A low voltage battery charger is provided that includes a fuel cell stack that generates electrical energy by a reaction of hydrogen and oxygen and an air blower that supplies air to the fuel cell stack. A high voltage DC converter converts an output voltage generated at the fuel cell stack to a high voltage. A high voltage battery is charged with the output voltage converted by the high voltage DC converter. A low voltage DC converter converts the output voltage generated at the fuel cell stack to a low voltage and a low voltage battery is charged with the output voltage converted by the low voltage DC converter. A controller then variably adjusts a charge voltage of the low voltage battery based on an air amount supplied to the fuel cell stack through the air blower.