A power supply device providing required power to a loading device. The power supply device includes a battery, an adjustable power converter and a control circuit. The battery, having a current meter, selectively outputs required power to the loading device. The power converter receives and converts an AC voltage into a first DC voltage or a second DC voltage, further receives an adjusting signal. The power converter outputs the second DC voltage when the adjusting signal is enabled. The magnitude of the second DC voltage is lower than the output voltage of the battery. The control circuit is coupled to the battery and the power converter. When the power of the battery is gauged, the control circuit enables the adjusting signal for the power converter to output the second DC voltage, so that the battery discharges the loading device for the current meter to re-gauge the power of the battery.