The concept of a passive hybrid system consisting of fuel cell stacks, Li-ion battery packs, and two diodes was tested. The diodes take the place of the DC/DC converter that is usually used to align fuel cell and battery voltages, thereby directly connecting the fuel cell and the battery. Prototype equipment was built for collecting characteristic data on the static and dynamic behavior of the hybrid system. The measurement results indicate that increasing the operation efficiency and simplifying the system were possible by applying the direct hybridization concept. The dynamic behavior results showed an interesting combination of output signals from the fuel cell stack and the battery pack. The quick response of the battery output completely compensated for the delay in fuel cell output response, indicating that the direct hybrid system is also applicable to high-frequency electric loads such as brushless DC motors. The ability to recharge without a DC/DC converter was also successfully validated for the direct hybrid system. From the measurement results, the design method, and the system sizing, this passive hybrid system appears promising for application in the Antares DLR-H2 electric aircraft test bed. (C) 2012 Elsevier B.V. All rights reserved.