TECHNOLOGY FOCUS - ELECTRICAL POWER AND ENERGY - Preferred Device: The power source unit further comprises a sensor cell (SC) (54) in the fuel cell unit (1) coupled to the CU, the sensor cell being responsive to changes in the hydrogen gas concentration at the exit of the fuel cell unit (1), where the change in cell voltage of the sensor cell is indicative of the change in hydrogen gas concentration. The CU is coupled to the SC (54) and is responsive to the cell voltage of the sensor cell. The pressure release valve (56) is adapted to open at a pressure that is lower than the pressure in the fuel source (14). The valve (56) is a pressure release valve of the umbrella type. The flow restrictor is a constriction in the gas flow passage. The flow restrictor is fixed. The flow restrictor is variable. It is a charger. The modular power source further comprises a chargeable battery that can deliver electricity when neither grid voltage nor fuel to the fuel cell unit is available. The grid connector (13; 15; 16) is a replaceable member. The fuel cell unit comprises air breathing polymer electrolyte fuel cells in planar configuration. It comprises a clip-on fuel tank (42). It further comprises a slot for accommodating a rechargeable battery, that can be charged by the power source, and that can deliver electric power if neither grid nor fuel cells are operable. Preferred System: Alternatively. The system (40) for providing electric power to an electronic device comprises a power source unit (1), comprising: a main body (10); a grid connector (13; 15; 16) provided on the main body (10) and comprising power transforming electronics, for enabling charging of a battery provided in an electronic device, or for direct powering of the device, by-passing the battery; a fuel cell unit (11); means (17) for accommodating a fuel tank (42) for the fuel cell unit (11); characterized in that the fuel cell unit is an air breathing polymer electrolyte fuel cell unit (11) in a planar configuration capable of delivering electric power when grid voltage is not available; and in that the system (40) further comprises: an electric power relay connector member (20) adapted to (i) replace a battery in an electronic device, (ii) to simulate the presence of a battery in the electronic device, and (iii) to relay current from the power source to the electronic device; and means (24, 26) for connecting the power source to the electric power relay connector member (20). It also comprises a power source unit (1), comprising: a main body (10); a grid connector (13; 15; 16) provided on the main body (10) and comprising power transforming electronics, for enabling charging of a battery provided in an electronic device, or for direct powering of the device, by-passing the battery; an air breathing polymer electrolyte fuel cell unit (11) in a planar configuration and capable of delivering electric power when grid voltage is not available; means (17) for accommodating a fuel tank (42) for the fuel cell unit (11); an electric power relay connector member (20) adapted to replace a battery in an electronic device, so as to simulate the presence of a battery in the electronic device, for relaying current from the power source to the electronic device; and means (24, 26) for connecting the power source to the electric power relay connector member (20).