Abstract: The battery energy storage system comprising: a bus connection portion comprising a positive bus connection end and a negative bus connection end; a first battery pack comprising a plurality of single batteries connected in series or in parallel; an output capacitor is connected in series to the first battery pack, the output capacitor and the first battery pack are connected between the positive bus connection end and the negative bus connection end; a H-bridge circuit having an output end connected to the output capacitor; a first isolated DC/DC conversion circuit having an input end electrically connected to the first battery pack, an output end electrically connected to an input end of the H-bridge circuit; a first relay electrically connected between the first battery pack and the input end of the first isolated DC/DC conversion circuit, when the first isolated DC/DC conversion circuit has fault, the first relay is disconnected.