Combined effect of cycle aging and temperature on the variation of the open-circuit voltage of lithium cobalt oxide batteries

A key focus in studying lithium-ion batteries (LiBs) is the estimation of their actual capacity. To this end, many algorithms rely on the relationship between the open-circuit voltage (OCV) and state of charge (SOC) or absolute state of discharge, q. This relationship can be influenced by factors such as temperature (in a reversible way) and battery degradation (in an irreversible way). Although several studies investigated variations in the OCV-SOC or OCV-q relationship due to temperature or cycle aging using lookup tables or analytical expressions with adjustment factors, a comprehensive analytical model that simultaneously incorporates both factors and defines its parameters remains absent. To address this gap, the present work extends an existing analytical OCV-q model to capture variations in the OCV-q relationship as a function of both battery temperature and cycling level. To this aim, a comprehensive experimental campaign was conducted on a LiB, characterizing its OCV curve across various temperatures and cycling levels. Finally, simulations validated the accuracy of the proposed OCV-q model, yielding a mean relative OCV error below 0.8 % across all tests. Furthermore, the model demonstrated the ability to estimate the actual battery capacity with an estimation error of less than 2.5 % in all cases.