A power system for driving a direct current (DC) electric motor that is controlled by a driving voltage (VD), the power system comprising: a plurality of cell modules (110, 210, 310), each cell module (110, 210, 310) comprising one or more cells (111, 211, 311); a plurality of connectors (120, 220) for connecting the cell modules (110, 210, 310) to form a configurable battery to generate the driving voltage (VD); wherein the connectors (120, 220) are configured to connect the cell modules (110, 210, 310) into configurable clusters; and a voltage level controller (21) configured to control the configuration of the configurable clusters to provide the driving voltage (VD) at one of a set of selectable voltage levels, wherein for each voltage level of said set a share of a current drawn from each of the cell modules (110, 210, 310) with respect to a total current drawn from the configurable battery is lower than for each higher voltage level of said set.