A two-stage charging device including a power supply device and a battery module is provided. The battery module is equipped with a linear charging device, a controller and a battery. During the charging process, the controller determines the
voltage difference between the main supply
voltage provided by the power supply device and the
battery voltage, and then outputs an adjusting
signal to the power supply device according to the obtained
voltage difference. Then the power supply device adjusts the supply voltage according to the adjusting
signal to maintain the voltage difference between the supply voltage and the
battery voltage approximately at a pre-set voltage level. The
battery voltage will rise along with the passing of the charging time. If the voltage difference between the main supply voltage and the battery voltage is smaller than a lower voltage level, the controller will increase the voltage level of the adjusting
signal so as to increase the power supplied by the power supply device. Conversely, if the voltage difference between the supply voltage and the battery voltage is larger than an upper voltage level, the controller will reduce the voltage level of the adjusting signal to enable power supply device to reduce the power supplied by the power supply device. Therefore, by making appropriate adjustments according to the level of the adjusting signal, the main supply voltage can be maintained at a voltage level slightly above the battery voltage.