Correlation between capacity loss and measurable parameters of lithium-ion batteries

State of health (SOH) of batteries is generally defined as the ratio of the remaining capacity after the battery fully charged to the normal capacity. In fact, the capacity of batteries that are employed as a power source in an electric vehicle cannot be measured online. Therefore, establishing the relation of the capacity loss to the measurable parameters is a crucial factor that solves the issue of the online SOH estimation. In this study, both the experimental test and the 2nd equivalent circuit model are applied to study the fading characteristic of the lithium-ion battery. Based on the measured capacity and the high-accurate identified parameters of the model, the quantitative relation is explored between the capacity change and the measurable parameters of the model. Results show that the available capacity decreases linearly with the increasing ohmic resistance of the battery. This linear relation provides the theoretical foundation of online estimating SOH. In addition, the main factors contributing to the capacity loss of the battery are also found. The primary factor is the ohmic resistance due to its contribution to the capacity loss always exceeding 60% during the entire life cycles. Besides, the polarization resistance is the second factor accounted for the fading, including the electrochemical polarization resistance and concentration polarization resistance. This conclusion provides a reasonable strategy to find the method that can evaluate and predict the battery cycle life.