A power-on method for a circuit system having a power supply terminal and a charge input terminal, comprising: coupling the power supply terminal to a battery and the charge input terminal to an adaptor; detecting a voltage of the battery; when the voltage of the battery is below a threshold, turning on a second switch such that the adaptor supplies the circuit system through the charge input terminal and the second switch to start the circuit system and charge the battery; when the voltage of the battery is above the threshold, turning off the second switch, and turning on a third switch, such that the charge input terminal is coupled to the battery through the third switch, and the circuit system is powered by the battery; and outputting a switch signal to control a first switch coupled between the adaptor and the charge input terminal for controlling charge capacity.