The “dual-carbon” target determines the strategic direction of China’s energy transition and provides opportunities and impetus for the grid connection of renewable energy. However, with a high proportion of renewable energy generation connected to the grid, the frequency fluctuation characteristics and regulation of the new power system become increasingly complex. Also the difficulty of frequency modulation has further increased, so that traditional frequency modulation methods cannot meet the needs of new power systems. Lithium battery energy storage can solve the above problems with its fast and accurate charging and discharging. Therefore, based on the Thevenin equivalent model of lithium battery energy storage, an improved battery cell model considering the influence of capacity attenuation, ambient temperature, current rate, etc., is established. At the same time, the identification method and theory of model parameters are given. In terms of the impact on the parameters, a method for FM-oriented lithium battery SOC estimation is formed based on these factors. Finally, the model of the energy storage battery is verified by Matlab simulation. Simulation shows that the model has high accuracy and can realize real-time estimation of SOC and other characteristics of energy storage lithium batteries. This work is supported by Beijing Natural Science Foundation Project (No. 21JC0026).