An energy storage system having a number of trays with each tray having a number of battery cells in which power is controllably stored and discharged. A first Battery Management System (BMS) is electrically coupled to a tray contained in a rack of trays. A second BMS is electrically coupled to and controls the first BMS. The first BMS includes a control unit electrically coupled to and controlling the battery cells. It further includes a switch unit electrically coupled to the control unit and selectively applying driving power according to a control signal from the second BMS.