A power supply system for an electronic device includes a first and a second connector, a switch, and a controller. The first connector is used for connecting to a first power source. The second connector is used for connecting to a second power source, wherein the second power source is rechargeable. The switch establishes a connection between the first connector and the second connector so as to allow the first power source to charge the second power source. The controller detects a presence of the first power source and the second power source, and controls the switch to periodically establish the connection at the absence of the second power source, and to continuously establish the connection at the presence of the second power source.