With the gradual development of energy storage system for largescale, the combined capacity expansion technology of battery energy storage system has been widely concerned by the society. Due to poor safety and economy, the traditional capacity expansion technology of batteries is difficult to be applied in the field of large capacity energy storage. The distributed energy storage architecture is beneficial to realize the module segmentation control of large capacity battery energy storage system. Comprehensively considering that the remaining capacity and state of charge(SOC) of battery modules are very different in real condition, this paper puts forward an optimal battery control strategy applied to distributed battery energy storage system, defining the physical meaning of the relative state of charge. Based on the balance control of relative state of charge between modules, the system realizes the optimal utilization of the capacity of different battery modules. Simulation and experimental results verify the effectiveness of the proposed control method.