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

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

Battery power requirements in high-performance electric vehicles

International standards and guidelines regarding characterisation and cycle life testing of batteries in electric vehicles (EVs) currently do not take into account high-performance driving. Using simulation software, track driving in a high-performance vehicle is simulated, and speed-time profiles are recorded. These as well as established driving cycles are used in conjunction with an EV model to determine power profiles at battery terminals. The difference in the resulting power profiles suggest that the evaluation of batteries for the HP segment requires separate characterisation and cycle life tests

Electrical and thermal behaviour of pouch-format lithium ion battery cells under high-performance and standard automotive duty-cycles

Six pouch-format cells comprising a carbon anode and nickel-cobalt-manganese (NCM) cathode are characterized. Their 1C discharge capacity and open circuit voltage are determined. Internal Resistance is investigated via Hybrid Pulse Power Characterization tests and Electrochemical Impedance Spectroscopy. They are subsequently subject to two different electrical loading profiles, one representing high-performance (HP) driving applications, the other representing urban and extra-urban driving scenarios. The cells are instrumented with thermocouples to determine their surface temperature during cycling. The experimental results show that HP scenarios result in higher temperatures and temperature gradients, requiring bespoke thermal management strategies and suggesting increased degradation over prolonged use

Duty-cycle characterisation of large-format automotive lithium ion pouch cells for high performance vehicle applications

The long-term behaviour of lithium ion batteries in high-performance (HP) electric vehicle (EV) applications is not well understood due to a lack of suitable testing cycles and experimental data. As such a generic HP duty cycle (HP-C), representing driving on a race track is validated, and six NMC graphite cells are characterised with respect to cycle-life. To enable a comparison between the HP-EV environment and conventional road driving, two test groups of cells are subject to an experimental evaluation over 200 duty cycles that includes a representative HP-C and a standard duty cycle from the IEC 62660-1 standard (IECC). After testing, both test groups display increased energy capacity, increased pure Ohmic resistance, lower charge transfer resistance an extended OCV operating window. The changes are more pronounced for cells subject to the HP-C. Based on capacity tests, Electrochemical Impedance Spectroscopy (EIS), pseudo-OCV tests, and Pulse Multisine Characterisation, it is concluded that the changes in cell characteristics are most likely caused by cracking of the electrode material caused by high electrical current pulses. With continued cycling, cells cycled with the HP-C are expected to show degradation at an increased rate due to raised temperatures, and more pronounced electrode cracking

Battery cycle life test development for high-performance electric vehicle applications

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 use cases and the duty-cycles embodied within established battery test standards will lead to unrepresentative estimates for battery life and performance within a HP application. A strategic requirement exists to define a methodology that may be used to create a representative HP duty-cycle. Within this paper two methods HP duty-cycle design are evaluated and validated. Extensive simulation results into the electrical performance and heat generation within the battery highlight that the new HP duty-cycles provide a more representative duty-cycle compared to traditional battery test standards. The ability to more accurately predict the performance requirements for the battery system within this emerging and strategically important BEV sector will support a range of engineering functions. In addition, the ability to more accurately define the use-case for a HP-BEV will underpin ongoing experimentation and mathematical modelling to quantify the associated cell ageing and degradation that may occur within HP vehicle applications

Thermal analysis of fin cooling large format automotive lithium-ion pouch cells

Conductively cooling the surface of lithium-ion pouch cells may simplify the external cooling mechanism, as heat transfer mediums are not routed across the cell surface. In this paper, the thermal performance of cooling cells with metallic fins is analysed using a developed test rig and thermal model. Results indicate that single edge fin cooling with aluminum sheets is effective in limiting surface temperature gradients to below circa 5℃ for cells subject to realistic EV and mild PHEV duty cycles. For aggressive track racing EV cycles, double edge fin cooling is required to limit surface temperature gradients to below 12℃

Data for Battery cycle life test development for high-performance electric vehicle applications

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 use cases and the duty-cycles embodied within established battery test standards will lead to unrepresentative estimates for battery life and performance within a HP application. A strategic requirement exists to define a methodology that may be used to create a representative HP duty-cycle. Within this paper two methods HP duty-cycle design are evaluated and validated. Extensive simulation results into the electrical performance and heat generation within the battery highlight that the new HP duty-cycles provide a more representative duty-cycle compared to traditional battery test standards. The ability to more accurately predict the performance requirements for the battery system within this emerging and strategically important BEV sector will support a range of engineering functions. In addition, the ability to more accurately define the use-case for a HP-BEV will underpin ongoing experimentation and mathematical modelling to quantify the associated cell ageing and degradation that may occur within HP vehicle applications

A new approach to the internal thermal management of cylindrical battery cells for automotive applications

Conventional cooling approaches that target either a singular tab or outer surface of common format cylindrical lithium-ion battery cells suffer from a high cell thermal resistance. Under an aggressive duty cycle, this resistance can result in the formation of large in-cell temperature gradients and high hot spot temperatures, which are known to accelerate ageing and further reduce performance. In this paper, a novel approach to internal thermal management of cylindrical battery cells to lower the thermal resistance for heat transport through the inside of the cell is investigated. The effectiveness of the proposed method is analysed for two common cylindrical formats when subject to highly aggressive electrical loading conditions representative of a high performance electric vehicle (EV) and hybrid electric vehicle (HEV). A mathematical model that captures the dominant thermal properties of the cylindrical cell is created and validated using experimental data. Results from the extensive simulation study indicate that the internal cooling strategy can reduce the cell thermal resistance by up to 67.8 ± 1.4% relative to single tab cooling, and can emulate the performance of a more complex pack-level double tab cooling approach whilst targeting cooling at a single tab

Dataset to support article: 'A new approach to the internal thermal management of cylindrical battery cells for automotive applications'

Conventional cooling approaches that target either a singular tab or outer surface of common format cylindrical lithium-ion battery cells suffer from a high cell thermal resistance. Under an aggressive duty cycle, this resistance can result in the formation of large in-cell temperature gradients and high hot spot temperatures, which are known to accelerate ageing and further reduce performance. In this paper, a novel approach to internal thermal management of cylindrical battery cells to lower the thermal resistance for heat transport through the inside of the cell is investigated. The effectiveness of the proposed method is analysed for two common cylindrical formats when subject to highly aggressive electrical loading conditions representative of a high performance electric vehicle (EV) and hybrid electric vehicle (HEV). A mathematical model that captures the dominant thermal properties of the cylindrical cell is created and validated using experimental data. Results from the extensive simulation study indicate that the internal cooling strategy can reduce the cell thermal resistance by up to 67.8 ± 1.4% relative to single tab cooling, and can emulate the performance of a more complex pack-level double tab cooling approach whilst targeting cooling at a single tab