A battery backup system includes a battery pack, a charger, a discharger, a voltage detection and control circuit, a first protection switch, a second protection switch and a third protection switch. The battery pack provides a backup power to a DC bus. The charger, connected to the battery pack, receives an external power. The discharger has an input end and an output end. The voltage detection and control circuit detects whether the voltage of the DC bus is greater than first or second voltage drop point. The first, the second and the third protection switches respectively are connected between the battery pack and the input end, between the DC bus and the output end, and between the charger and the input end. The first protection switch is turned on/off according to whether the voltage of the DC bus is less than or greater than the first voltage drop point.