Button Cell
Small battery
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Button, Coin, or Watch Cells
A button cell, also known as a watch battery or coin battery, is a small single-cell battery designed like a squat cylinder, typically measuring between 5 to 25 mm in diameter and 1 to 6 mm in height, resembling a button. The bottom part of the cell, usually made of stainless steel, serves as the positive terminal. The insulated metallic top cap acts as the negative terminal.
Button Cells Use in RTC Modules as Power Source
Button cells are used to power small portable electronics such as watches and pocket calculators. Larger variants are often referred to as coin cells. Devices using button cells are typically designed for a long service life, often exceeding a year in continuous use in watches. These cells have a low self-discharge rate, making them reliable over long periods if unused. However, zinc–air batteries, used in high-power devices like hearing aids, offer a higher capacity but dry out within weeks, even if not in use.
Button cells are usually single-use primary cells with common anode materials being zinc or lithium and cathode materials including manganese dioxide, silver oxide, carbon monofluoride, and cupric oxide. Mercuric oxide button cells were common in the past but are no longer available due to toxicity concerns.
Button cells pose a significant danger to small children if swallowed, as they can cause severe internal burns, leading to injury or death. Duracell, for instance, has added a bitter coating to their batteries to mitigate this risk.
Properties of Cell Chemistries
Cells made from different chemical compositions but in the same size are mechanically interchangeable. However, the chemical makeup affects the service life and voltage stability. Using the wrong type of cell can result in a short lifespan and improper functioning. For instance, stable voltage is crucial for light metering in cameras, for which silver cells are usually preferred.
Different types of cells, like alkaline, silver oxide, and lithium, have their unique properties and uses. Silver cells, for instance, may maintain a stable voltage until they suddenly drop at end of life, which varies among different types and manufacturers. Mercury batteries, although supplying stable voltage, are banned in many countries due to their toxicity.
Zinc-air batteries use air as a depolarizer, offering higher capacity but drying out within weeks once the air-tight seal is removed. Comparing properties of some cells with an 11.6 mm diameter and 5.4 mm height shows varied specifications in terms of internal resistance, weight, and capacity.
Type Designation
The international standard IEC 60086-3 defines a coding system for "Watch batteries". Different manufacturers have their naming systems, but types like LR44, AG13, and 357 are commonly known. IEC and some other standards encode the cell size uniquely.
Electrochemical System
The IEC standard system uses letters to identify the chemical composition of the battery. For example, a silver (oxide) cell has an alkaline electrolyte. Common types like L, S, and C are used in watches, calculators, and small PDA devices. Rechargeable versions carry different prefixes like ML or LiR, using rechargeable lithium technology.
Package Size
The package size of button batteries is indicated by a numeric code representing the battery's diameter and height. For example, CR2032 indicates a lithium battery with a 20 mm diameter and 3.2 mm height.
Examples:
- CR2032: Lithium, 20 mm diameter, 3.2 mm height, 220 mAh
- SR516: Silver, 5.8 mm diameter, 1.6 mm height
- LR1154/SR1154: Alkaline/Silver, 11.6 mm diameter, 5.4 mm height
Some coin cells, especially lithium ones, are made with solder tabs for permanent installation in devices.
Letter Suffix
Additional letters may appear in the type designation to indicate the electrolyte used:
- P: Potassium hydroxide electrolyte
- S: Sodium hydroxide electrolyte
- SW: Low drain type for quartz watches
- W: High drain type for quartz watches, calculators, and cameras
Rechargeable Variants
In addition to disposable button cells, rechargeable batteries in similar sizes are available but typically with lower capacity. These can be used in battery holders if the voltage is compatible. They are commonly used to back up the settings of equipment which is usually mains-powered, such as central heating controllers.
Rechargeable batteries generally have different letter codes but share the same numeric codes as their disposable counterparts. For instance, CR2032 is a disposable battery while ML2032, VL2032, and LIR2032 are rechargeable variants.
Health Issues
Accidental Ingestion
Button cells are particularly dangerous to young children if swallowed. They can cause severe internal burns and damage within hours of ingestion. Cases have been reported where children suffered life-changing injuries, or even death, from swallowing button batteries.
Duracell has added a bitterant compound to some of their lithium button cells to discourage ingestion. However, the best preventive measure is to keep button cells out of reach of children.
Mercury and Cadmium
Some button cells contain mercury or cadmium, which are toxic. Efforts have been made globally to ban the import and export of mercury-containing products. For instance, the European Parliament Environment Committee voted for such a ban, effective from 2020.
Common Manufacturer Code
Manufacturers often use codes like AG for alkaline and SG for silver cells, followed by numbers to distinguish different sizes and types.
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