an AC/DC power supply connected to an AC source and converting an AC current to a first DC current for output;

a transformer having a primary side and a secondary side, the primary side connected to the AC/DC power supply, the secondary side outputting a second DC current based on the first DC current and connected to a battery in a removable way;

a compensator connected to both ends of the primary side of the transformer and sending a compensating current to increase a DC current entering the primary side of the transformer;

a controller connected to the transformer and the compensator;

a comparator unit connected between the secondary side of the transformer and the battery to determine a charging status of the battery, the comparator unit comprising;

a voltage divider comprising first, second, and third resistors connected in series, the first resistor connected to the secondary side of the transformer and the battery, and the third resistor connected to the ground;

a reference voltage circuit comprising a reference voltage IC to provide a reference voltage;

a first comparator having a positive input connected to the reference voltage IC to obtain the reference voltage, and having a negative input connected between the first resistor and the second resistor to obtain the voltage drop across the second and third resistors; and

a second comparator having a positive input connected to the reference voltage IC and output terminal of the first comparator via a diode, and having a negative input connected between the second resistor and the third resistor to obtain the voltage drop across the third resistor; and

an isolating controller having an emitting end and a receiving end, the emitting end connected to the comparator unit, and the receiving end connected to the controller;

wherein the isolating controller operates the controller according to the charging status of the battery, so that the compensator sends the compensating current to change the magnitude of the second DC current.