Mercury battery

A mercury battery (also called mercuric oxide battery, or mercury cell) is a non-rechargeable electrochemical battery, a primary cell. Due to the content of mercury, and the resulting environmental concerns, the sale of mercury batteries is banned in many countries. Both ANSI and IEC have withdrawn standards for mercury batteries. Mercury batteries were made in button types for watches and calculators, and in larger forms for other applications.

Chemistry

Mercury batteries use either pure mercuric oxide or a mix of mercuric oxide with manganese dioxide as the cathode. Mercuric oxide is a non-conductor so some graphite is mixed with it; the graphite also helps prevent collection of mercury into large droplets. The anode is made of zinc and separated from the cathode with a layer of paper or other porous material soaked with electrolyte. During discharge, zinc oxidizes to zinc oxide and mercuric oxide gets reduced to elementary mercury. A little extra mercuric oxide is put into the cell to prevent evolution of hydrogen gas at the end of life. Mercury batteries are very similar to silver-oxide batteries.
Sodium hydroxide or potassium hydroxide are used as an electrolyte. Sodium hydroxide cells have nearly constant voltage at low discharge currents, making them ideal for hearing aids, calculators, and electronic watches. Potassium hydroxide cells, in turn, provided constant voltage at higher currents, making them suitable for applications requiring current surges, e.g. photographic cameras with flash, and watches with a backlight. Potassium hydroxide cells also have better performance at lower temperatures. Mercury cells have very long shelf life, up to 10 years.
A different form of mercury battery uses mercuric oxide and cadmium. This has a much lower terminal voltage around 0.9 volts and so has lower energy density, but it has an extended temperature range, in special designs up to 180 C.[3][4]

Electrical characteristics

Mercury batteries using mercury(II) oxide cathode have a very flat discharge curve, holding constant 1.35 V (open circuit) voltage until about last 5% of their lifetime, when their voltage drops rapidly. The voltage remains within 1% for several years at light load, and over a wide temperature range, making mercury batteries useful as a reference voltage in electronic instruments and in photographic light meters. Mercury batteries with cathodes made of a mix of mercuric oxide and manganese dioxide have output voltage of 1.4 V and more sloped discharge curve.

Substitutes

The ban on sale of mercury oxide batteries caused numerous problems for photographers, whose equipment frequently relied on their advantageous discharge curves and long lifetime. Alternatives used are zinc-air batteries, with similar discharge curve but much shorter lifetime (a few months) and poor performance in dry climates, alkaline batteries with voltage widely varying through their lifetime, and silver-oxide batteries with higher voltage (1.55 V) and very flat discharge curve, making them possibly the best, though expensive, replacement. Special adapters with a voltage dropping germanium diode are available, to adapt silver oxide batteries for use in older equipment designed for mercury batteries, such as cameras and light meters which require a stable, exact voltage.