NOVELTY - Device has an auxiliary coolant circuit assembled by shunting on a main coolant circuit. The auxiliary circuit traverses pre-anodic and pre-cathodic exchangers (18, 10), and post-anodic and post-cathodic exchangers (19, 21). The auxiliary circuit has an inlet duct (33) that traverses a lower temperature part of a heater (30) to cool a coolant to a temperature lower than temperature of a coolant in the main circuit.    USE - Used for cooling a power module of a fuel cell that is hydraulically coupled with a cooling loop of a power train of a motor vehicle, where the fuel cell supplies the electrical engine of an electric rear wheel drive vehicle.    ADVANTAGE - The inlet duct of the auxiliary circuit traverses the lower temperature part of the heater to cool the coolant to temperature lower than the temperature of the coolant in the main circuit, to cool components of the power module and the drive train, at different levels of temperature with same coolant, in a simplified and effective manner. The device permits to upgrade heat generated by the power module, when the module is used as auxiliary to main unit of a vehicle with existing heat engine, for preheating the heat engine and for heating cab interior of the motor vehicle in order to reduce over-consumption and emissions of engine, during cold engine start phase. The device permits circulation through a single coolant pump in the main and auxiliary circuits, at different temperature levels. The device does not necessitate circulation of other fluid e.g. refrigerant, and hence the device is inexpensive. The device permits to reduce the time taken by the fuel cell for attaining its optimal operating temperature without requiring an additional unit.    DETAILED DESCRIPTION - An INDEPENDENT CLAIM is also included for: method of cooling a power module.    DESCRIPTION OF DRAWING(S) - The figure shows a schematic view of a cooling device of a power module of a fuel cell hydraulically coupled with a cooling loop of a power train of a motor vehicle.    Pre-cathodic exchanger (10)    Pre-anodic exchanger (18)    Post-anodic exchanger (19)    Post-cathodic exchanger (21)    Heater (30)    Inlet duct (33)