Batteries and ultracapacitors have significantly different energy storage and power delivery capabilities. Electrical traction motors in hybrid electric vehicles have characteristic power and energy demands, and a single energy storage technology may not be optimized to meet both the minimum power and energy demands. In this paper, we investigate the effect of combining batteries and ultracapacitors, both actively and passively, to produce a more versatile electrical energy storage system for hybrid electric vehicles. Hybridized energy storage systems result in increased component life cycles, decreased internal resistance losses, and reduced cost and mass when compared to either battery-only or ultracapacitor-only configurations.