A battery was coustructed by including a polymer proton conductor, poly(propargyl alcohol) doped with perchloric acid, between electrodes of magnesium and gold. The characteristics of the cell, showing an open circuit voltage of 2.0-2.2 V, were studied as the function of the relative humidity, It was found that the system behaves as a fuel cell and that the discharge current of the battery is due to magnesium oxidation at the anode and proton reduction at the cathode.