The optimal design of multisource systems, hybrid systems in particular, requires an adequate choice of the energy management strategy. This latter usually impacts source sizing and lifetime. The present paper deals with an energy management approach based on a frequency sharing of the mission. Firstly, the limits of a symmetric frequency energy management are presented in the case of a hybrid system associating a fuel cell with a Li-Ion battery. Subsequently, an original energy “asymmetric” management strategy for the optimal sizing of this association is presented. This strategy is then tested on the “Hydrogen” platform at the LAPLACE research laboratory. Finally, the two energy management strategies are compared in the context of an integrated design by optimization; the asymmetric strategy offers significant gains in terms of system weight, which is important for embedded applications.