This study focuses on the system dynamics and the state-of-charge (SOC) evaluation of a lithium battery module. The analysis and comparison of the differences between the linear model and the nonlinear model are carried out. Using AC impedance approach, the mathematical model can be established via a surveyed equivalent circuit. The linear model is thus constructed. With the nonlinear behavior investigated from experimental results, the nonlinear mathematical model is further established. Next, the measured nonlinear electric capacity of the battery module can be employed to derive the approach of nonlinear SOC evaluation. This research can precisely develop the nonlinear system model and evaluate the nonlinear SOC by analyzing the experimental data.