Fuel cells as alternative propulsion systems in vehicles can achieve higher driving ranges and shorter refueling times compared to pure battery-electric vehicles, while maintaining the local zero-emission status. However, to take advantage of pure battery electric driving, an externally rechargeable battery can be combined with a fuel cell range extender. As part of a research project, an efficient air supply system for a fuel cell range extender was developed. To this end, a 25 kW PEM fuel cell system test bench was set up. The different parameter influences of the test bench, in particular of the air supply system, were analyzed and evaluated in terms of stack/system efficiency and functionality. The control software of the test bench was specifically developed for the flexible operating parameter variation. All adjustable variables of the system (air ratio, stack temperature, pressure, etc.) were varied and evaluated at steady-state operating points. Likewise, the system was analyzed during dynamic operation and fault cases in adverse operating conditions (water condensation, oxygen deficiency) were identified. The system's warm-up process was also evaluated in regard to efficiency and functionality, since at lower temperatures, a larger air mass flow is needed to counteract water condensation inside the stack. Finally, the hydrogen purging losses were quantified at different operating pressures purging intervals. The experimental results show that, above all, the air supply has a significant influence on the efficiency of the system and is decisive for the proper operation and further improvement of the system. In summary, depending on the load point and operating conditions, a system efficiency between 42 % and 56 % was achieved.