Button cell
A button cell, also known as a coin cell, is a small, round, flat electrochemical cell characterized by a diameter greater than its height, typically measuring less than 25 mm across and under 5 mm thick, with stacked electrodes rather than radial configuration.[1][2] These batteries provide reliable, compact power sources with high output-to-mass ratios, enabling their use in portable electronics such as watches, calculators, remote controls, and hearing aids since the 1980s.[3][4] Common chemistries include alkaline (1.5 V nominal voltage), silver oxide (1.55 V), and lithium-based variants (3 V), with sizes standardized by designations like CR2032—indicating lithium chemistry, 20 mm diameter, and 3.2 mm height—or LR44 for alkaline types.[5][6] Button cells are primarily disposable, though rechargeable versions exist, and their lithium formulations offer superior energy density but introduce significant safety risks, particularly ingestion hazards for children, where lodged batteries can generate hydroxide ions causing rapid esophageal necrosis and potentially fatal injuries within two hours.[6][7][8] Earlier mercury-containing types have been largely phased out due to toxicity concerns, reflecting shifts toward safer chemistries amid regulatory pressures.[2]History
Invention and early adoption
The mercury button cell, the earliest form of the button cell battery, was developed in 1942 by American inventor Samuel Ruben in collaboration with P.R. Mallory & Co. This zinc-mercuric oxide primary cell featured a compact, cylindrical design with mercuric oxide as the cathode and zinc as the anode, separated by an alkaline electrolyte, delivering a stable 1.35 V output suitable for low-drain applications.[9] The innovation addressed prior limitations in miniature power sources by providing consistent voltage across temperature variations, including tropical conditions, through the use of mercury oxide's electrochemical properties.[10] Commercialized by Mallory shortly after its invention, the mercury button cell saw initial adoption in military and portable electronics during World War II, where its shelf-stable nature—retaining charge for years without leakage—proved advantageous for devices requiring reliability in harsh environments.[9] By the late 1940s and early 1950s, it transitioned to civilian uses, powering early hearing aids and small radios, as its sub-miniature size (diameters as small as 5-10 mm) enabled integration into previously battery-constrained devices.[10] Widespread early adoption accelerated in the 1950s with the rise of transistorized electronics, including pocket calculators and wristwatches, where the cell's flat discharge curve ensured precise timing and operation over extended periods.[11] Production scaled through licensees like Eveready and Union Carbide, with annual output reaching millions by the mid-1950s, though environmental concerns over mercury content would later prompt phase-outs starting in the 1970s.[12]Evolution of electrochemical systems
The earliest electrochemical systems in button cells relied on mercuric oxide-zinc chemistry, which provided a stable nominal voltage of 1.35 V and a flat discharge profile suitable for precision devices like watches and hearing aids. These cells, developed in the late 1940s, dominated early applications due to their reliability and long shelf life, with mercury oxide serving as the cathode and zinc as the anode in an alkaline electrolyte.[13] By the 1960s and 1970s, environmental and health concerns over mercury toxicity prompted the development of alternatives, including silver oxide-zinc systems. Silver oxide batteries, offering 1.55 V and approximately twice the energy density of alkaline equivalents, were commercialized in Japan by Maxell in 1976 for small electronics, featuring silver oxide cathodes that maintained stable voltage under low-drain conditions.[14] Alkaline manganese dioxide-zinc chemistries emerged concurrently as cost-effective options, with button cells like the LR44 providing 1.5 V but exhibiting a more sloped discharge curve compared to mercury types.[15] Lithium-based primary cells marked a significant advancement in the 1970s, introducing higher energy densities (up to 3 V nominal) and longer service life for intermittent high-drain uses. Early lithium button cells, such as those with lithium anodes and manganese dioxide or thionyl chloride cathodes, were adopted for watches and calculators, with metallic lithium enabling self-discharge rates below 1% per year.