variations within the system as a mean to reduce the differential current flows and heat generation with the pack. This research highlights that even for a RESS that is initially well designed; the impact of vibration-induced ageing may require greater levels of cell balancing and thermal management for this chemistry type.
The results summarised in Table 44 to Table 47 highlight that both the SOC and orientation are as important parameters to consider when designing a RESS as the contribution of the vibration induced profile. It is expected that variations in SOC within the RESS will be observed, especially for a BEV, where a large depth of discharge (DOD) is required to maximise vehicle range. Consequently, SOC may be a parameter that engineers consider more greatly than orientation. However, to maximize the volumetric energy density and minimise the footprint of the RESS, engineers may need to account for the impact of cell orientation on the performance of the RESS. Consequently, the author suggests that as part of the technology selection process, OEMs should study the susceptibility of the chosen cells to mechanically induced vibration profiles at different SOC and cell orientation to mitigate their effects through improved system design.
Conclusions
5.7
This study developed a test methodology, testing practices, test fixtures and safety protocols to allow for the evaluation of two different 18650 lithium-ion cell chemistries to vibration that was representative of 100,000 miles of customer durability. Unlike the previous studies identified within Chapter 2, these investigations evaluated the electrical and mechanical performance of the cells via impedance, capacity, OCV and natural frequency measurements at the SOT and EOT. The findings of this study have been published within [36, 37]. Additional studies utilising the cells and results from this study that are outside the scope of this thesis, have also been published. These studies, which are presented in [39, 40], include an investigation into the impact of vibration aged cells on a BMS strategy and the effect of vibration on surface layers.
Both vibration profiles employed within this study, which were devised to represent 100,000 miles of vehicle operation, resulted in a performance decrease within the NMC 18650 cells. However, the two different vibration profiles of USABC Procedure 10 and WMG-MPG resulted in two different results with respect to the effect of SOC and cell orientation. Of the NMC samples evaluated to USABC Procedure 10, cells in the Z:Z orientation typically displayed the least amount of degradation, whilst cells in the Z:Y orientation displayed the greatest. Whilst samples evaluated to the Z:X and Z:Z orientation displayed the least and greatest amount of degradation when
exposed to the WMG-MPG profile, respectively. Of the samples evaluated to USABC Procedure 10, items conditioned to 75 % SOC displayed the greatest degradation, whilst WMG/MBK, items conditioned to 25 % SOC displayed the greatest degradation. Samples conditioned to 50 % SOC typically displayed the least degradation regardless of the test profile.
Typically, both the electrical performance and the mechanical properties of the NCA 18650 lithium-ion cells were relatively unaffected when evaluated in accordance with USABC Procedure 10. No external damage or electrolyte leakage was observed in any of the test cells post vibration testing. No significant change in RO, or cell
capacity was observed as a result of vibration at the 95 % confidence level. OCV was not affected by vibration within this investigation. Cell degradation as a function of vibration was observed within the RDC of the cells oriented in the Z:X axis.
However no significant change in the RDC resistance was noted at the 95 %
confidence level in either the Z:Y and Z:Z oriented samples. Samples tested in the horizontal orientations of Z:X or Z:Y did not illustrate an increase in RCT, which was
observed to increase within both the Z:Z and control samples. A similar reduction in energy capacity, increase in RO and increase in RDC was witnessed within the
reference samples. These results indicate that the change in these electrical attributes is a function of other environmental conditions.
When comparing the orientation results of the NCA samples assessed in investigation 2 to the NMC items from investigation 1, no significant correlation in performance was observed. Drawing on the literature review presented in Chapter 2 and the experimentation undertaken, at this stage, the underlying causality between the application of vibration energy and cell orientation is not fully understood. It is recommended that the definition of these relationships is the focus of on-going research, using novel cell imaging and autopsy methods, to quantify changes in material composition and structure as per the study presented in [40]. Expanding the experimental programme to also include cells of different form-factor and chemistries will identify if the experimental results presented here are transferable to other cell technologies.
