This work develops a methodology to facilitate the optimum design of spacecraft power processing systems. Emphasis is placed on the battery charge and discharge systems. A comparison of several battery charge and discharge topologies is presented. Characteristics which effect the overall system performance are addressed including size, weight, efficiency, dynamic performance, electromagnetic interference, and reliability. A detailed comparison, using nonlinear design optimization techniques, is performed for three candidate topologies for application to the NASA Earth Observing System (EOS) satellite battery discharger. Experimental verification is provided. A novel zero-voltage-switching (ZVS) bidirectional converter topology is presented. A bidirectional battery charger / discharger has several advantages, including a potentially substantial system weight savings. However, it is shown that most of the advantages can not be realized unless zero-voltage-switching is incorporated. The design of a novel ZVS, multi-module, multi-phase, bidirectional converter with application to the NASA EOS satellite battery charger / discharger system is presented. The system not only yields high efficiency and light weight, but also possesses superior dynamic characteristics when compared to either a conventional bidirectional converter or separate charge and discharge converters. Design considerations for system control of the NASA EOS satellite are presented. A methodology is presented to insure system stability with an unknown complex load. / Ph. D.