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A watch battery is a tiny form-factor battery, which is commonly used in such small electronic devices such as wristwatches, hearing aids, laser pointers, keyless entry remote controls, and small pocket calculators. They are also called button cell, silver button cell, or coin cell batteries. A watch battery ordinarily is a single cell with a nominal voltage in the 1.5 to 3 volt range. Considering its extremely small size, a watch battery has an extremely long life.
The International Electrotechnical Commission (IEC) has published standard IEC 60086-3, which defines an alphanumeric coding system for all watch batteries. This coding system, consisting of a three to five letter code, is based in part on the electrochemical system used in the battery. The coding system also describes the battery's diameter and height, and the shape of the battery casing. One of the most commonly used watch batteries is the C-type battery. This 3-volt lithium battery is used not only in quartz watches, but also in some personal data assistant (PDA) devices, as well as in computer motherboard clocks.
Standard watch batteries are typically not rechargeable, though some brands of rechargeable watch batteries are available. They aren't commonly used because their charge capacity is inferior to non-rechargeable batteries, and they tend to self-discharge much more quickly. Additionally, watch batteries are used in devices where power consumption is so low that there isn't any tangible benefit in using a rechargeable. In many cases, the watch battery actually outlives the device it was meant to power.
Most watch batteries in common use today are either zinc or silver oxide batteries. Mercury-based button cells, once very popular as watch batteries, are now banned in many countries due to concerns about their impact on the environment. This ban has caused a particular problem for photographers. Much of their sensitive equipment still relies on the very flat, stable discharge curve and long life of mercury cells.
Photographers have since come to substitute the mercury batteries with zinc-air batteries, which do have a similarly flat discharge curve to mercury cells but a substantially shorter life span. Zinc-air batteries also perform poorly in very arid climates, which is of concern to some photographers. Another alternative for mercury cells is alkaline watch batteries, but their voltage varies so widely throughout the battery's lifetime that photographers tend to dislike them. The best—-and most expensive-—alternative for photographers is silver-oxide batteries. These have a more acceptably flat discharge curve, though still less flat than mercury.
Care must be taken in regard to the battery requirements of any device using a watch battery. Using the wrong type or voltage of button cell may not only ruin a device but can also pose a danger to the user. Manufacturer's instructions should always be followed to the letter to ensure safe and efficient operation of the device.