NOVELTY - Fuel is introduced into anode flowpath such that fuel flows through first and second fuel cell stacks in first direction. Oxygen is introduced into cathode flowpath. Flow of fuel through anode flowpath is reversed to flow through fuel cell stacks in second direction. Voltage difference associated with operation of fuel cell system is sensed. The sensed voltage difference is correlated with fuel concentration in anode flowpath. A bleed valve is manipulated to allow a portion of fluid present in anode flowpath to escape when threshold value of voltage difference is attained based on correlation.    USE - Method of operating flow shifting fuel cell system (claimed) used for vehicle.    ADVANTAGE - Identifies and regulates the concentration of nitrogen buildup in an operating fuel cell. Controls the bleed of nitrogen in the anode loop of the flow shifting fuel cell system. Simplifies bleed algorithm for the flow shifting anode flowpath to maximize stable average stack voltages while minimizing hydrogen loss during nitrogen bleed. Improves operability of the flow shifting fuel cell system through the use of bleed valves that minimizes system complexity and efficiency impacts.    DETAILED DESCRIPTION - The flow shifting fuel cell system is configured to comprise first and second fuel cell stacks, each comprising individual fuel cells each comprising anode (310), cathode (330) and electrolyte (320) disposed between anode and cathode, the cathode flowpath (336) configured to couple cathodes of individual fuel cell (300) to an oxygen source, the anode flowpath (316) configured to couple anodes of individual fuel cell to fuel source, and a sensor for measuring voltage. The anode flowpath is configured to be connected between the first and second stacks such that fuel present within anode flowpath cycles back and forth between first and second stacks through the bleed valve coupled to the anode flowpath.    DESCRIPTION OF DRAWING(S) - The drawing shows the sectional view of the single fuel cell of fuel cell stack.    Fuel cell (300)    Anode (310)    Anode flowpath (316)    Electrolyte (320)    Cathode (330)    Cathode flowpath (336)