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One of the present challenges of Polymer electrolyte membrane fuel cells (PEMFCs) is to enhance their durability. It is known that maintaining the load conditions of PEMFCs appropriately could improve the lifetime of PEMFC. We proposed the method of smoothing the output current of PEMFCs by charging or discharging electrical double layer capacitors (EDLCs). To achieve such purpose, we developed a multi-port bidirectional DC-DC converter, which is able to control the energy flow among a fuel cell, EDLC, and load, and to regulate terminal voltages proper to these devices. The aim of this study is to evaluate the frequency characteristics of the proposed hybrid power system. The evaluation contributes to design a long life fuel cell system. The frequency characteristics of the proposed converter system for mitigating PEMFC degradation have been evaluated. To investigate the relationship between the load and the fuel cell currents, frequency response characteristics are measured. The converter can smooth the output current of the fuel cell although the load current fluctuate below 100 Hz.