TECHNOLOGY FOCUS - INSTRUMENTATION AND TESTING - Preferred Method: The hydrogen produced during the electrolysis of water is stored in the hydrogen path of the fuel cell system, preferably in the region of the hydrogen-side electrode, and/or the oxygen produced during the electrolysis of water is stored in the oxygen path of the fuel cell system, preferably in the region of the oxygen-side electrode. The pressure in the hydrogen path is measured during the electrolysis. The electrolysis is terminated within a predetermined period from the beginning of the electrolysis when the pressure in the hydrogen path does not exceed a predetermined minimum value. The electrolysis is terminated when the pressure in the hydrogen exceeds the predetermined maximum value. The hydrogen produced during the electrolysis of water is dissipated into a hydrogen reservoir, when the pressure in the hydrogen path exceeds predetermined path of hydrogen reservoir value. The temperature of the hydrogen-side gas, oxygen-side gas, hydrogen-side electrode, the oxygen-side electrode, membrane, hydrogen path, oxygen path and/or a coolant of a cooling system of the membrane electrode unit is measured during the electrolysis, where the electrolysis is terminated when the temperature exceeds a predetermined maximum value and the maximum cooling capacity of the cooling system is achieved. The electrical energy storage of the vehicle is checked for the storage capacity prior to the start of the electrolysis and/or the requirement electrical energy of an electrical load of the vehicle is checked, where the electrolysis is started only when other electrical energy storage of the vehicle does not have more storage capacity and/or does not require electrical load of the vehicle of the electrical energy, which is converted from braking energy, or the electrolysis is restricted in its performance or terminated, if other electrical energy storage of the vehicle has storage capacity and/or require electrical load of the vehicle of the electrical energy, which is converted from braking energy. The method comprises a process step (c), which comprises (c1) converting the hydrogen formed during the electrolysis and/or oxygen to water to generate electrical energy by changing the membrane electrode unit from the electrolysis operation to fuel cell operation, and (c2) converting the additionally supplied hydrogen and/or oxygen to water to generate electrical energy by further operation of the membrane electrode unit in the fuel cell operation, where the oxygen-side electrode is supplied using supplemental oxygen only after the start of the step (c1) and/or the hydrogen-side electrode is fed using additional hydrogen only after the start of the step (c1). Preferred Components: The recuperation device is an electric motor operable in generator mode. The fuel cell system comprises a hydrogen circuit, which is adapted to supply hydrogen to the hydrogen-side electrode and hydrogen is optionally not converted when passing- or flowing over the hydrogen-side electrode and hydrogen-side electrode is conducted again. The hydrogen path comprises at least one pressure sensor for measuring the gas pressure in the hydrogen path, preferably hydrogen circuit, and/or the oxygen path comprises at least one pressure sensor for measuring the gas pressure in the oxygen path. The fuel cell system comprises at least one temperature sensor for measuring the temperature of the hydrogen-side gas, oxygen-side gas, hydrogen-side electrode, oxygen-side electrode, membrane, hydrogen path, oxygen path and/or a coolant of a cooling system of the membrane electrode unit. The energy recovery system comprises a control device, which is adapted: to terminate the electrolysis, when the pressure measured by a pressure sensor does not exceed a predetermined minimum value within a predetermined period from the start of the electrolysis, when the pressure measured by the pressure sensor exceeds a predetermined maximum value and/or when temperature measured by a temperature sensor during the electrolysis exceeds a predetermined maximum value and the maximum cooling capacity of the cooling system is achieved; to terminate or slow down the electrolysis in its performance when electrical energy storage of the vehicle, preferably an accumulator has storage capacity and/or require electrical load of the vehicle of the electrical energy, which is converted from braking energy; and/or to discharge hydrogen produced during the electrolysis of water into a hydrogen reservoir, when the pressure measured by a pressure sensor in the hydrogen path exceeds predetermined path of hydrogen reservoir value, when the electrolysis is started only when other electrical energy storage of the vehicle, preferably accumulator does not have more storage capacity and/or does not require electrical load of the vehicle of the electrical energy, which is converted from braking energy. EXAMPLE - No suitable example given.