Battery

Primary cells are designed to be discharged only once. This is despite the fact that all electrodes must theoretically participate in a reversible reaction when current is generated. The reason that the primary cell reaction is not reversible has to do with reactions that prevent or limit the efficiency of recharging. For example, a magnesium anode decomposes to produce magnesium ions and electrons. The magnesium ions react with water to produce magnesium hydroxide, which causes the cell to swell, and hydrogen gas. Any attempt to recharge the cell would only generate more hydrogen gas at the oxide surface, because the voltage required to generate hydrogen is less than that required to redeposit the magnesium.