Abstract: A power supply system includes three sets of batteries for supplying direct current to a load. Three sets of switches connect mutually respective sets of batteries to the load. A controller operates the switching means so that at any time at least two of the sets of batteries are connected in parallel to the load. The controller alternates each set of switches in open and closed positions with the switches being operated in a predetermined, cyclic manner, so that during the cycle, each of the sets of batteries provides current to the load for a portion of the cycle and is disconnected from the load for the other portion of the cycle. Utilizing the sets of batteries in pairs so that each set of batteries provides only about half the current requirement and of repeatedly disconnecting each set of batteries for a short period of time while the other two sets of batteries supply the current which maximizes the life and performance of the batteries.