A mercury battery, also called a mercuric oxide battery or a mercury cell, is a non-rechargeable electrochemical battery. These batteries have been used in the shape of button cells for watches, hearing aids, and calculators, and in larger forms for other devices, including walkie-talkies.
Mercury watch battery
Mercury batteries are convenient because of their size. This is a small watch mercury battery.
Mercury batteries have the advantages of a long shelf life of up to 10 years and steady voltage output. Although these batteries were very common in the mid-20th century, the Mercury-Containing and Rechargeable Battery Management Act (the Battery Act) passed in 1996 in the United States has largely phased out mercury batteries due to environmental concerns.
Mercury batteries use either pure mercuric oxide or a mixture of mercuric oxide with manganese dioxide as the cathode. Mercury oxide cells are constructed with a zinc anode, a mercury oxide cathode, and potassium hydroxide or sodium hydroxide as the electrolyte. Since mercuric oxide is a non-conductor, some graphite is mixed with it. This helps prevent the collection of mercury into large droplets. During discharge, zinc oxidizes to zinc oxide, and mercuric oxide gets reduced to elemental mercury. A little extra mercuric oxide is put into the cell to prevent evolution of hydrogen gas at the end of its life.
In a mercury battery, sodium hydroxide or potassium hydroxide is 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, provide constant voltage at higher currents, making them suitable for applications requiring current surges, such as photographic cameras with flash and watches with a backlight. Potassium hydroxide cells also have better performance at lower temperatures.
The Battery Act
In 1996, the Mercury-Containing and Rechargeable Battery Management Act (the Battery Act; Public law 104-142) was signed into law in the United States. The intended objective of the act was to reduce heavy metals in municipal waste, streams, and ground water. This resulted from the disposal of mercury in single-use batteries, as well as of other toxic metal content such as lead from lead-acid batteries and the cadmium in rechargeable batteries. The law therefore sought to phase out the use of mercury in batteries due to the environmental damage it caused.