A secondary battery pack is described that is rechargeable and has a large capacity. The secondary pack has multiple battery units and at least one controller. Each battery unit of the battery pack includes a cell unit and a cell balancing system. In one embodiment, the cell balancing system has at least three detachable connections for connecting the cell balancing system to the cell unit, the controller or to other cell balancing units. The connections allow the cell balancing systems to be detached from the battery units and reused.