The paper describes a test set-up, based on the use of a Digital Signal Processor (DSP), developed for the evaluation of the operational characteristics of an autonomous Auxiliary Power Unit (APU). The considered APU includes a 4.8 kW commercial Proton Exchange Membrane (PEM) fuel cell and a set of electrochemical batteries. Both power sources are connected to a common AC bus through power electronic converters. The DSP-based test set-up is also conceived so to incorporate the basic functions of the APU energy management system. The paper also presents some experimental results, with particular reference to the assessment of the PEM fuel cell performances. © 2007 IEEE.