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Supercapacitors are commonly based on porous activated carbon electrodes and on electrostatic charge storage mechanism. The carbon electrodes are supposed to be chemically and electrochemically inert and the electrostatic nature of the charge storage mechanism is highly reversible. These properties should ensure to supercapacitors an infinite shelf life. But in practice, supercapacitor cells exhibit a performances fading when they are used during months. The purpose of this paper is to study the influence of the performances fading of supercapacitors on the operation of a hybrid electric vehicle system composed of supercapacitors and battery. Therefore, results of supercapacitors ageing are presented in order to highlight the performances fading. Then an analytical impedance model is presented and used for the system simulation in Matlab/Simulink. The influence of the supercapacitor parameters changes with ageing will be studied and their impact on the complete system operation will be discussed.