 This paper presents an improved Thevenin model for lithium ion batteries called the dual polarization, DP, model. This model adds an additional RC component to the traditional Thevenin model to better represent the electrochemical polarization and concentration polarization. The model parameters were identified using a genetic algorithm and the results showed that the DP model had the best dynamic performance and provided the most accurate SOC estimation compared to other models. Additionally, the sensitivity of the SOC initial value was also examined and it was found that the error resulting from the SOC initial value was significantly reduced and the true SOC was converged within an acceptable error. This article was authored by Jinxing Fan, Hongwen He and Rui Shihong.