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Monitoring state of health (SOH) is critical to battery safety and accurate SOH estimation methods are of great importance. In this paper, an electrochemical aging model is built and the formation of solid-electrolyte-interphase (SEI) film is used to evaluate the battery health state. SOH is usually defined by the ratio between the current battery capacity and the fresh battery capacity, and we show that the SEI film resistance has a high correlation with battery capacity based on Pearson correlation analysis. Therefore, after estimating SEI film resistance, the capacity can be calculated indirectly. In order to estimate the SEI film resistance from the complex electrochemical aging model, the ensemble Kalman filter is used in this study. Simulation results show that the SEI film resistance can be estimated accurately at different aging states, and the capacity can be calculated indirectly. The proposed SOH estimation method has …