Vehicle Battery Charging System and Battery Charging Method Thereof

The present disclosure relates to a vehicle battery charging system and a battery charging method thereof. The present disclosure provides a battery charging method of a battery charging system including an on-board charger to convert an AC voltage from an outside into a DC voltage, a first battery, and a second battery having a lower rated voltage than that of the first battery, the battery charging method including: operating a first charging mode of turning on a first switch between the on-board charger and the first battery and turning off a second switch between the on-board charger and the second battery when a voltage of the second battery exceeds a first threshold value; and operating a second charging mode of turning off the first switch and turning on the second switch when the voltage of the second battery does not exceed the first threshold value.