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In this paper, a flatness-based nonlinear control method is proposed to control a multisource/multiload electrical hybrid system (EHS). The EHS is composed of a fuel cell and a supercapacitor-bank (SCB) as the main and auxiliary sources. They supply two independent loads, connected to a dc-bus through unidirectional buck converters. The proposed method is able to control the fuel cell output power and its dynamics. It also allows limiting the current of an SCB during charging and discharging processes. The proposed control strategy has the advantage of not requiring any commutation between different control algorithms when the operating mode of the system changes (from the normal mode to the overload mode, for example). As the fuel cell output characteristic (FCOC) varies with the physical and environmental parameters, an observer is also proposed and designed to estimate either the fuel cell voltage-power (V-P) output characteristic or voltage-current (V -I) output. The use of the proposed observer allows achieving an efficient control of the system and avoiding overcharging or discharging of an SCB. Experimental results demonstrate the operation of the proposed control strategy and observer in all situations. Experimental test on a system is done with two separated loads of 5 and 12 V, via a dSPACE platform.