Button cell

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.

Properties of cell chemistries

Cells of different chemical composition made in the same size are mechanically interchangeable. However, the composition can affect service life and voltage stability. Using the wrong cell may lead to short life or improper operation (for example, light metering on a camera requires a stable voltage, thus silver cells are usually specified). Sometimes different cells of the same type, size and capacity are optimized for different loads by using different electrolytes, so that one may have longer service life than the other when supplying a relatively high current.

Alkaline batteries are made in the same button sizes as the other types, but typically provide less capacity and less stable voltage than more costly silver oxide or lithium cells.

Silver cells may have an output voltage that is stable until it suddenly drops at end of life. This varies for individual types; one manufacturer (Energizer) offers three silver oxide cells of the same size, 357–303, 357-303H and EPX76, with capacities ranging from 150 to 200 mAh, voltage characteristics ranging from gradually reducing to fairly constant, and some stated to be for continuous low drain with high pulse on demand, others for photo use.

Mercury batteries also supply a stable voltage, but are now banned in many countries due to their toxicity and environmental impact.

Zinc-air batteries use air as the depolarizer and have much higher capacity than other types, as they take that air from the atmosphere. Cells have an air-tight seal which must be removed before use; they will then dry out in a few weeks, regardless of use.

For comparison, the properties of some cells from one manufacturer with diameter 11.6 mm and height 5.4 mm were listed in 2009 as:Energizer website
, with datasheets for many batteries of several chemistries
*Silver: capacity 200 mAh to an end-point of 0.9 V, internal resistance 5–15 Ω, weight 2.3 g
*Alkaline (manganese dioxide): 150 mAh (0.9), 3–9 Ω, 2.4 g
*Mercury: 200 mAh, 2.6 g
*Zinc-air: 620 mAh, 1.9 g

Examining datasheets for a manufacturer's range may show a high-capacity alkaline cell with a capacity as high as one of the lower-capacity silver types; or a particular silver cell with twice the capacity of a particular alkaline cell. If the powered equipment requires a relatively high voltage (e.g., 1.3 V) to operate correctly, a silver cell with a flat discharge characteristic will give much longer service than an alkaline cell—even if it has the same specified capacity in mAh to an end-point of 0.9 V. If a device seems to "eat up" batteries after the original supplied by the manufacturer is replaced, it may be useful to check the device's requirements and the replacement battery's characteristics. For digital calipers, in particular, some are specified to require at least 1.25 V to operate and others 1.38 V.Caliper Battery Life
. Davehylands.com. Retrieved on 2015-11-08.

While alkaline, silver oxide, and mercury batteries of the same size may be mechanically interchangeable in any given device, use of a cell of the right voltage but unsuitable characteristics can lead to short battery life or failure to operate equipment. Common lithium primary cells, with a terminal voltage around 3 volts, are not made in sizes interchangeable with 1.5 volt cells. Use of a battery of significantly higher voltage than equipment is designed for can cause permanent damage.

Type designation

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.

Electrochemical system

The first letter in the IEC standard system identifies the chemical composition of the battery, which also implies a nominal voltage:

For types with stable voltage falling precipitously at end-of-life (cliff-top voltage-versus-time graph), the end-voltage is the value at the "cliff-edge", after which the voltage drops extremely rapidly. For types which lose voltage gradually (slope graph, no cliff-edge), the end-point is the voltage beyond which further discharge will cause damage to either the battery or the device it is powering, typically 1.0 or 0.9 V.

Common names are conventional rather than uniquely descriptive; for example, a ''silver (oxide) cell'' has an alkaline electrolyte.

''L'', ''S'', and ''C'' type cells are today the most commonly used types in quartz watches, calculators, small PDA devices, computer clocks, and blinky lights. Miniature zinc-air batteries – ''P'' type – are used in hearing aids and medical instruments. In the IEC system, larger cells may have no prefix for the chemical system, indicating they are zinc-carbon batteries; such types are not available in button cell format.

The second letter, ''R'', indicates a round (cylindrical) form.

The standard only describes primary batteries. Rechargeable types made in the same case size will carry a different prefix not given in the IEC standard, for example some ''ML'' and ''LiR'' button cells use rechargeable lithium technology.

Package size

Package size of button batteries using standard names is indicated by a 2-digit code representing a standard case size, or a 3- or 4-digit code representing the cell diameter and height. The first one or two digits encode the outer diameter of the battery in whole millimeters, rounded down; exact diameters are specified by the standard, and there is no ambiguity; e.g., any cell with an initial ''9'' is 9.5 mm in diameter, no other value between 9.0 and 9.9 is used. The last two digits are the overall height in tenths of a millimeter.

Examples:
*CR2032: lithium, 20 mm diameter, 3.2 mm height, 220 mAh
*CR2032H; lithium, 20 mm diameter, 3.2 mm height 240 mAh
*CR2025: l