In this paper, the existing commonly used lithium-ion battery models are analyzed, and a second-order RC network equivalent circuit model which is convenient for engineering application is established. The corresponding battery models are built in MATLAB, and the measured data are used to identify the battery model parameters offline, and the accuracy of the model is verified. Considering that the values of model parameters are not constant in the process of battery charging and discharging, but constantly change due to factors such as charging and discharging rate and battery SOC, in order to improve the accuracy of the model, the recursive least square method with forgetting factors is adopted to identify the model parameters online, and the optimal range of forgetting factors is determined by comparing the influences of different forgetting factors through simulation analysis. The experimental results show that as the forgetting factor decreases from 1, the accuracy of the model will first increase and then decrease. The appropriate forgetting factor range of this model is about 0.90~0.95, and the best value should be around 0.94. At this time, the average voltage error of the model is only 0.00043V, which proves the correctness and high accuracy of the identification method in this paper.