Open-circuit voltage (OCV) is widely used to estimate the state-of-charge (SoC) in many SoC estimation algorithms. However, the relationship between the OCV and SoC cannot be exactly same for all batteries. Because the conventional OCV–SoC differs among batteries, there is a problem in that the relationship of the OCV–SoC should be measured to estimate accurately the SoC. Therefore, a modified OCV–SoC relationship based on the conventional OCV–SoC is proposed. Problems resulting from the defects of the extended Kalman filter (EKF) can be avoided by preventing the relationship from varying. Also, in order to improve the performance of the algorithm, measurement noise models of the Kalman filter are applied. Thus, the measurement noise models allow the Kalman filter to overcome defects from the simplified battery modelling and to separate the sequence for estimation of the state and weight filter. The SoC and the capacity of a lithium-ion battery are estimated using the dual EKF with the proposed method.