This paper proposes a parallel grid-connected system of non-master-slave and current-sharing battery energy storage. The converter of the system is designed as a bi-directional extended conversion structure， so the system becomes a four-quadrant operation battery energy storage device with characteristics such as low harmonics pollution and high power factor. On this basis， the operating modes of the system are designed to three modes: battery grid charging mode， battery grid discharging mode and emergency power supply mode. According to the different working modes， the control unit controls the operating status of the converter. It can achieve basic functions of traditional battery energy storage system， as well as emergency power used independently on the grid faults. In order to achieve the control targets of steady state and transient state， the control management strategy is given. EMTDC simulation results are provided to verify the effectiveness of the proposed system and control strategy.