In this paper, an impedance model of the proton exchange membrane fuel cell stack (PEMFCS) is proposed. The proposed study employs an equivalent circuit of the PEMFCS derived by the frequency response analysis (FRA) technique. An equivalent circuit for the fuel cell stack is developed to evaluate the effects of ripple currents generated by the power-conditioning unit. The calculated results are then verified by means of experiments on two commercially available fuel cells: Avista Labs SR-12 (500 W) and Ballard Nexa (1.2 kW) PEMFC system. The relationship between ripple current and fuel cell performance, such as power loss and fuel consumption is investigated. Experimental results show that the ripple current can contribute up to a 10% reduction in the available output power. (c) 2005 Elsevier B.V. All rights reserved.