An apparatus includes a first cell subpack having a plurality of cells arranged in series and a second cell subpack connected in series to the first cell subpack. The second cell subpack includes a plurality of cells arranged in series and at least one cell arranged in parallel with one of the plurality of cells, arranged in series, of the second cell subpack, where the first cell subpack and the second cell subpack use a first voltage rail to provide at least a first voltage level and a second voltage rail to provide a second voltage level, where the first voltage level is different from the second voltage level.