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Ultracapacitors are energy storage devices that can be connected in parallel with batteries to create a hybrid power system. This hybrid configuration provides a higher power density than a single battery and helps extend the life of the batteries. An important issue is the voltage regulation of a battery-supplied dc bus subjected to a long high-power transient. In the aerospace field, the bus voltage must be maintained within certain margins. Batteries subjected to high-power transients generate terminal voltages with voltage dips that can exceed the voltage regulation band. The purpose of this article is to see to what extent an ultracapacitor placed in parallel with a battery can assist in maintaining a bus voltage within the regulatory limits and to develop a closed-form mathematical expression assuming the injection of constant power pulses. This expression can be used to size the ultracapacitor, meet a specific voltage regulation requirement for a given battery, or provide the constraints on the voltage requirements that can be achieved.