A power supply system for preventing battery packs connected in parallel from charging each other is provided. Each of a plurality of battery packs includes a plurality of batteries, a sensing resistor, a detector circuit, a discharging transistor, a charging transistor, and a controller circuit. The sensing resistor has a first end connected to a negative terminal of the battery packs and a second end connected to a negative electrode of the battery circuit. The detector circuit is connected to the first end and the second end of the sensing resistor. The discharging transistor has a first end connected to a positive terminal of the battery packs and a second end connected to a first end of the charging transistor. According to a current of the sensing resistor, the controller circuit controls the discharging transistor and the charging transistor to be turned on or off.