Design of a battery-ultracapacitor semi-active hybrid energy source for powering a pulsed current load in presented in the paper. The energy source consists of a rechargeable LiFePO4 battery pack paired with an ultracapacitor pack, connected to the load via a DC-DC converter. The system is controlled such that battery supplies a nearly constant current, equal to the average load current, while the ultracapacitor supplies the dynamic component of the load current. Placing the DC-DC converter on the ultracapacitor, as opposed to placing it on the battery, decouples the ultracapacitor's voltage from the battery/load voltage. As a result, smaller and cheaper ultracapacitor can be employed, compared to a passive hybrid connection. Simulations of both passive and semi-active systems are presented in the paper and differences between the two systems are discussed thoroughly.