Abstract: A control strategy for battery energy storage system based on modular multilevel converter（MMC）is proposed,which is applicable for the operation under both symmetrical and asymmetrical grid voltage conditions.It contains the control of output power,balance of state of charge（SOC）and elimination of DC output current.The battery SOC balancing control contains the SOC balancing among three phases,between arms and within each arm.The SOC balancing control methods among three phases and between arms are implemented by controlling the circulating currents.The SOC balancing control method within each arm is implemented by modifying the fundamental frequency component of the sub-module output voltage.The basic operational principles have been analyzed in detail and the control strategy has been designed accordingly.Finally,the proposed theories are verified through the MATLAB simulations and a scaled down experimental prototype.