NOVELTY - The method involves estimating concentration of hydrogen gas in an anode of a fuel cell i.e. hydrogen fuel cell, by utilizing a hydrogen gas concentration model. A determination is made to check whether the estimated concentration of the hydrogen gas is below a predetermined threshold. The hydrogen gas is caused to be injected into the anode from a hydrogen source (36) i.e. high pressure tank, while shutting-down a fuel cell system (10) i.e. hydrogen fuel cell system, when the estimated concentration of the hydrogen gas is less than the predetermined threshold.    USE - Method for determining time for injecting hydrogen gas into an anode of a fuel cell i.e. hydrogen fuel cell, in a fuel cell system i.e. hydrogen fuel cell system, mounted on a vehicle i.e. fuel cell vehicle, during shutdown of the fuel cell system and off condition of the vehicle (all claimed) by using a hydrogen gas concentration model.    ADVANTAGE - The method enables injecting the hydrogen gas into an anode side of a fuel cell stack during shut-down condition at a desired time to consistently minimize hydrogen/air front occurring in the fuel cell stack. The method enables measuring pressure in an injector to determine the hydrogen gas flow through the injector instead of estimating the amount of hydrogen gas while avoiding the need for estimating the amount of hydrogen gas by making multiple assumptions regarding hydrogen movement through diffusion, convection, leakage and membrane permeation. The method enables performing a concentration-based injection control to perform injection process only when the hydrogen concentration is less than the threshold. The method enables performing a time-based injection control to wake up a controller and to perform the injection process regardless of hydrogen concentration. The concentration-based injection control reduces fuel and battery consumption, while impacting a startup time and multiple emissions on a next start-up. The concentration-based injection control ensures that the hydrogen concentration reaches a same level after injection regardless of the initial hydrogen concentration because of hydrogen concentration feedback.    DETAILED DESCRIPTION - An INDEPENDENT CLAIM is also included for a system for determining time for injecting hydrogen gas into an anode of a fuel cell system during shut-down of the fuel cell system.    DESCRIPTION OF DRAWING(S) - The drawing shows a schematic view of a fuel cell system.    Fuel cell system (10)    Fuel cell stack (12)    Compressor (14)    Bleed valve (30)    Hydrogen source (36)