An apparatus is provided that includes a string of power sources and step-down circuitry. The power sources are connected in series to provide a first voltage rail at a first voltage, and the string of power sources is divided into power-source segments. The step-down circuitry electrically is configured to provide a second voltage rail at a second, lesser voltage. The step-down circuitry includes control circuitry configured to monitor states of charge of the power-source segments, and control selection of one of the power-source segments to deliver an output from which the second voltage rail is provided, based on the states of charge. One or more first electronic components are electrically coupled to the first voltage rail, and designed to operate at the first voltage. And one or more second electronic components are electrically coupled to the second voltage rail, and designed to operate at the second voltage.