Button cell


Button cell
Button, coin, or watch cells

A watch battery or button cell is a small single cell battery shaped as a squat cylinder typically 5 to 12 mm in diameter and 1 to 6 mm high—like a button on a garment, hence the name. Button cells are used to power small portable electronics devices such as wrist watches, pocket calculators, and hearing aids. Some cells larger than the dimensions above are also called button cells, but are less commonly used. Lithium cells are generally similar but somewhat larger; they tend to be called either lithium cells or batteries or coin cells rather than button cells.

Devices using button cells are usually designed to use 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 and hold their charge for a long time if not used. Higher-power devices such as hearing aids, where high capacity is important and low self-discharge less so as the cell will usually be used up before it has time to discharge, may use zinc-air cells which have much higher capacity for a given size, but discharge over 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 hazard of mercury.

Cells have a metal can forming the bottom body, with a circular insulated top cap. The can is the positive and the top the negative terminal.

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, and silver cells are usually specified). Sometimes different cells of the same type and size and specified capacity in mAh are optimised for different loads by using different electrolytes, so that one may have longer service life than the other if supplying a relatively high current.

Contents

Properties of different types

Silver cells may have very stable output voltage until it suddenly drops very rapidly at end of life. This varies for individual types; one manufacturer (Energizer) offers 3 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.

Alkaline batteries are made in the same button sizes as other types, but typically provide less capacity and less stable voltage (it drops gradually in use) than more costly silver oxide or lithium cells. They are often sold as cheap watch batteries to, and sometimes by, people who do not know the difference.[1]

Zinc-air batteries use air as the depolarizer and have much higher capacity than other types (they use air from the atmosphere which does not need to be supplied in the battery). A seal is removed before use to allow air to enter the cell; the cell will then self-discharge in a few weeks even if not used up.

For comparison, a cell of diameter 11.6 mm and height 5.4 mm from one reputable manufacturer has the following properties.[2] In many cases there are several batteries of the same chemistry and size with different capacities and properties; figures listed are merely indicative.

  • Silver: capacity 200 mAh to an end-point of 0.9 V, internal resistance 5–15 ohms, weight 2.3 g
  • Alkaline (manganese dioxide): 150 mAh (0.9), 3-9 ohms, 2.4 g
  • Mercury 200mAh, 2.6 g
  • Zinc-air 620 mAh, 1.9 g

Examining datasheets for a manufacturer's range[2] may find 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 some particular alkaline cell. If the powered equipment requiring a relatively high voltage (e.g., 1.3V) 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 0.9V. If some 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 callipers, in particular, some are specified to require at least 1.25V to operate, others 1.38V.[3][4]

Datasheets for some cheaper cells, particularly alkaline, are not available, so it is not possible to say whether capacities are about the same as for documented types.[5] Discussions on web forums suggest that they can be very poor.[6]

In some ways the size is the most important property of a button cell: cells of different chemistry are to a considerable extent interchangeable. In practice only cells of fairly similar voltages are made in any given size; there is no "CR1154" 3V lithium battery mechanically interchangeable with a 1.5V silver or alkaline size 1154 cell. Use of a battery of significantly higher voltage than equipment is designed for can cause permanent damage, but use of a cell of the right voltage but unsuitable characteristics can only lead to short battery life or failure to operate equipment.

Type designation

LR44 alkaline cell.

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 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 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
L Alkaline Manganese dioxide Alkali Zinc 1.5 1.0
S Silver Silver oxide Alkali Zinc 1.55 1.2
P Zinc-air Oxygen Alkali Zinc 1.4 1.2
C Lithium Manganese dioxide Organic Lithium 3 2.0
B Carbon monofluoride Organic Lithium 3 2.0
G Copper oxide Organic Lithium 1.5 1.2
M,N(withdrawn) Mercury Mercuric oxide Alkaline Zinc 1.35/1.40 1.1

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 it drops extremely rapidly. For types which lose voltage gradually (slope graph, no cliff-edge) the end-point is the voltage beyond which it is deemed that equipment will not work properly, typically 1.0 or 0.9V.

Common names are conventional rather than uniquely descriptive; for example, a cell called a "silver [oxide] cell" rather than "alkaline" actually 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 type "P" are used in hearing aids and medical instruments.

Package size

Several sizes of button and coin cell with 2 9v batteries as a size comparison.

Package size of button batteries using standard names is indicated by a two-digit code representing a standard case size, or a three- or four-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.

Diameter codes (1st 1 or 2 digits)
Number
code
Nominal
diameter (mm)
Tolerance
4 4.8 ±0.15 mm
5 5.8 ±0.15 mm
6 6.8 ±0.15 mm
7 7.9 ±0.15 mm
9 9.5 ±0.15 mm
10 10.0 ±0.20 mm
11 11.6 ±0.20 mm
12 12.5 ±0.25 mm
16 16 ±0.25 mm
20 20 ±0.25 mm
23 23 ±0.50 mm
24 24.5 ±0.50mm

Examples:

  • CR2032: lithium, 20 mm diameter, 3.2 mm height
  • 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:

Also:

  • W; the battery complies with all the requirements of the international IEC 60086-3 [7] standard for watch batteries.
Type CR2032 watch battery (lithium anode, 3 V, 20.0 mm × 3.2 mm)
Leaked and corroded button cell

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.

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 1990 or January 2000, 9Y = November 1999).

Common manufacturer code

A code used by some manufacturers is AG (alkaline) or SG (silver) followed by a number, where 1 equates to standard 621, 2 to 726, 3 to 736, 4 to 626, 5 to 754, 6 to 920 or 921, 7 to 926 or 927, 8 to 1120 or 1121, 9 to 936, 10 to 1130 or 1131, 11 to 721, 12 to 1142, and 13 to 1154. To those familiar with the chemical symbol for silver, "Ag", this may suggest incorrectly that AG cells are silver.

Common applications

Rechargeable variants

Most button cell batteries are not rechargeable due to the inferior capacity and/or high self-discharge rate of rechargeable batteries. There are a few rechargeable button cell batteries, such as lithium-ion rechargeable batteries intended to replace CR-series lithium manganese dioxide batteries in devices with relatively high current consumption or some devices with solar panels which can be used to keep the battery charged under certain circumstances.

Rechargeable NiCd button cells were often components of the backup battery of older IBM PC compatibles as well as Amiga computers.

See also

References

  1. ^ [1] A card with 30 branded Hyundai button cells made in China in 5 sizes, stating that they are alkaline but with pictures of watches, calculators, etc. is sold on a card for prices ranging from about £1 to £4 in the UK
  2. ^ a b Energizer website, with datasheets for many batteries of several chemistries
  3. ^ Buying Button Cells for Digital Calipers
  4. ^ Caliper Battery Life
  5. ^ [2] Alkaline button cells in a range of sizes are sold as made by Hyundai, but no technical information can be found
  6. ^ moneysavingexpert.com discussion reporting very poor performance of cheap button cells
  7. ^ IEC 60086-3 standard for watch batteries

External links


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