Publisher Summary This chapter discusses numerous types of mercury-zinc batteries: mercury-zinc primary batteries and mercury-zinc cardiac pacemaker batteries. In mercury-zinc primary batteries button cells with solid cathodes are designed as anode limited. The anode is amalgamated powdered or gelled zinc and contains a gelling agent, and is housed in a triclad metal top with the copper in direct contact with the zinc. The anode is separated from the mercuric oxide cathode by a separator or absorbent material and a semi-permeable membrane barrier. The design of the absorbent/separator system depends on the intended use of the cell. An important application is hearing aid cells. In mercury-zinc cardiac pacemaker batteries anode consists of amalgamated zinc, which is in contact with the welded top assembly, the inner face of which is also amalgamated to achieve electrochemical compatibility with the anode. It is surrounded by the absorbent barrier, which is impregnated with a solution of sodium hydroxide saturated with zinc oxide. This type of electrolyte minimizes hydrolysis of the zinc, which causes production of hydrogen and results in capacity loss. The annular cathode consists of mercuric oxide mixed with silver powder, which forms a solid amalgam with the mercury appearing during discharge. It is in contact with the welded can assembly, which forms the positive terminal of the battery. The sealing ring and insulators, which divide the battery into two compartments, reduce the risk of short-circuit and electrical leakage.