Formulation of Model for Estimation of Battery Capacity Degradation Based on Usage History 2013-01-0501

As the electric vehicle (EV) market expands and we enter the period of fully fledged diffusion of the vehicles, evaluation of battery performance when secondhand vehicles are sold and when batteries are put to alternative uses will become increasingly important.

However, the accurate measurement of battery performance for the purpose of battery evaluation represents a challenge when the batteries are fitted in a battery pack consisting of multiple cells. The authors therefore formulated a degradation estimation model for the evaluation of battery performance based on battery usage history. To formulate the model, parameters expressing the internal state of the battery are estimated from the battery's usage history; battery capacity is estimated with consideration of these parameters. The model was applied to prototype battery cells that had been degraded on a test bench, and the results showed that the method reduced error in estimation by two-thirds against the square-root law, the method that is conventionally used. The model was also applied in the same way to batteries fitted in a test vehicle, and its accuracy was verified.

The logic employed by the degradation estimation model was tested using prototype batteries, but it was not possible to test whether the model was able to be employed in relation to EV used in a variety of different environments. The FIT EV, which Honda is leasing, is provided with a battery traceability system that collects battery history data when the vehicle is in operation and when the batteries are being charged. In future, the authors intend to study the performance of the model using battery history data collected by this system.