Primary batteries: non-rechargeable. Dry cell (Leclanche): Zn anode, MnO2 cathode, NH4Cl paste. E approximately 1.5 V. Mercury cell: Zn/HgO, constant voltage (1.35 V), used in hearing aids. Secondary batteries: rechargeable. Lead-acid: Pb anode, PbO2 cathode, H2SO4 electrolyte. Cell reaction: Pb + PbO2 + 2H2SO4 <=> 2PbSO4 + 2H2O. E approximately 2 V per cell (6 cells = 12 V car battery). Ni-Cd: Cd anode, NiO(OH) cathode, KOH electrolyte. Fuel cells: continuous supply of reactants. H2-O2 fuel cell: 2H2 + O2 -> 2H2O. E approximately 1 V. High efficiency (no Carnot limitation), zero pollution (water as byproduct). Used in spacecraft.