Abstract
Research and/or engineering question/objective
Constant monitoring of Electric vehicle (EV) battery’s behaviour and in particular heat generation is vital for the safety of the vehicle and occupant. EV safety is considered important area of research in automotive industry. Recent safety issues after EV crash raised need for detecting thermal runaway at earlier stages to protect occupants and minimize damage to battery pack. The ultimate aim of this research is to develop a new model integrating electrical, thermal and physical behaviour to improve vehicle and occupant safety in case of crash/impact.
Methodology
A unique model is developed to represent battery operation in case of crash/impact. This model integrates electrical, thermal and physical behaviour and is used to define temperature changes at different discharge rates due to impact. For impact analysis layered structure FEM model of lithium ion battery is developed which is tested using solid sphere load to understand voltage-strain and voltage-temperature relationships. In this model battery heat transfer is considered which is further extended for heat generation estimation. For validation of proposed model simulation and experimental data is compared.
Results
In this work impact analysis is carried out using solid sphere which impact battery at constant speed and this is repeated for different battery location to estimate temperature variations. Results show battery heat generation is not uniform and have different values at different impact locations, which is due to varying state of charge (SOC) of battery. Also capacity degradation is observed which is slow and does not give sudden power loss but it can cause gradual build-up of heat inside battery which can damage battery and lead to permanent damage to cell or catastrophic event.
Limitations of the study
There are some limitations of proposed model in this paper, for simplification of model heat dissipation is neglected and no cooling system is used, secondly in this model ohmic heat, reversible heat and irreversible heat is neglected. The purpose to develop this model is to avoid complexity and achieve accuracy, so that parameters used in this research are determined through experimental work and from literature.
What does the paper offer that is new in the field including in comparison to other work by the authors?
Detailed literature review suggests that there is lot of work on individual electrical, thermal or physical models of lithium ion battery and few researchers investigated electro-thermal or thermo-physical models. This study offers a unique model to study the potential relationship among electrical, thermal and physical model of lithium ion battery for EV applications. To ensure safety and maximum accuracy cell level charge-discharge behaviour is observed which is later used for heat generation estimation. The combinations among proposed model, impact analysis and validation of results make this research unique.
Conclusions
It is shown from simulation results and experimental work that heat generation rate vary with charge and discharge rate. At high discharge rate forced heat dissipation is required for normal operation of lithium ion battery also convection heat transfer coefficient plays an important role for safe operation of battery. However Impact analysis shows good results to estimate heat generation due to mechanical load and will be used for crash analysis of EV battery. The results obtained using proposed model suggests that this model can be further extended for safety analysis of EV battery by incorporating more battery parameters.
KEYWORDS : Heat generation, FEM, electro-thermal model, thermo-physical model