A system for a rechargeable battery pack includes one or more lower voltage cells connectable to a charging circuit via a lower voltage terminal and one or more higher voltage cells electrically coupled in series to the one or more lower voltage cells via a bi-directional charge regulator. A higher voltage terminal is electrically coupled to the one or more higher voltage cells, and a common ground terminal is coupled to the one or more lower voltage cells and to the one or more higher voltage cells.