Abstract: At present, the modelling of electrochemical energy storage systems generally follows the simple equivalent model based on one battery cell. The inconsistency of internal battery parameters is rarely considered in modelling of packs and systems,which results in large error in battery pack simulation under low state of charge（SOC） conditions, and the error becomes more obvious with the aging of the energy storage system. Based on the structure of the real battery packs in lithium-ion battery packs for energy storage station, this paper analyzes the factors that affect the voltage error of the simple equivalent model and the voltage characteristic connection between battery cells and battery packs, analyzes the causes of voltage differences between battery cells,proposes the series capacitor equivalent model, introduces SOC and capacity of battery cells into voltage correction as the main incentive parameters, and establishes a parallel model considering unbalanced current between battery strings. Simulation result shows that compared to the simplified equivalent model,the proposed model can improve simulation accuracy in low SOC, which provides technology support for battery system simulation models composed by series and parallel battery cells.