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Design of a battery-ultracapacitor semiactive hybrid for powering pulsed current loads is presented in this paper. The semiactive hybrid energy source consists of an ultracapacitor assisted Li-Ion battery, connected to the load via a dc-dc converter. The system is controlled such that the battery is supplying a near-constant current to satisfy the average load demand while the ultracapacitor supplies the dynamic component of the load current. The control algorithm is based on a well-known capacitance-emulating approach. As a result, a high-energy battery can be used despite the high-power load demand spikes. In addition, the battery-load voltage matching is not required and the control algorithm does not require load current sensing. It is shown that the proposed system performance is equivalent to the performance of a passive hybrid, employing a very high nonfeasible capacitance. Moreover, it is revealed that the battery current is independent of the capacitor size, which affects the load voltage ripple only. Extended simulation and experimental results are presented to demonstrate the feasibility of the approach.