 This paper presents an improved Thevenin model for lithium-ion batteries called the dual polarization, DP, model. This model adds an additional resistor 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 model was tested against experimental data from a hybrid pulse power characterization, HPPC, test on a LIM-204 battery module. The results showed that the DP model had the best dynamic performance and provided the most accurate state of charge, SOC, estimation compared to other models. Additionally, the sensitivity of the SOC initial value was also examined and it was found that the errors resulting from the SOC initial value were significantly reduced and a true SOC was converged within an acceptable error. This article was authored by Jinxin Fan, Hongwen He, and Rui Xiong.