The motivation underpinning this thesis as described in Chapter 1 is to investigate the instantaneous performance and long-term degradation of LIBs within the context of HP-BEV and racing applications from an engineering perspective. Following a critical review of the existing literature relating to LIB characterisation, performance and degradation testing within Chapter 2, it was concluded that the existing testing methods for performance and degradation testing currently do not fully address the requirements of HP-BEV applications, and that the results obtained from existing research are not transferrable to this use case. Therefore, a lack of experimental investigations into the performance and degradation of LIBs within HP-BEV applications was identified as a first knowledge gap.
Furthermore, it was found that the absence of experimental studies within this field could be partially attributed to a lack of representative testing profiles suitable for experimental investigations. As such, a prerequisite for testing is to develop a new testing profile. To address this, Chapter 3 critically reviewed methodologies to construct representative test cases. It was found that there is no systematic approach to generate a duty cycle for the use of LIB performance and degradation testing from vehicle battery recordings. Based on these shortcomings, it was concluded that the existing framework for performance and degradation testing is insufficient for HP-BEV applications.
8.1 Contributions to knowledge
The key contribution of this work is the definition and implementation of a framework for LIB performance and degradation testing for applications, where existing testing standards are unrepresentative of the typical usage profile. This framework is illustrated within Figure 8-1.
FIGURE 8-1–PROPOSED FRAMEWORK FOR PERFORMANCE AND DEGRADATION TESTING
Degradation
169 The development and implementation of this framework on the use case of HP-BEV applications has addressed the research gaps identified within Chapters 2 and 3, as illustrated within Table 8-1. This was achieved through a series of four research tasks.
Procedure On-Road
TABLE 8-1–KNOWLEDGE GAPS IN THE EXISTING FRAMEWORK FOR LIB PERFORMANCE AND DEGRADATION TESTING;
- ADDRESSED,- NOT ADDRESSED,- INCONCLUSIVE
The first process was defined in “Research Task 1: Collate a database of battery duty cycles representative of HP-BEV racing applications.” This task is representative of the data collection step of the new framework illustrated within Figure 8-1. Data collection within this thesis consists of extensive modelling and simulation work to produce a duty cycle database of previously unexplored HP-BEV driving on racing circuit, the specifics of which are covered in Chapter 4. It requires the modelling and parameterisation of a conceptual HP-BEV in accordance with the sponsoring companies’ ambitions, the modelling of racing circuits, and the parameterisation of an AI racing driver. Although the resulting database is based off simulation, this approach offers the most accurate approximation to collecting real-world data with the information available to the author and lays the foundation for the development of a generic duty cycle.
The second stage of the process was defined in “Research Task 2: Define a methodology, from which a duty cycle that is suitable for LIB performance and degradation testing may be derived.”
This encapsulates the cycle construction methodology and cycle construction steps of the new framework. Chapter 5 presented the successful adaption of two signal design approaches to develop methodologies that allow for the derivation of testing profiles, which accurately capture signal traits that influence battery performance and degradation, thus addressing the research gap pertaining to LIB test cycle construction as indicated through the -symbol in column 4 within Table 8-1. Both methodologies presented within Chapter 5 are suitable for the design of duty cycles for battery performance evaluation within HP-BEV applications. Subsequently these methods yield two suitable duty cycles, one of which (HP-MSC) was selected for further testing.
The new HP duty cycles provide a more representative test profile compared to traditional battery test standards. The resulting methodology including database generation and cycle design were published in the Journal of Energy Storage [1].
170 For HP-BEVs, the ability to more accurately predict the performance requirements for the battery system within this emerging and strategically important BEV sector will further support a range of engineering functions, such as the ability to optimise the energy capacity and power capability of a battery pack and to select and design the most appropriate thermal management strategy.
This, in turn, may reduce system costs whilst concurrently increasing energy and power density of the resulting pack. Since it’s conception, the new HP duty cycle has facilitated further research regarding the evaluation of cell cooling strategies [3,7], and for the validation of a 1D electrochemical-thermal model of the Xalt 53Ah pouch cell [125]. On a wider impact, the devised methodologies are not limited to automotive use cases but are transferrable to all applications, where LIB performance and degradation are a concern. Therefore, the first two research tasks address the knowledge gap of a systematic approach to create duty cycles specifically for LIB performance and degradation testing.
