NOVELTY - The method involves controlling an output of a fuel cell (10) based on a predetermined value, and maintaining the value. A voltage of a bidirectional high voltage direct current (DC) -DC converter (21) connected between a battery and a main bus terminal (11) is reduced. The output of the fuel cell is controlled based on the predetermined value. The battery is forcibly charged via an output current of the fuel cell generated by maintaining the reduced voltage of the converter, where oxygen in a cathode is exhausted and voltage of a fuel cell becomes lower than the voltage of the converter.    USE - Method for controlling a load-following operation of a fuel cell hybrid system for a vehicle during emergency operating conditions. Uses include but are not limited to motor vehicles e.g. passenger automobiles such as sports utility vehicles, buses, lorries, commercial vehicles, watercraft such as boats and ships, and aircraft, hybrid vehicles, plug-in hybrid electric vehicles, hydrogen- powered vehicles, and diesel-powered vehicle.    ADVANTAGE - The method enables controlling the voltage of a bidirectional high voltage DC-DC converter connected to an energy storage device as an auxiliary power source while controlling the air supply at the same time, thus effectively avoiding a low power and low efficiency region of the fuel cell during operation, providing more efficient idle stop-start control and improving durability of the fuel cell system by the avoidance operation of the open-circuit voltage (OCV) region and fuel efficiency of the vehicle by the avoidance operation of the low efficiency region. The method enables allowing output of the fuel cell to be generated when the oxygen in the cathode is exhausted to charge the high voltage battery, so as to achieve more efficient idle stop-start control.    DETAILED DESCRIPTION - The battery is a high voltage battery (20) i.e. main battery, and a low voltage battery (40). An INDEPENDENT CLAIM is also included for a system for controlling an operation of a fuel cell hybrid system.    DESCRIPTION OF DRAWING(S) - The drawing shows a schematic block diagram of a fuel cell hybrid system to which a control system is applied.    Fuel cell (10)    Main bus terminal (11)    Diode (13)    High voltage battery (20)    Bidirectional high voltage DC-DC converter (21)    Low voltage battery (40)