In terms of the dynamic changes of battery model parameters in a single-model filtering algorithm, the filter estimation accuracy can be poor, and filtering is scattered due to the different internal state parameters of lithium-ion batteries in different aging states, which affects the state of charge (SOC). In order to address these issues, an Interacting Multiple Model (IMM) algorithm was proposed in this study, which adopted an Unscented Kalman Filter (UKF) to better approximate the nonlinear characteristics of the state equation while better stabilizing the filter and having lower computational requirements. Accordingly, the IMM was used to solve the problem of the accurate estimation of the SOC under the dynamic change of model parameters. Moreover, an electrochemical impedance spectrum was used to establish the electrochemical model, after which the lithium-ion equivalent electrochemical circuit model was established, which improved the complexity problem due to its high accuracy but complicated the calculation of the multi-order equivalent circuit model. By conducting experiments and simulations, the algorithm of IMM-UKF was shown to achieve an effective estimation of the battery SOC, even when the state parameters of lithium-ion batteries were uncertain.