The Jet Propulsion Laboratory Mars Exploration Program Office is currently planning a series of exciting missions to the Red Planet. During each launch opportunity, the missions to Mars will include a Rover mission. During the earlier Rover missions to Mars such as the Mars Pathfinder mission carrying the Sojourner Rover in 1997, the main rover power source was a solar array. The power subsystem of the Sojourner Rover included a solar panel for power during the day, a non-rechargeable lithium battery for power during the night, and a power electronics board for power conditioning and distribution. Starting with the year 2003 the rover missions to Mars will incorporate a rechargeable energy storage device rather than a non-rechargeable power source. Included in the power electronics board, will be a battery controller/charger. The battery controller/charger will be able to monitor and control three parallel 4-cell battery strings. It will also incorporate a cell bypass circuit to ensure that the individual cells within a battery string are at the same state-of-charge. During the design phase of the power subsystem a breadboard version of a one 4-cell string battery charger was fabricated using discrete electronic components on a circuit board. The breadboard was to be used as proof of concept and was not optimized for weight or volume. The breadboard was tested initially using nickel cadmium cells and subsequently using commercial lithium ion rechargeable cells. In this paper a description of the single string charger and the breadboard setup will be presented along with the performance data of the bypass circuit.