A battery charging apparatus connects secondary batteries of the battery pack, plurality of charge terminals and two discharge terminals to form plurality charge paths and a discharge path, which has a main switch provided on the discharge path, a main controller connecting the battery pack and the main switch, plurality of sub switches provided on the charge paths, plurality of sub controllers connecting secondary batteries of the battery pack and corresponding sub switches, a thermal controller connecting the main controller and a temperature switch. The temperature switch connects the sub switches. The main controller and sub controllers monitor charge state and discharge state of the battery pack to set the main switch and sub switches cut off the discharge path and the charge paths. The thermal controller monitors temperature of the battery pack and main switch to set the main switch and the sub switches cut off the discharge path and the charge paths.