Abstract: To study lithium ion battery state of health (SOH), according to the definitions of SOH and state of charge (SOC) and 2RC (resistance-capacitance) battery equivalent circuit model, the SOH estimation model of the battery voltage curve based on the constant current charging stage was established. Based on the analysis of the battery cycle life test data, the SOH was estimated by the battery voltage curve of the constant current charging stage and compared to the testing data. The relative error of SOH estimation was in the range of 2% before the SOH value was reduced to 80%, which was well consistent with the experimental results. The results show that the proposed method is feasible and accurate.