The power pack includes a plurality of parallel groups. Each parallel group includes a plurality of electrical power sources connected in parallel. The pack also includes a plurality of series groups. Each series group connects in series one of the power sources from each of the parallel groups. The pack also includes a series balancing circuit configured to balance the voltage of the power sources in one of the series groups such that one or more power sources in the series group recharges one or more other power sources in the same series group.