Publisher Summary This chapter explains the types of mercury batteries: mercury-zinc (primary) batteries and mercury-zinc cardiac pacemaker batteries followed by other different types of mercury batteries. Union Carbide and Mallory are two of the major producers of mercury-zinc cells and batteries. There are, of course, other important producers, such as Berec (UK), Crompton-Parkinson (Hawker Siddeley) (UK) and Varta (West Germany). Both Union Carbide and Mallory use a vented construction, which renders their products leak-proof and free from bulging under all normal working conditions. Self-venting occurs automatically if, for example, excessive gas is produced within the cell under sustained short-circuit conditions. Both manufacturers assemble cells in a wide variety of structures to provide batteries of varying size, voltage, and capacity. A wide range of purpose-built batteries isobtained. For maximum reliability, inter-cell connection is by spot-welded strips of nickel-plated steel. Outer containers are made from materials ranging from cardboard to metal. The type I mercury-zinc cardiac pacemaker batteries has a nominal voltage of 1.35 V and a nominal capacity of 1000mAh defined for a load of 1250 Ω, with a cut-off voltage of 0.9 V. Other types of mercury battery discussed are mercuric oxide-indium-bismuth and mercury-cadmium (primary) batteries supplied by Crompton-Parkinson; mercury-cadmium batteries supplied by Mallory; and mercury-cadmium batteries supplied by Varta.