[16] These systems prioritized safety through non-aqueous electrolytes, reducing corrosion risks inherent in aqueous chemistries.[13] Regulatory pressures accelerated the phase-out of mercury-containing button cells; U.S. manufacturers voluntarily committed to eliminating mercury by 2011, though many markets transitioned earlier via substitutes like silver oxide and lithium variants to comply with toxicity standards.[17] This shift favored lithium chemistries for their superior volumetric efficiency—often exceeding 300 Wh/L in modern iterations—and prompted innovations in cathode materials, such as lithium iodine for medical implants, enhancing longevity in low-power applications.[18] Despite these advances, legacy devices originally designed for mercury's precise voltage profile sometimes required voltage adapters or recalibration when retrofitted with higher-voltage alternatives.[19]Chemistry and Types
Primary cell chemistries
Primary button cell chemistries encompass alkaline manganese dioxide, silver oxide, lithium-based systems such as lithium manganese dioxide and lithium carbon monofluoride, and zinc-air configurations. These variants provide nominal voltages ranging from 1.4 V to 3 V, with differences in energy density, discharge stability, and application suitability driven by their electrochemical reactions. Alkaline and silver oxide types dominate low-voltage (1.5 V) needs, while lithium offers higher voltage and longevity, and zinc-air excels in capacity for air-permeable designs.[20][1] Alkaline button cells employ a zinc anode, manganese dioxide cathode, and alkaline electrolyte, yielding a nominal 1.5 V with capacities of 50–150 mAh in typical sizes like LR44. They suit moderate-drain devices such as calculators and toys but show voltage decline under load compared to alternatives.[1][21] Silver oxide cells utilize zinc anodes paired with silver oxide cathodes, maintaining a stable 1.55 V discharge profile superior to alkaline for precision low-drain uses like wristwatches and medical instruments. Capacities align closely with alkaline equivalents, around 150–200 mAh, but with flatter voltage curves enhancing reliability in voltage-sensitive circuits.[22][14] Lithium primary button cells feature metallic lithium anodes with cathodes like manganese dioxide (CR series) or carbon monofluoride, delivering 3 V nominal voltage, high energy densities up to 700 Wh/kg, and shelf lives exceeding 10 years. Common examples include the CR2032 with 220 mAh capacity, ideal for real-time clocks, key fobs, and sensors requiring consistent power over extended periods.[20][23] Zinc-air button cells oxidize zinc anodes using atmospheric oxygen at porous cathodes, providing 1.4 V and high practical capacities (e.g., 500–600 mAh in hearing aid sizes) after tab removal activates oxygen access. Primarily for hearing aids, they offer extended runtime but limited to applications tolerant of activation delay and humidity sensitivity.[24][25]Rechargeable variants
Rechargeable button cells, also known as secondary coin cells, employ chemistries such as lithium-ion (Li-ion) and nickel-metal hydride (NiMH), enabling multiple charge-discharge cycles unlike primary variants.[26] These cells typically maintain nominal voltages of 3.6–3.7 V for Li-ion types (charged to 4.2 V) or 1.2 V for NiMH, with capacities ranging from 29 mAh to 145 mAh depending on size, such as the ML1220 or LIR2032 equivalents.[27] Manufacturers like VARTA, Panasonic, and Maxell produce these for applications requiring longevity without frequent replacement, including real-time clocks (RTCs) in electronics and sensors.[28] Li-ion rechargeable coin cells, often designated with "LIR" or "ML" prefixes, utilize cathodes like lithium manganese dioxide or lithium cobalt oxide, paired with lithium-based anodes, to achieve up to 500–1000 cycles before capacity drops to 80% of initial value.[26] [28] For instance, VARTA's CoinPower series offers low internal resistance (<100 mΩ) and minimal self-discharge (under 3% per month at 20°C), supporting stable performance in compact devices.[26] Panasonic's coin-type rechargeables exhibit low self-discharge rates and operate across -20°C to 60°C, with cycle lives exceeding 500 charges under specified conditions.[27] These cells require dedicated chargers to prevent overvoltage, as improper charging can lead to thermal runaway, though built-in protections mitigate risks in quality units.[29] NiMH and nickel-cadmium (NiCd) rechargeable button cells, less prevalent in modern coin formats, provide 1.2 V output and capacities around 20–50 mAh for sizes like the ML621, but suffer higher self-discharge (up to 20% monthly) and memory effect compared to Li-ion alternatives.[30] Overall, rechargeable variants offer 20–50% lower energy density than primary lithium cells (e.g., CR2032 at ~220 mAh vs. LIR2032 at ~65–80 mAh), prioritizing reusability over raw capacity for cost efficiency in low-drain uses.[31] Users must avoid charging primary cells, as this poses explosion or fire hazards due to incompatible chemistries.[32]Design and Standards
