NOVELTY - The reservoir (8) has a flexible membrane (15) separating the reservoir into compartments (16, 17) respectively filled with coolant and air. The compartment (17) is connected to an on/off controlled pressurization valve (20) in relation with an overpressure circuit (21) to favor the circulation of coolant towards a heat controlling circuit (4) during the operation of a fuel cell (1). The compartment (17) is connected to an on/off controlled pressurization valve (22) in relation with an overpressure circuit (23) to favor the flow of the coolant in the reservoir while the cell is out of service.    USE - Used in a fuel cell (claimed) such as proton-exchange membrane fuel cell and range extender fuel cell, which assures the hauling of a motor vehicle and permits to convert chemical energy into electrical energy from a fuel such as hydrogen, and a comburant such as oxygen or a gas e.g. air, containing oxygen.    ADVANTAGE - One compartment favors the flow of coolant in the reservoir when the fuel cell is out of service, thus permitting to conserve the electrical characteristics of the coolant and hence permitting good operation of the fuel cell. The compartment permits to eliminate the operation of emptying of the fluid contained in the cell during the maintenance operation of the cell. The compartment reduces the temperature rise time of the cell for attaining the optimal operation temperature. The compartment is connected to the on/off controlled pressurization valves in relation with the overpressure circuits, thus permitting to assist a pump of the heat controlling circuit during the starting and hence improving the yield of the fuel cell and accelerating the evacuation time when the cell is out of service.    DETAILED DESCRIPTION - INDEPENDENT CLAIMS are also included for the following:    (1) a device for evacuating a coolant from a fuel cell    (2) a method for evacuating a coolant in a fuel cell.    DESCRIPTION OF DRAWING(S) - The drawing shows a block diagram of a coolant evacuating device before the operation of a fuel cell.    Fuel cell (1)    Heat controlling circuit (4)    Coolant reservoir (8)    Flexible membrane (15)    Compartments (16, 17)    On/off controlled pressurization valves (20, 22)    Overpressure circuits (21, 23)