NOVELTY - Power source unit (1) comprises a fuel cell unit (11), a source (14) of gaseous fuel, a pressure release valve (56), and a flow restrictor (FR) (52).    USE - The power source unit (1) is useful for providing electric power to an electronic device (claimed).    ADVANTAGE - The unit provides a power source for electronic devices comprising responsive to changes in fuel cell performance and adapted to momentarily stop or decrease the fuel cell current for a period of time in case of a malfunction.    DETAILED DESCRIPTION - Power source unit (1) for providing electric power to an electronic device comprises a fuel cell unit (11) and a source (14) of gaseous fuel, characterized by a control unit (CU) responsive to changes in fuel cell performance and adapted to momentarily stop or decrease the fuel cell current for a period of time in case of a malfunction; a pressure release valve (56) at the exit of the fuel cell unit (11), the valve being adapted to open in response to an increased pressure inside the fuel cell unit; and a FR (52) coupled between the fuel source and the fuel cell unit (11) adapted to provide a pressure within the fuel cell unit during normal operation that is lower than the opening pressure for the valve. INDEPENDENT CLAIMS are:    (1) a modular power source (1) for portable electronic devices 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 power source, 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); and    (2) a system (40) for providing electric power to an electronic device comprising an electric power relay connector member (20) adapted to replace a battery in an electronic device; a power source unit (1); and means (24, 26) for connecting the power source to the electric power relay connector member (20).    DESCRIPTION OF DRAWING(S) - The drawing shows a schematic view of the power source unit (1).    System (40)    Fuel tank (42)    FR (52)    SC (54)    Pressure release valve (56)