A battery charger design for the NASA Earth Observing System (EOS) Space Platform has been developed and tested. This thesis discusses the design of the battery charger power stage and its current and voltage control loops. The charger was designed to minimize the mass and to maximize the efficiency. In addition to restoring energy to the batteries, this charger regulates the spacecraft bus voltage during the transition between eclipse and sunlight. The battery charger design and analysis was facilitated by use of the model for the pulse-width-modulated (PWM) switch and the new continuous-time model for current-mode control. Analyses of the battery charger small-signal behavior are compared to hardware measurements to verify modeling accuracy.