Battery charging system and method

A battery charging system includes a PFC converter which converts an alternating current input voltage which is input from a power supply to a direct current output voltage, a DC-DC converter which changes an output voltage of the PFC converter, a battery which is charged by the DC-DC converter, and a controller which calculates power conversion efficiency based on the input current and the input voltage, a charged voltage of the battery, and a charging current for the battery, and updates the output voltage of the PFC converter according to the power conversion efficiency. A battery charging method includes steps of: calculating power conversion efficiency using the input current and the input voltage from the power supply, and the charging current and the charged voltage of the battery; and updating the output voltage of the PFC converter according to the calculated power conversion efficiency.