Based on the derivation of the HP-MSC, Chapter 6 addressed “Research Task 3: Devise an experiment to conduct LIB performance and degradation testing to investigate differences between HP-BEV applications and standard testing procedures.” This relates to the test definition step within Figure 8-1. It describes the experimental procedures for characterisation, performance and degradation testing. The selection and parameterisation of the characterisation tests that provide a detailed description of cell behaviour follows the best practices as discussed within the literature review in Chapter 2. The performance and degradation tests are designed to benchmark the HP-BEV use case against standard testing procedures.
The results of the experimental work and their analysis are presented within Chapter 7 and fulfil
“Research Task 4: Analyse the experimental results and determine any use-case specific behaviour between HP-BEV applications and standard testing procedures.” The completion of research tasks 3 and 4 narrow the knowledge gap pertaining to LIB performance and degradation testing within HP-BEV applications. The results provide a direct comparison for battery cell thermal performance, and degradation behaviour between the HP and a standard test case. The results highlight the necessity for the proposed framework for performance testing, as indicated through the -symbol in column 4 within Table 8-1. From the experimental results obtained within this work, it is inconclusive whether this framework needs to be extended to degradation testing for this particular use case, as indicated through the -symbol in column 4 within Table 8-1. Considering the existing body of literature, however, the implementation of the framework for degradation testing would be expected to be beneficial. This assertion needs to be tested in further studies and could be achieved through continued testing until EOL is reached for both test groups. With regard to the degradation study, this work offers a hypothesis, based on
171 existing research, for the atypical degradation process observed. The testing methodology, results, and explanation for the likely degradation process from this experimental work have been published in the Journal of Energy Storage [2].
These findings impact the wider scientific community in a number of ways. The increase in cell temperature surface gradients for HP applications compared to standard testing procedures, and potential impact on degradation, require more capable thermal management strategies.
Secondly, the atypical OCV, capacity and degradation progression pose additional challenges for the SOH and SOC estimation algorithms in a BMS. The changes observed within this study are not detectible without EIS and dQ/dV measurements, which are both time consuming and the implementation of which is not always feasible for commercialised vehicles due to the time-scales and equipment involved.
8.2 Future direction and further work
Both, the development and implementation of the proposed framework provide opportunities for additional research and further work.
Pertaining to the data collection phase, within this thesis the database of HP-BEV duty cycles is obtained through simulation and modelling. As such, the representation of the simulated duty cycles compared to real world driving scenarios is limited by the fidelity of the models that comprise the HP-BEV, AI driver, and racing circuits. The inclusion of additional validated component models or real-world battery data would improve the fidelity of the database. Within this study only one type of vehicle and driver were considered. An expansion of the database through additional driver and vehicle models, would allow the database to be split into several classes. From this database, a larger number of duty cycles can be constructed to represent a wider variety of HP use cases.
It was shown that the duty cycle construction methodologies that constitute part of the framework could be used to produce representative profiles for HP-BEV applications. Although in principle, these methodologies are transferrable to other use cases, their usefulness for these applications should be validated.
The battery cell characterisation and degradation testing has shown that the proposed framework is required within the context of performance testing. For degradation testing, the usefulness of the framework, based on experimental data acquired within this thesis, is
172 inconclusive. The continuation of the degradation study until the EOL of the batteries in both test groups would resolve this unknown.
Regarding the presented hypothesis explaining the atypical aging behaviour, a post mortem study should be conducted to validate the explanation of electrode cracking, or to provide an alternative rationale for the observed changes. A repetition of the study without active cooling would provide additional information regarding the thermal performance of the cell, and quantification of a safe HP-operation capacity. As this study only investigates one specific cell, this work should be extended to additional LIB chemistries and formats to investigate the transferability of the results on those factors.
The proposed framework has only been implemented for HP-BEVs. The usefulness of this framework to other automotive use cases or industrial applications should be investigated.
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