A battery module for use with a vehicle electrical system. The battery module includes a bi-directional battery voltage converter, a first battery, and a second battery. A first relay selectively connects the first battery to the bi-directional battery voltage converter. A second relay selectively connecting the second battery to the bi-directional battery voltage converter. A controller selectively energizes the first relay, selectively energizes the second relay, and controls a direction of current through the bi-directional battery voltage converter.