Calendar aging of commercial graphite/LiFePO4 cell - Predicting capacity fade under time dependent storage conditions

In applications such as electrical transportation, most of the battery life is spent under storage. Understanding and estimating aging under storage, also named as calendar aging, is therefore a prerequisite for cell life prediction. This work investigates aging behavior upon storage of a commercial 15 Ah lithium-ion graphite/iron phosphate cell. Performance decline during 450 days of storage under nine stationary conditions is analyzed using non-destructive electrochemical tests. Temperature is found to be more detrimental than State of Charge (SoC). Most often, degradation models express the accumulated degradation with respect to time and aging conditions. In this article, a simple modeling approach is proposed focusing on the degradation rate to predict capacity fade. This permits predicting cell degradation under time dependent storage conditions (SoC and temperature) which are usually experienced in real applications. Model prediction is compared to experimental calendar aging data obtained over 625 days in a controlled time dependent temperature storage conditions. Predictions are in good agreement with experimental results as the absolute error on capacity prediction never exceeds 3% over 400 days and 5% over 625 days