Fault analysis in battery module design for electric and hybrid vehicles

In this paper systems integration issues, such as electrical and thermal design and management of full battery packs - often containing hundreds of cells - are discussed. The design and construction of a 9 kWh battery pack for a motorsports application is used as an example. The pack contained 504 lithium cells arranged into 2 sidepods, each containing 3 modules, with each module in a 12P7S configuration. This paper focuses on describing problems related to cells being connected in parallel, known as massively parallel packs. We also demonstrate how a full vehicle test can be used to identify malfunctioning strings of cells for further investigation. It is shown that normal inter-cell contact resistances can cause currents to flow unevenly within the pack, leading to cells being unequally worked. This is supported by a Matlab/Simulink model of one battery module, including contact resistances, which was able to reproduce the results that were seen in experimental tests. Over time the unequal current flowing through cells can lead to significant differences in cells´ state of charge and open circuit voltages, large currents flowing between cells even when the load is disconnected, and ultimately, some cells discharging and aging more quickly than others and jeopardising the energy storage capacity and lifetime of the entire pack.