Button cell

A button cell, watch battery, or coin battery is a small battery made of a single electrochemical cell and 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 electronic devices such as wrist watches, pocket calculators, and remote key fobs. 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.
Button cells are usually disposable primary cells, but some are rechargeable secondary cells. Common chemistries include zinc, lithium, manganese dioxide, and silver oxide. 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. 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 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 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:

Silver: capacity 200 mAh to an end-point of 0.9 V, internal resistance 5–15 Ω, weight 2.3 g
Alkaline : 150 mAh, 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 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.
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

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:

Letter code	Common name	Positive electrode	Electrolyte	Negative electrode	Nominal voltage	End-point voltage	Maximum voltage	Comments
-	Zinc-carbon	Manganese dioxide	Zinc chloride	Zinc	1.5	?	1.72
L	Alkaline	Manganese dioxide	Alkali	Zinc	1.5	1.0	1.65	Much higher capacity than zinc-carbon.
S	Silver	Silver oxide	Alkali	Zinc	1.55	1.2	1.63	SR synonymous to AG. Much flatter discharge curve than L. Slightly higher nominal capacity, much higher effective capacity for devices with higher cutoff voltage.
P	Zinc-air	Oxygen	Alkali	Zinc	1.4	1.2	1.68	High capacity. Activated by removing seal from vent. Electrolyte can dry out.
C	Lithium	Manganese dioxide	Organic	Lithium	3	2.0	3.7	Very common. Gradual loss of voltage with discharge.
B		Carbon monofluoride	Organic	Lithium	3	2.0	3.7	Similar to C, with better performance and lower self-discharge at higher temperatures; lower initial voltage, flatter discharge curve.
G		Copper oxide	Organic	Lithium	1.5	1.2	2.3
Z	Nickel oxyhydroxide	Manganese dioxide, nickel oxyhydroxide	Alkali	Zinc	1.5	?		-
E	Thionyl chloride	Thionyl chloride	Organic	Lithium	3.6	?	3.9
F	Iron sulfide	Iron disulfide	Organic	Lithium	1.5	?	1.83	Low temperature, high capacity
M, N	Mercury	Mercuric oxide	Alkali	Zinc	1.35/1.40	1.1	?	-

For types with stable voltage falling precipitously at end-of-life, the end-voltage is the value at the "cliff-edge", after which the voltage drops extremely rapidly. For types which lose voltage gradually, the end-point is the voltage beyond which further discharge will cause damage to the battery and possibly the device it is powering, typically 1.0 or 0.9 V.
Common names are conventional rather than uniquely descriptive; for example, a silver 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 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.
For rechargeables, the IEC prefixes are:

H - alloy-nickel oxide with aqueous electrolyte, 1.2V
K - cadmium-nickel oxide with aqueous electrolyte, 1.2V
PB - lead-lead dioxide with sulfuric acid electrolyte, 2V
IC - lithium-cobalt oxide with organic electrolyte, 3.8V
IN - lithium-nickel oxide with organic electrolyte, 3.8V
IM - lithium-manganese oxide with organic electrolyte, 3.8V

Prefix	Rechargeable	Chemistry	Nominal voltage	Comments
LPM, LiR, RJD, RCR	yes	Li-ion	3.7	sometimes called "rechargeable CR"; higher voltage than CR can cause damage when used as direct replacement, may be lowered with a serial diode or regulator
ML	yes	LiMnO2	3	"manganese"; -20..+60 °C, 2.8..3.2V charging; common for long-term memory/RTC backups; plateaus at 2.5V
MS	yes	LiMnSi	3	"manganese silicon"; -20..+60 °C, 2.8..3.3V charging; better charge/discharge characteristics than VL or ML; common for memory/RTC backups
VL	yes	LiV2O5	3	"vanadium", eg, VL1220, VL2020, VL2330, VL3032; -20..+60 °C, 3.25..3.55V charging; lower voltage drop on load
CTL	yes	CoTiLi	2.3	"cobalt titanium"; -20..+60 °C, 2.5..2.7V charging; common in solar watches, sometimes called "watch capacitor"
MT	yes	LiMnTi	1.5	"manganese titanium"; -10..+60 °C, 1.8..2.6V charging
V, MH	yes	NiMH	1.2	can replace LR/SR cells if lower voltage than 1.5V is not a problem
TL	no	LiSoCl2	3.6	"thionyl chloride"; for low temperatures