A battery charging method for an on-board charger (OBC) includes: applying, by a controller, an output voltage of a first converter to a second converter by controlling the output voltage based on a result of a comparison between a battery load voltage and a battery rated voltage; controlling, by the controller, the second converter to perform a frequency modulation on the output voltage using a low-speed frequency modulation method or a high-speed frequency modulation method based on a result of the control of the output voltage; and generating, by the controller, a frequency harmonic of a system frequency in a battery current based on a modulation method used to perform the frequency modulation. An output voltage of the first converter is controlled based on a load voltage state of a battery and applied to the second converter.