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This work investigates the electric characteristics of a carbon-coated LiFePO4 cell with emphasis on their specific open-circuit voltage (OCV) characteristics, which include very flat OCV curves over the state-of-charge (SOC) and pronounced hysteresis phenomena. Examining discharging/ charging OCV measurement data of a LiFePO4/C cell elucidates these phenomena. A simple equivalent circuit model is newly derived and used in an OCV hysteresis effect-based SOC estimation in the extended Kalman filter (EKF) algorithm. The interval of the relationship between the OCV and SOC is decreased to 5% ΔSOC steps to improve the performance of the algorithm. Additionally, to compensate the model errors caused by the equivalent circuit model and variation in parameters, a measurement noise model and data rejection are implemented because the model accuracy is critical in the EKF algorithm in order to obtain a good estimation. All SOC estimation results of the proposed work satisfy the specification within ±5%.