High-performance electric vehicle duty cycles and their impact on lithium ion battery performance and degradation

Abstract

High performance (HP) battery electric vehicle (BEV) and racing applications represent significantly different use cases than those associated with conventional consumer vehicles and road driving. The differences between HP-BEV use cases and the duty cycles embodied within established lithium ion battery cell (LIB) test standards will lead to unrepresentative estimates for battery life and performance within HP-BEV applications. Furthermore, the behaviour of LIBs in these applications is not well understood due to a lack of suitable testing cycles and experimental data. The research presented within this thesis addresses this knowledge gap through the definition and implementation of a new framework for LIB performance and degradation testing. The new framework encompasses the definition of a methodology through which a suitable duty cycle may be derived, and subsequent definition of the experimental procedures required to conduct LIB performance and degradation testing. To underpin the development of a suitable duty cycle, a method is presented to simulate race circuits, a HP-BEV and a driver model to generate a database that defines a range of HP duty cycles that are deemed representative of the real-world use of a HP-BEV. Subsequently, two methods to design a HP duty cycle are evaluated and validated. One of the methods studied (HP Random Pulse Cycle) extends an established driving-cycle construction technique, based on the derivation of micro-trips. The second method (HP Multisine Cycle) utilises a time-frequency domain-swapping algorithm to develop a duty cycle with a target amplitude spectrum and histogram. The design criteria for both construction techniques are carefully selected based on their potential impact on battery degradation. The new HP duty cycles provide a more representative duty cycle compared to a traditional battery test standard and facilitate experimental work, which will more accurately describe the performance and degradation rate of cells within HP-BEV use. Utilising the newly developed HP-Multisine Cycle, an experimental procedure for LIB performance and degradation testing is presented. Six lithium ion cells are characterised, followed by a performance and degradation study. The performance study investigates the thermal behaviour of the cells when subjected to HP-BEV scenarios and a standard testing cycle (IECC). Results show an increase in excess of 200% in surface temperature gradients for the HP use case compared to the standard testing cycle. The degradation study compares the degradation progression between the HP-BEV environment and conventional testing standards. Two test groups of cells are subject to an experimental evaluation using the HP Multisine Cycle and the IECC. After 200 cycles, both test groups display, counter to expectations, an increased energy capacity, increased pure Ohmic resistance, lower charge transfer resistance and an extended OCV operating window. The changes are more pronounced for the cells subjected to the HP Multisine Cycle. It is hypothesised that the ’improved’ changes in cell characteristics are caused by cracking of the electrode material caused by high electrical current pulses. With continued cycling, the cells cycled with the HP Multisine Cycle are expected to show degradation at an increased rate. The results from the experimental studies provide new insights into the thermal management requirements and evolution of cell characteristics during use within HP-BEVs, and highlight the limitations in the understanding of the complex cell degradation in this area. The new framework addresses the lack of suitable testing cycles and experimental investigations for the HP-BEV environment. The methodologies presented are not limited to the automotive sector but may be used in all areas, where existing testing standards are unrepresentative of the typical usage profile, and LIB degradation and performance are a concern