Physical properties and size designations
Button cells possess a compact, squat cylindrical form with flat, parallel end faces, distinguishing them from taller cylindrical batteries; the overall height is typically less than the diameter, enabling their "coin-like" profile. The casing consists of thin stainless steel or nickel-plated steel, providing structural integrity, corrosion resistance, and electrical contact, with the top (positive) surface often serving as one terminal and the bottom (negative) as the other, sealed to prevent electrolyte leakage. These cells lack protruding terminals, relying on direct surface contact for electrical connection, and their lightweight construction—typically 1 to 4 grams depending on size and chemistry—facilitates integration into small devices.[33][34] Dimensions adhere to International Electrotechnical Commission (IEC) standard 60086-1, which specifies nomenclature and physical tolerances for primary batteries, including round types. Common diameters range from 4.8 mm to 30 mm, while heights vary from 1.0 mm to 7.7 mm, with tolerances of ±0.2 mm for diameter and ±0.15 mm for height in most sizes to ensure interchangeability. Larger variants, such as those up to 25 mm in diameter, are suited for higher-capacity applications, whereas thinner cells (e.g., 1-2 mm height) prioritize minimal space.[35][36] Size designations follow IEC and ANSI/NEDA conventions, encoded in a alphanumeric string denoting chemistry, shape, and dimensions for precise identification. For lithium manganese dioxide primary cells, the "CR" prefix indicates "C" for the lithium-based chemistry (specifically 3 V nominal with manganese dioxide cathode) and "R" for round (button) shape; subsequent digits specify diameter in millimeters (first one or two digits) followed by height in tenths of a millimeter (last two digits). Examples include CR2032 (20 mm diameter, 3.2 mm height) and CR1220 (12 mm diameter, 2.0 mm height). Alkaline variants use "LR" (L for alkaline, R for round), such as LR44 (11.6 mm diameter, 5.4 mm height). These codes ensure compatibility across manufacturers, with deviations rare and noted in datasheets.[37][38][39]| Designation | Chemistry Prefix | Diameter (mm) | Height (mm) | Typical Capacity (mAh) |
|---|---|---|---|---|
| CR2032 | CR (Lithium/MnO₂) | 20 | 3.2 | 220-240 |
| CR1220 | CR (Lithium/MnO₂) | 12 | 2.0 | 35-50 |
| LR44 | LR (Alkaline) | 11.6 | 5.4 | 110-130 |
| SR626SW | SR (Silver Oxide) | 6.8 | 2.6 | 25-30 |
Coding and marking conventions
Button cells follow nomenclature defined in IEC 60086-3, which specifies alphanumeric designations for watch batteries, including chemistry, shape, and dimensions. The code begins with a letter indicating the electrochemical system—such as "L" for alkaline zinc-manganese dioxide, "S" for silver oxide, "C" for lithium primary, or "P" for zinc-air—followed by "R" denoting the round, flat button shape.[6] Subsequent digits represent size: typically two digits for diameter in millimeters, followed by two digits for height in tenths of a millimeter, as in CR2032 (20 mm diameter, 3.2 mm height).[39] For certain standard sizes, particularly smaller watch batteries like the LR44 series, the numerical portion uses a two-digit size code corresponding to specific dimensions, such as approximately 11.6 mm diameter and 5.4 mm height for the "44" designation.[21] These codes ensure interchangeability across manufacturers adhering to IEC 60086-1, which standardizes dimensions and performance. Markings on button cells, mandated by IEC 60086-1, include the type designation, nominal voltage (e.g., 1.5 V for alkaline or silver oxide, 3 V for lithium), polarity indicators (+ on the flat face, - on the bottom), and the manufacturer's name or symbol. Additional markings may feature production date codes (e.g., Julian date) or capacity in mAh, though not always required. Lithium variants often include handling warnings due to risks, but core markings prioritize identification and safe orientation in devices.[40]| Code Prefix | Chemistry | Typical Voltage |
