Abstract: A power supply system includes a plurality of batteries or banks of batteries for supplying direct current to a load, a plurality of switching devices each coupling a different one of the batteries to the load and each responsive to an energizing signal for conducting current from the respective battery to the load, and control apparatus for successively energizing each of the switching devices one at a time so that each battery provides current to the load only for a short period of time after which another battery supplies the current, etc. The time during which each switching device is energized is selected to maximize the life of the batteries.