The determination of state of charge based on extended kalman filter using per-unit system and time constant principle

In order to cope with the parameter variations for aged battery, which result in erroneous SOC estimation, the SOC estimation algorithm of lithium-ion batteries based on extended Kalman filter (EKF) using per-unit system and time constant principle is proposed. In this per-unit system, the absolute values of capacity, voltage and current for aged batteries are converted into relative ones and are applied to dynamic model of the EKF algorithm. Moreover, the parameter values (R_Diff, C_Diff) are applied to dynamic and measurement model of EKF algorithm by time constant principle. In comparison with Ah-counting and EKF results, the estimation results using the proposed EKF algorithm have error within ±3%. Few aged batteries (1.3 Ah) for 18650 types are used for verification of the proposed method.