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
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:| 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 |
Package size
The package size of button batteries can be indicated by a 2-digit code representing a standard case size, or a 3- or 4-digit code representing the cell diameter and height, where 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, since no other value between 9.0 and 9.9 is used. The last two digits are the overall height in tenths of a millimeter.| Number code | Nominal diameter | Tolerance |
| 4 | 4.8 | ±0.15 |
| 5 | 5.8 | ±0.15 |
| 6 | 6.8 | ±0.15 |
| 7 | 7.9 | ±0.15 |
| 9 | 9.5 | ±0.15 |
| 10 | 10.0 | ±0.20 |
| 11 | 11.6 | ±0.20 |
| 12 | 12.5 | ±0.25 |
| 16 | 16.0 | ±0.25 |
| 20 | 20.0 | ±0.25 |
| 23 | 23.0 | ±0.50 |
| 24 | 24.5 | ±0.50 |
| 44 | 11.6 | ±0.20 |
Examples:
- CR2032: lithium, 20 mm diameter, 3.2 mm height, 220 mAh
- CR2032H; lithium, 20 mm diameter, 3.2 mm height 240 mAh
- CR2025: lithium, 20 mm diameter, 2.5 mm height, 170 mAh
- SR516: silver, 5.8 mm diameter, 1.6 mm height
- LR1154/SR1154: alkaline/silver, 11.6 mm diameter, 5.4 mm height. The two-digit codes LR44/SR44 are often used for this size
Letter suffix
After the package code, the following additional letters may optionally appear in the type designation to indicate the electrolyte used:- P: potassium hydroxide electrolyte
- S: sodium hydroxide electrolyte
- No letter: organic electrolyte
- SW: low drain type for quartz watches without light, alarm, or chronograph functions
- W: high drain type for all quartz watches, calculators and cameras. The battery complies with all the requirements of the international IEC 60086-3 standard for watch batteries.
Other package markings
Apart from the type code described in the preceding section, watch batteries should also be marked with- the name or trademark of the manufacturer or supplier;
- the polarity ;
- the date of manufacturing.
Date codes
Often a 2-letter code 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.
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.
Common manufacturer code
A code used by some manufacturers is AG or SG followed by a number, as follows| G code | IEC code |
| xG0 | 521 |
| xG1 | 621 |
| xG2 | 726 |
| xG3 | 736 |
| xG4 | 626 |
| xG5 | 754 |
| xG6 | 920 or 921 |
| xG7 | 926 or 927 |
| xG8 | 1120 or 1121 |
| xG9 | 936 |
| xG10 | 1130 or 1131 |
| xG11 | 721 |
| xG12 | 1142 |
| xG13 | 1154 |
To those familiar with the chemical symbol for silver, Ag, this may suggest incorrectly that AG cells are silver.
Rechargeable variants
In addition to disposable 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 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 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.
Health issues
Accidental ingestion
[Image:Keep-out-of-reach-of-children.svg|thumb|170px|KEEP OUT OF REACH OF CHILDREN icon required by IEC 60086-4]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. In severe cases, damage can cause a passage between the esophagus and the trachea. Swallowed button cells can damage the vocal cords. They can even burn through the blood vessels in the chest area, including the aorta. In the United States, 44 child deaths were reported from button battery ingestion in 2002–2021. Ingestions are treated initially with honey or sucralfate as a temporizing measure, with endoscopic removal as definitive treatment.
In Greater Manchester, England, with a population of 2,700,000, two children between 12 months and six years old died, and five suffered life-changing injuries, in the 18 months leading up to October 2014. In the United States, on average, over 3,000 pediatric ingestions of button batteries are reported annually. The proportion of major and fatal outcomes is increasing. 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 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. Three child deaths in Australia reveal that in each case: i) the ingestion was not witnessed, ii) the source of the battery remains unknown, iii) initial misdiagnosis delayed appropriate intervention, iv) diagnosis was confirmed by X-ray, v) in each case the battery lodged in the child's oesophagus, vi) the offending batteries were 20mm lithium cells, vii) death occurred 19 days to 3 weeks after ingestion. The presenting symptoms of button cell ingestion may be misdiagnosed and attributed to common non life-threatening childhood maladies.