NOVELTY - The fuel cell system comprises a fuel tank (110), a fuel supply unit (120) connected to the fuel tank to supply a first fuel to the fuel tank, a fuel cell stack (130) connected to the fuel tank, a liquefied recycling unit (140) connected to the fuel cell stack to liquefy the first gas into a recycled fluid, a liquid level modulation unit (150) connected to the fuel tank and the liquefied recycling unit, a sensing unit electrically connected to the fuel cell stack, a control unit electrically connected to the sensing unit and the fuel supply unit, and a pressurizing unit.    USE - The fuel cell system is useful in domestic backup power systems, transportable power systems or portable electronic devices.    ADVANTAGE - The fuel cell system ensures precise and uniform supply of fuel in its components thus avoiding the damage of the fuel cell stack and allowing the user to restart the system after resupplying or replacing the fuel supply unit.    DETAILED DESCRIPTION - The fuel cell system comprises a fuel tank (110), a fuel supply unit (120) connected to the fuel tank to supply a first fuel to the fuel tank, a fuel cell stack (130) connected to the fuel tank, a liquefied recycling unit (140) connected to the fuel cell stack to liquefy the first gas into a recycled fluid, a liquid level modulation unit (150) connected to the fuel tank and the liquefied recycling unit, a sensing unit electrically connected to the fuel cell stack, a control unit electrically connected to the sensing unit and the fuel supply unit, a pressurizing unit providing an operating pressure to the fuel cell system to move the first gas to the liquefied recycling unit to be recycled as the recycled fluid, and a critical level detector disposed in the fuel tank. The fuel tank provides a second fuel to the fuel cell stack, and the fuel cell stack generates a first gas. The liquid level modulation unit guides the recycled fluid into the fuel tank to maintain a stable fuel level of the second fuel in the fuel tank. The sensing unit senses: a current signal provided by the fuel cell stack; and a voltage signal provided by the fuel cell stack; and a temperature signal of the fuel cell stack. The control unit calculates a current quantity according to the current signal. The first fuel is supplied to the fuel tank from the fuel supply unit when the current quantity reaches a predetermined quantity. A consistency of the first fuel is 50 vol.%, and a consistency of the second fuel is lower than 10 vol.%. The liquefied recycling unit comprises a liquefier, a fan and a recycling tank. The pressurizing unit moves the first gas to the liquefier, the fan cools the liquefier, the liquefier liquefies the first gas into the recycled fluid, and the recycled fluid is moved to the recycling tank to be stored. The liquid level modulation unit moves the recycled fluid from the recycling tank to the fuel tank when the level of the second fluid in the fuel tank downs. The liquid level modulation unit comprises a supply tube and an exhaust tube, where the supply tube connects the recycling tank to the fuel tank and the exhaust tube connects the recycling tank to the fuel tank. The recycled fluid travels from the recycling tank passing through the supply tube to the fuel tank when the level of the second fluid in the fuel tank is lower than the free end of the exhaust tube. The recycled fluid is prevented from traveling to the fuel tank when the level of the second fluid in the fuel tank reaches the free end of the exhaust tube. An air pressure in the fuel tank is 0.15 kg/cm2 when the level of the second fluid in the fuel tank reaches the free end of the exhaust tube. The recycling tank comprises an air outlet. A second gas passes through the exhaust tube and the recycling tank and leaves the recycling tank through the air outlet. The control unit supplies the first fuel to the fuel tank according to the voltage signal and the temperature signal. The critical level detector sends a critical signal to the control unit and the control unit stops the fuel cell system according to the critical signal when the level of the second fuel in the fuel tank reaches a critical level. An INDEPENDENT CLAIM is included for a method of supplying fuel to a fuel tank.    DESCRIPTION OF DRAWING(S) - The figure shows a schematic view of a fuel cell system.    Fuel tank (110)    Fuel supply unit (120)    Fuel cell stack (130)    Liquefied recycling unit (140)    Liquid level modulation unit. (150)