Because of the fast response rate and long term operation of grid side energy storage system(ESS) when it participates in the secondary frequency regulation of regional power grid

it is necessary to establish a battery model that can reflect the short-term polarization of the ESS and the long-term accuracy of SOC estimation. Thus

a modeling method of dynamic variable parameter lithium battery is proposed by using the relationship between the independent variable SOC and the internal parameters of the battery. Firstly

the state equation of single lithium battery is discretized to obtain the partnership for a new generation of vehicles(PNGV) model with variable parameters. Then

the capacity of single battery is expanded by using the typical cascade scheme of the ESS unit. Finally

the SOC estimation method in the model is improved by using the modified Kalman filter method. The simulation uses Matlab/Simulink software to carry out the HPPC experiment

and compares the differences of different ESS unit models under the condition of secondary frequency regulation. The results show that

the error of the variable parameter model in the fitting simulation is less than 1.4%

and the accuracy of SOC estimation in the long-term operation condition simulation is9.7% higher than the conventional estimation

which provides a more accurate lithium battery model for study on control strategy of the ESS participating in the secondary frequency regulation.