Modeling the capacity degradation of LiFePO4/graphite batteries based on stress coupling analysis

A coupling-analysis-based model to predict the capacity degradation of LiFePO4 batteries under multistress accelerated conditions has been developed. In this model, the joint effect on the battery capacity degradation of any 2 out of 5 stress factors, which include ambient temperature, end of discharge and charge voltage (EODV and EOCV), and discharge and charge rate, is studied through coupling validation tests. Coupling generally exists among these 5 stress factors, and the coupling intensity has a certain relationship with the stress levels. There is a critical stress level at which the coupling can be considered negligible, and when the stress level goes higher, coupling aggravates battery degradation exponentially. Additionally, the study also indicates that battery life shows stronger sensitivity to discharge rate and EOCV than to charge rate and EODV. The developed capacity degradation model based on the input of real operating conditions and coupling intensity calibration achieves error less than 15% when the cycling goes into the stable decay period, and the error converges gradually as the cycling continues. (C) 2011 Elsevier B.V. All rights reserved.