A power supply system includes: a power supply terminal, for receiving first power from an external power source or outputting second power to a load; at least one battery, for receiving and storing first power in a charging mode, and outputting second power to the power supply terminal in a discharging mode; a charger, enabled in the charging mode for charging the battery by first power; a switch, turned on in the discharging mode for transmitting second power to the power supply terminal; a current detection circuit, for detecting a current of the power supply terminal to determine whether the power supply system is operated in the charging or discharging mode. The current detection circuit controls the charger to be enabled in the charging mode and disabled in the discharging mode, and controls the switch to be turned on in the discharging mode and turned off in the charging mode.