The present disclosure provides an energy storage system and an ID assignment method of the energy storage system. There is provided an energy storage system including: a first energy storage device that includes a first battery rack and a first battery control device connected to the first battery rack; a second energy storage device that includes a second battery rack and a second battery control device connected to the second battery rack; and a system BMS that is electrically connected to the first energy storage device and the second energy storage device, in which the first battery control device includes a first rack BMS that acquires and stores a first ID, and a first sensor unit that transmits the stored first ID to the second battery control device, and the second battery control device includes a second sensor unit that receives the first ID from the first sensor unit, and a second rack BMS that based on the received first ID generates a second ID and stores the generated second ID.
