A secondary battery, so called because the conversion of chemical to electrical energy is reversible and the battery is thus rechargable. An automobile battery usually consists of 12–17 cells with plates (electrodes) made of sponge lead (negative plate or anode) and lead dioxide (positive plate or cathode) that is in the form of a paste. The electrolyte is sulfuric acid. The chemical reaction that yields electric current is Pb + PbO2 + 2H2SO4 ←→ 2PbSO4 + 2H2O + 2e. More complicated and expensive types have nickel-iron, nickel-cadmium, silver-zinc, and silver-cadmium as electrode materials. A sodium-liquid sulfur battery for high-temperature operation as well as a chlorine-zinc type using titanium electrodes have also been developed.As part of the U.S. effort to replace gasoline with another form of energy, DOE is supporting short- and long-term research on batteries for electric vehicles at Argonne National Laboratories. Three types intended to deliver 20–30 kwh are in the short-term program: improved lead-acid, nickel-zinc and nickel-iron. The long-term program includes lithium-metal sulfide, sodium-sulfur (β-battery), zinc-chlorine, and metal-air. Independent research indicates that a zinc–nickel oxide system has encouraging possibilities. A lead-acid battery for storing energy from solar cells has been reported to have a life of 5–7 years.