In conclusion, the experimental results highlight that depending on cell chemistry the potential for key electrical and mechanical properties within the cell to diverge, over time, due to the application of vibration energy that is consummate with a typical road vehicle life. Unless this phenomenon is well understood at the design stage of the vehicle, it may drive further complexity into design of the RESS in addition to causing in-service warranty claims.
6 Study 4 - Vibration Durability Behaviour of EV Batteries
via Multi-Axis Testing Techniques
Introduction
6.1
This study investigates if the electromechanical attributes of NCA 18650 battery cells and a Tesla Model S module (composed of the same 18650 cell type and chemistry) are adversely affected by exposure to vibration commensurate with that experienced by EVs through road induced excitation. The vibration excitation employed within this study is underpinned through real-world vehicle measurements sequenced to represent 10 years of vehicle European structural durability. Unlike the investigation presented in Chapter 5, this study applied the vibration to the test items in six degrees of freedom (6DOF) using a multi-axis shaker table (MAST). This method of real-world mechanical testing is known to be more representative of the vibration experienced by automotive components, as 6 motions of vibration (X, Y, Z, roll, pitch and yaw) are applied simultaneously. Similar to the studies presented in Chapter 5, cell and module characterisation within the electrical domain is performed via quantification of impedance, the open-circuit potential of the DUT and its energy capacity. Conversely, the mechanical properties of the test items are inferred through measurement of the cell’s natural frequency, or impact excitation modal analysis, in the case of the Tesla module. Experimental results are presented that highlight that the electromechanical performances of the 18650 NCA cells do not, in the main, display statistically significant degradation when subjected to vibration representative of a typical 10-year European vehicle life.
Unlike the studies presented in Chapter 5, which apply vibration to each axis sequentially and uni-axially within the frequency domain via the use of PSD profiles, this study applies the measured vibration within the time domain. The advantage of this methodology is that errors resulting from test time compression are avoided [33]. Also, because the vibration motion is applied in the time domain, it is more representative of real-world in-vehicle loading. As discussed within [3, 162, 163] within the context of traditional vehicle testing and component evaluation, the application of combined axial motions will often highlight additional failure modes that would otherwise not be observed through single-axis testing.
Due to the challenge accessing test equipment of this type, within a University context to undertake doctoral research, this study was performed at MPG within the Component Test Laboratory (CTL) Cube 2 test facility.
This Chapter is structured as follows; the objective and aims of this study are defined in Section 6.2. Section 6.3 presents the test methodology and associated theory. Sections 6.4 and 6.5 introduce and analyse the test result for the 18650 cells and Tesla Model S module respectively, whilst the discussion and conclusions are given in Sections 6.6 and 6.7.
Objective and Aims of Study
6.2
6.2.1 Objective
To determine if NCA 18650 battery cells and a current productionised Tesla Model S 18650 battery module can be electrochemically and mechanically aged by mechanical induced vibration. This shall be performed via the use of a multi axis shaker table and measurements from a Smart ED RESS, replicated in the time domain, that are representative of a 10 year durability life. The study investigates if orientation influences the observed degradation within the 18650 battery cells resulting from vibration.
6.2.2 Aims
To determine if vibration applied in 6DOF that is representative of approximately 10 years of European customer EV usage can age NCA 18650 battery cells and a Tesla Model S module.
To measure if cell orientation in relation to the vehicle axis can affect the vibration durability life of the NCA 18650 battery cells.
To compare the vibration degradation of NCA 18650 cells with the observed degradation trends observed from the previous study defined in Chapter 5.
Method of Vibration Durability Testing in 6DOF of 18650 Cells and Tesla
6.3
Model S Module
This Chapter defines the test method employed to determine the durability and aging behaviour of 18650 NCA cells and a Tesla Model S module when subjected to vibration in 6DOF. The test process and structure of this Chapter is summarised in Figure 57.