Abstract: The invention is a metal air fuel cell consisting of a cathode contained in a housing, the housing having an air passage through which air (O2 gas) can pass to the cathode. The air passage is sealed by a gas (i.e. O2) permeable membrane. The fuel cell further includes an anode made of a metal selected from the group of metals including aluminum, zinc, magnesium, and alloys thereof. The cathode and anode are electrochemically coupled by an electrolyte such that the cathode and anode are capable of electrochemically reacting to consume O2 gas at a volume rate of V when producing a desired electrical current of I. The gas permeable membrane has a gas permeability rate and a surface area through which O2 gas can pass through the gas permeable membrane to the cathode, the surface area and the gas permeability rate of the gas permeable membrane selected to permit O2 gas to pass through the membrane at a rate Vm substantially equal to V at the desired current I.