An apparatus is electrically connected to a battery unit consisting of multiple batteries, an energy conversion system, and power-driven equipment. The apparatus sets number requirements of batteries to be connected to a discharge unit and a charge unit of the battery unit. When the power-driven equipment is started, the apparatus receives an open-circuit voltage of each battery detected by a sensor connected to the battery, and establishes connections of the batteries with the discharge unit or the charge unit according to the open-circuit voltages of the batteries and the number requirements. During the discharge unit supplying power to the power-driven equipment and the energy conversion system charging the charge unit, the apparatus monitors variations of the open-circuit voltages of the batteries, and switches the connections of batteries with the discharge unit or the charge unit according to associations between open-circuit voltages, cut-off discharge voltages, and nominal voltages of the batteries.