an AC/DC circuit connected to an AC source and converting an AC current to a DC current;

a transformer having a primary side and a secondary side, the primary side connected to the AC/DC circuit to receive the DC current and the secondary side outputting an elevated voltage based on the DC current;

an output interface connected to the secondary side of the transformer for the connection of a battery and outputting the elevated voltage to charge the battery;

a charging controller comprising:

a pulse width modulation (PWM) circuit connected to the AC/DC circuit and the primary side of the transformer for controlling the magnitude of the DC current flowing to the primary side of the transformer;

a sensor connected to the secondary side of the transformer for detecting the magnitude of the elevated voltage of the output interface, and comprising a first, a second, a third and a fourth resistor sequentially connected in series, wherein an end of the first resistor is connected to the secondary side of the transformer, and the fourth resistor is connected to a ground;

an isolating controller connected to the PWM circuit via a receiving end and connected to the sensor via an emitting end; and

a switch connected to the sensor and the PWM circuit; wherein the output voltage detected by the sensor controls whether the switch should be turned on or off, and the DC current flowing to the primary side of the transformer is controlled by the PWM circuit along with the isolating controller; and

a compensator connected to the secondary side of the transformer and the charging controller, said compensator comprising:

a diode having a negative terminal and a positive terminal, the negative terminal connected to the secondary side of the transformer;

a capacitor connected between the positive terminal of the diode and the ground;

a fifth resistor connected between the diode and the capacitor; and

a sixth resistor connected to the switch and connected to a node between the second and third resistors of the sensor;

wherein the elevated voltage is further increased by the charging controller to compensate for the energy wasted by the circuit when the battery is connected to the output interface.