Button cell

Small battery

A button cell, watch battery, or coin battery is a small single cell battery shaped as a squat cylinder typically in diameter and high – resembling a button. Stainless steel usually forms the bottom body and positive terminal of the cell; insulated from it, the metallic top cap forms the negative terminal. Button cells are used to power small portable electronics devices such as wrist watches and pocket calculators. Wider variants are usually called coin cells. Devices using button cells are usually designed around a cell giving a long service life, typically well over a year in continuous use in a wristwatch. Most button cells have low self discharge, holding their charge for a long time if not used. Relatively high power devices such as hearing aids may use a zinc–air battery, which has a much higher capacity for a given size, but dries out after a few weeks even if not used. Button cells are single cells, usually disposable primary cells. Common anode materials are zinc or lithium. Common cathode materials are manganese dioxide, silver oxide, carbon monofluoride, cupric oxide or oxygen from the air. Mercuric oxide button cells were formerly common, but are no longer available due to the toxicity and environmental effects of mercury. Button cells are dangerous for small children, as when swallowed they can cause severe internal burns and significant injury or death. Duracell has attempted to mitigate this by adding a bitter coating to their batteries.

International standard IEC 60086 3 defines an alphanumeric coding system for "Watch batteries". Manufacturers often have their own naming system; for example, the cell called LR1154 by the IEC standard is named AG13, LR44, 357, A76, and other names by different manufacturers. The IEC standard and some others encode the case size so that the numeric part of the code is uniquely determined by the case size; other codes do not encode size directly. Examples of batteries conforming to the IEC standard are CR2032, SR516, and LR1154, where the letters and numbers indicate the following characteristics.

Often a 2 letter code (sometimes on the side of the battery) where the first letter identifies the manufacturer and the second is the year of manufacture. For example:
YN – the letter N is the 14th letter in the alphabet – indicates the cell was manufactured in 2014. There is no universal standard. The manufacturing date can be abbreviated to the last digit of the year, followed by a digit or letter indicating the month, where O, Y, and Z are used for October, November and December, respectively (e. g., 01 = January 2010 or 2000, 9Y = November 2019 or 2009).

In addition to disposable (single use) button cells, rechargeable batteries in many of the same sizes are available, with lower capacity than disposable cells. Disposable and rechargeable batteries are manufactured to fit into a holder or with solder tags for permanent connection. In equipment with a battery holder, disposable or rechargeable batteries may be used, if the voltage is compatible. A typical use for a small rechargeable battery (in coin or other format) is to back up the settings of equipment which is normally permanently mains powered, in the case of power failure. For example, many central heating controllers store operation times and similar information in volatile memory, lost in the case of power failure. It is usual for such systems to include a backup battery, either a disposable in a holder (current drain is extremely low and life is long) or a soldered in rechargeable. Rechargeable NiCd button cells were often components of the backup battery of older computers; non rechargeable lithium button cells with a lifetime of several years are used in later equipment. Rechargeable batteries typically have the same dimension based numeric code with different letters; thus CR2032 is a disposable battery while ML2032, VL2032 and LIR2032 are rechargeables that fit in the same holder if not fitted with solder tags. It is mechanically possible, though hazardous, to fit a disposable battery in a holder intended for a rechargeable; holders are fitted in parts of equipment only accessible by service personnel in such cases.

Button cells are attractive to small children; they may put them in their mouth and swallow them. The ingested battery can cause significant damage to internal organs. The battery reacts with bodily fluids, such as mucus or saliva, creating a circuit which can release an alkali that is strong enough to burn through human tissue. Swallowed batteries can cause damage to the lining of the esophagus, and can create a hole in the esophagus lining in two hours. Coin cells of diameter 20 mm or greater cause the most serious injuries, even if expended and intact. In Auckland, New Zealand as of 2018 there are about 20 cases per year requiring hospitalization. In 2020, Duracell announced that they were coating some of their lithium button cells with a bitterant compound to discourage children from ingesting them. An alternative solution is to design (or litigate) the offending cells, mostly 20mm lithium cells, out of the supply chain. Children most at risk of button battery ingestion are those aged 5 years and under.