Use of Effective Capacitance Variation as a Measure of State-of-Health in a Series-Connected Automotive Battery Pack

In electric and hybrid electric vehicles, series-connected battery packs are commonly used. Should the state-of-health (SOH) of one or several individual cells deteriorate, the entire battery pack is affected, reducing battery pack capacity, which in turn reduces the maximum distance able to be driven. In order to predict the SOH of the individual battery cells, this paper introduces the concept of effective battery capacitance. Effective capacitance is defined as the local slope of the voltage versus charge curve derived from a third-order polynomial relationship between these parameters. The location of maximum effective capacitance can be used as a means of identifying end-of-life and/or catastrophic failure of battery modules. Four different Toyota Prius battery packs were used in establishing this method in the proof of concept work. This paper presents a linear relationship between maximum effective capacitance and SOH; this relationship is confirmed with a larger dataset.