Publisher Summary This chapter provides na overview of mercy batteries. It discusses different types of batteries, such as mercury-zinc batteries, mercury-indium-bismuth, and mercury-cadmium primary batteries. Mercury batteries have an appreciably higher energy-to-weight ratio than carbon-zinc batteries, resulting from the high energy density of the materials used in their construction. Thus mercury batteries are only one-third the size of conventional dry batteries of the same capacity. The ampere hour capacity of mercury cells and batteries is relatively unchanged with variation of discharge schedule and, to some extent, with variation of discharge current. They have a relatively fiat discharge characteristic. Mercuric oxide batteries are available with two formulations designed for different field usage. Further, mercury-indium-bismuth and mercury-cadmium batteries are alkaline systems recommended for applications where high reliability in particularly onerous long-term storage and use conditions is a prime requirement. For example, the particular advantage of mercury-cadmium is that it will show good performance at low temperatures even below -20°C, where most other primary systems just would not function. Typical applications include reactor and high-temperature processing, telemetry systems and military uses.