|---|---|---|
| LR | Alkaline | 1.5 V |
| SR | Silver oxide | 1.55 V |
| CR | Lithium primary | 3 V |
| PR | Zinc-air | 1.4 V |
Applications and Performance
Common uses in devices
Button cells power a variety of low-drain, compact consumer electronics where space constraints and longevity are critical. Their flat, disc-like form factor enables integration into slim designs, while stable voltage delivery—typically 1.5 V for alkaline or silver-oxide types and 3 V for lithium variants—supports intermittent or continuous operation over extended periods, often 1–5 years depending on chemistry and load.[4][41] In timepieces, button cells are standard for quartz wristwatches and small clocks, providing precise, low-power oscillation for movements that require minimal current (around 1–5 μA). Silver-oxide cells, such as SR44, dominate here due to their high capacity (up to 200 mAh) and flat discharge curve, ensuring consistent accuracy over the battery's life.[41][4] Hearing aids frequently employ zinc-air button cells (e.g., PR41 or 312 size), which activate upon exposure to air for oxygen cathode reactions, delivering high energy density (up to 600 mAh) in a biocompatible package suited for in-ear placement. These cells support amplification circuits drawing 0.5–2 mA during use, with sealed tabs preventing premature activation.[41][42] Portable computing aids like pocket calculators use alkaline manganese dioxide button cells (e.g., LR44), which handle sporadic high-pulse demands for display and processing at currents up to 10 mA, while idling at microamp levels for memory retention.[41][4] In computing hardware, lithium coin cells (e.g., CR2032) act as CMOS backup for motherboards and real-time clock (RTC) modules, supplying 1–3 μA to preserve BIOS settings, date/time data, and volatile memory during main power shutdowns, often lasting 5–10 years.[43] Vehicle and access devices, including key fobs and keyless entry remotes, incorporate 3 V lithium cells for radio frequency transmission and encryption, enduring infrequent bursts (10–50 mA for milliseconds) with shelf lives exceeding 10 years.[41] Additional applications encompass digital thermometers, bathroom scales, and cameras for sensor powering and metering; laser pointers and small toys for intermittent illumination; and select medical devices like glucose monitors for low-power metering.[44][41][43]Advantages, limitations, and performance metrics
Button cells provide several advantages stemming from their compact, disc-shaped form factor, which measures typically 5–25 mm in diameter and 1–6 mm in thickness, enabling integration into slim devices like wristwatches, remote controls, and medical implants without significantly increasing overall volume.[45] Lithium-based primary cells, such as CR series, deliver a stable nominal voltage of 3 V—higher than the 1.5 V of alkaline equivalents—while maintaining flat discharge profiles for consistent performance in low-drain circuits.[46] Their long shelf life, often exceeding 10 years with self-discharge rates under 1% per year, minimizes premature capacity loss during storage, making them ideal for intermittently used or backup applications.[47][48] Despite these benefits, button cells face limitations in capacity and power delivery, with typical ratings of 20–250 mAh restricting them to devices drawing less than 1–3 mA continuously, as higher currents cause rapid voltage drop and reduced efficiency.[49] Primary chemistries like lithium manganese dioxide (CR) or silver oxide are non-rechargeable, necessitating frequent replacements in moderate-use scenarios and contributing to e-waste accumulation.[50] Environmental concerns arise from materials such as lithium, manganese, and mercury traces in older alkaline variants (LR), which pose recycling challenges despite low overall volume per cell.[51] Rechargeable options, like nickel-metal hydride (NiMH), suffer from lower energy density and higher self-discharge, limiting their practicality compared to primaries.[52] Performance metrics vary by chemistry and size, but common benchmarks include:| Chemistry | Nominal Voltage (V) | Typical Capacity (mAh, e.g., CR2032) | Energy Density (Wh/kg) | Operating Temperature Range (°C) | Self-Discharge Rate (%/year) |
|---|---|---|---|---|---|
| Lithium Manganese Dioxide (CR) | 3.0 | 220–240 | 200–300 | -30 to +60 | <1 |
| Alkaline (LR) | 1.5 | 130–160 | 100–150 | -20 to +54 | 2–5 |
| Silver Oxide (SR) | 1.55 | 150–200 | 150–250 | -10 to +60 | <3 |