In order to evaluate losses during energetical transfers, accurate battery models are needed in embedded power systems. The accuracy of those is all the more important as in hybrid vehicles, batteries are continuously charged and discharged. This paper deals with a dynamical model of NiMH battery, the originality of which lays on the use of non integer derivatives to model diffusion phenomena. With this method, we represent the battery by an equivalent electric scheme, whose components are identified by an experimental technique called “Electrochemical Impedance Spectroscopy (E.I.S.)". This identification process has been applied to a commercially available NiMH cell and its relevance checked by comparing digital simulations and laboratory experiments.