NOVELTY - A cathode reagent flow (32) is delivered to the cathode, which has two discrete sides. A first fraction (36) of this flow is sent via a first path to the first side. A second fraction (40) is sent via a second path to the second side. A stoichiometric quantity of cathode reagent is varied in the first and second fractional flows. A flow resistance in the flow paths is varied, hence varying this quantity. The resistance lies downstream or upstream of the cathode, and optionally in both of these locations.    USE - For dynamic control of the cathode gas flow rate supplied to the fuel cell unit used e.g. in a vehicle.    ADVANTAGE - The supply of a stoichiometric quantity of cathode reagent (air) can be varied between two predetermined values. This can take place under constant demand, and/or during transitions in power demand. The air delivery device is operated under constant conditions when the demand on the cell is constant. Operation at constant rates minimizes or eliminates the perception of e.g. fan or compressor noise by the user.    DETAILED DESCRIPTION - Two valves are used to cause the flow variations. The cathode reagent flow is delivered by a device moving air at constant flow rate for constant fuel cell power output. A minimal stoichometric flow of cathode reagent is maintained on each side of the cathode. An essentially-constant power demand is supplied by the fuel cell. The stoichiometric flow of cathode reagent in each fractional flow is varied between two predetermined stoichiometric magnitudes. The fuel cell stack comprises at least two fuel stacks. First and second cathode sides, are associated with a first and a second fuel cell stack, with fractions of the flow supplied to each. A proportion of the flow from the cathode flow leaving the first stack is sent to the cathode side of the second stack, the flow from the stacks being varied.    An INDEPENDENT CLAIM is included for the corresponding fuel cell stack.    DESCRIPTION OF DRAWING(S) - A schematic block diagram illustrates the air (or O2) circuit and its control, in a preferred implementation.(Drawing includes non-English language text).    fuel cell system (20)    first and second stacks (22, 24)    air delivery unit (26)    motor (28)    compressor (30)    fractions of the cathode flow (36, 40)    cathode flow (32)