A battery system and a method include an auxiliary power module configured to support auxiliary loads. A first contactor switch connected between first and second battery packs, and a second contactor switch is in series with the first contactor switch. A controller determines whether to open or close first and second contactor switches depending on whether the battery packs are being charged in a high voltage mode or a low voltage mode. The contactor switches are both closed when in the high voltage mode and at least one of the contactor switches is opened when in the low voltage mode. At least one of the first and second battery packs operate to power the auxiliary power module while charging at least one of the first and second battery packs regardless of whether the battery packs are in the high voltage mode or the low voltage mode.