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This paper presents a system integration and control strategy for managing power transients on a Nexa polymer electrolyte membrane fuel cell (FC) with the assistance of an ultracapacitor (UC) module. The two degrees of freedom provided by the use of two dc/dc converters enable the independent low-level control of dc bus voltage and the current split between the FC and UC. The supervisory-level control objectives are to respond to rapid variations in load while minimizing damaging fluctuations in FC current and maintaining the UC charge (or voltage) within allowable bounds. The use of a model predictive control approach which optimally balances the distribution of power between the FC and UC while satisfying the constraints is shown to be an effective method for meeting the supervisory-level objectives. The results are confirmed in experiments.