Modeling reversible self-discharge in series-connected Li-ion battery cells

Large battery systems like those in plug-in hybrid or pure electric vehicles contain strings of series-connected cells. Cell voltage imbalances within strings decrease useful string capacity and increase cell ageing. Balancing systems are often employed to alleviate imbalances but optimal design and sizing requires prediction of expected imbalance. This paper presents a methodology for simulating voltage imbalances caused by temperature-dependent reversible self-discharge processes in cells. Simulation results for a three-cell string with a 4°C operating cell temperature difference over 30 days show a maximum cell voltage divergence of less than 1mV. The same simulation with a malfunction-typical operating temperature difference of 20°C leads to a maximum voltage divergence of over 5mV. The results and underlying methodology are of value for battery pack manufacturers and designers of balancing systems.