Efficiency. An iron-air battery has an open circuit cell voltage of about 1.28V and a theoretical energy density of 764 Wh/kg. While the current densities are at least an order of magnitude higher than would be used for discharge of batteries for grid-scale electrical storage, there is a 0.5V difference between the charge and discharge voltages. This difference in charge/discharge voltages arises mainly from the poor performance of the air electrode. This voltage loss contributes significantly to the low round-trip efficiency of 50% of current iron-air batteries, 20% loss of capacity in 14 days, and 10% loss of faradic charge efficiency.