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This seems like a very common voltage. I remember older batteries used more round numbers like 1.5V or 3V or 9V.

3.7V seems very arbitrary but seems to be everywhere.

oshkosh
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    @DKNguyen's answer is correct - But note that 3.6V or 3.7V is the AVERAGE voltage of LiIon (Lithium Ion) cells across their discharge range. They are at 4.2V when fully charged, about 3V when essentially fully depleted and about 2.5V when dangerously depleted. – Russell McMahon Apr 05 '21 at 05:32
  • Discussion on roundness has been moved to chat where it may be continued if desired. – Russell McMahon Apr 05 '21 at 23:42
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    @RussellMcMahon Note further that batteries with "round numbers like 1.5V or 3V or 9V" also emit a range of voltages over their charge cycle and lifetime. – Tim Sparkles Apr 06 '21 at 19:04
  • @Tim Sparkles I agree. And most people are unaware of the range and factors concerned. | eg A zinc carbon can be reliably distinguished by an Alkaline by terminal voltage when new.| And charging voltages depend on. Hemiatry Nd rate and ... . Eg NiMH at C/ 10 or less and under say 30 C are charged at 1.45 V. And ... . – Russell McMahon Apr 06 '21 at 21:20

3 Answers3

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The voltage is dependent on the materials used in the cell chemistry and cannot be chosen per se.

3.7V is typical for Lithium chemistries. Similarly, 1.2V is typical for nickel-cadmium (NiCd), 1.2V for nickel-metal hydride (NiMh), 1.5V for alkaline batteries, and 2V for lead-acid.

Notice that 3V and 9V are multiples of 1.5V. 3V and 9V are not customized values simply because they are round figures. They have been chosen as such since they are batteries constructed from stacks of alkaline cells. The same is true for 6V and 12V lead-acid batteries made from stacks of 2V cells.

So, why are there so many lithium batteries today? They are rechargeable, have no memory, and have superior qualities with respect to energy, power density, power volume, and weight compared to the other forms of batteries mentioned above.

NOTE: Memory is a behavior that some battery chemistries display wherein their capacities decrease every time you recharge them if you are not properly maintaining them or discharging them to near-empty before recharging them. Think of it like letting the old gas at the bottom of your gas tank congeal and solidify and then filling it up again. You get less gas into and out of it every time.

Aaron John Sabu
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DKNguyen
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    I think NiCad cells are (were?) closer to 1.2 volts. – Peter Bennett Apr 05 '21 at 03:57
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    @PeterBennett Were they? i can't remember lol. It's been forever. – DKNguyen Apr 05 '21 at 03:57
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    Yeah, 1.5V is about what older zinc-carbon batteries produce. – gidds Apr 05 '21 at 12:15
  • @PeterBennett I certainly hope "were"; I would hate to see people still manufacturing them! – Hearth Apr 05 '21 at 14:45
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    @Hearth Not really, NiCd batteries are still ubiquitous in applications where you need a battery to survive all kinds of abuse and/or high discharge rates yet you cannot afford the weight of lead-acid. Aircraft (starter) batteries are a prime example. – TooTea Apr 05 '21 at 19:05
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    @Hearth Dewalt drills still use them. Bought a huge replacement pack last year, – DKNguyen Apr 05 '21 at 19:07
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    @TooTea I though NiMH had approximately equal performance to NiCd, but if they're really better I suppose limited use is tolerable if they're well recycled. – Hearth Apr 06 '21 at 01:05
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    NiCad can actually be refreshed rather easily, but it isn't common knowledge. NiMh can't be refreshed, nor can Li-Ion. A properly maintained NiCad can last much longer than contemporary replacements, and can handle deep discharge better than both Li-Ion and NiMh. – Nelson Apr 06 '21 at 04:42
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    Another popular battery is LiFePO4, they have a nominal voltage of 3.2V, it allows for high power output and ages slower than a lithium-ion polymer battery (LiPo, LCO) – Ferrybig Apr 06 '21 at 07:37
  • @DKNguyen Can you add an explanation of what "[they] have no memory" means in this context? For us learning laymen out here – TCooper Apr 07 '21 at 18:31
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    @TCooper Added. – DKNguyen Apr 07 '21 at 19:30
  • @DKNguyen Thanks! I was familiar with the concept of losing charge capacity over time, didn't know the term was memory – TCooper Apr 08 '21 at 17:14
  • @TCooper Well, there is loss of charge capacity due to just regular degradation and then there is that due to memory. – DKNguyen Apr 08 '21 at 18:27
  • @DKNguyen So battery memory specifically results from improper (or at least not optimal) use of the battery, rather than 'normal wear and tear'? And Li eliminates that concern, only degrading on a base line despite its use cases? – TCooper Apr 08 '21 at 18:51
  • @TCooper Yeah. Sometimes it's reversible too. You can't reverse normal battery degradation. – DKNguyen Apr 08 '21 at 18:53
  • @TCooper LiIon actually has one of the worst type of memory degradation: time. It will simply decay by minimum 25% every 2 years by just existing. Temperature, storage charge level, and obviously usage, all accelerate that decay rate. No Li-Ion battery lasts more than 6-8 years, with many failing within the 1-2 year range due to misuse (deep discharge, overcharging). – Nelson Apr 29 '21 at 07:58
  • @Nelson Really? Are you sure about this? No one seems to talk about memory with Lithium Ions like they did with NiCds. Note that memory here is different than just regular degradation of capacity from wear or shelf-life. I always attributed the lifetime of a Lithium Ion to the fact that the device being discussed is almost always a smartphone or laptop which undergoes much more frequent charge cycles. I don't know of any other device that undergoes a full charge cycle every day, or sometimes multiple charge cycles per day for years. – DKNguyen Apr 29 '21 at 18:12
  • @DKNguyen The information is quite widely available on google, but this site seems to have a lot of general information: https://batteryuniversity.com/learn/article/how_to_prolong_lithium_based_batteries NiCads have memory effect when charged, so if you charge a battery, it will lose it relatively quickly compared to other types. Li-Ion doesn't have THAT specific effect, but it definitely degrades beyond improper use. – Nelson Apr 30 '21 at 02:48
  • @Nelson That link specifically says there is no memory effect which is degradation caused by recharging a battery before it has been sufficiently discharged. I am referring specifically to your comment "LiIon actually has one of the worst type of memory degradation: time" – DKNguyen Apr 30 '21 at 02:49
  • The "memory effect" is something very specific. If a NiCad does not get fully discharged before charging, then its capacity will be reduced. Li-Ion doesn't have that specific problem, but it has other, even worse problems that require the battery to have a dedicated safety circuit. Raw Li-Ion batteries are very dangerous, but you have to go out of your way to get those. However, Li-Ion batteries are several times larger capacity than NiCad in the same size, even after the safety circuit. I highly recommend reading on Li-Ion batteries because they definitely have flaws. – Nelson Apr 30 '21 at 02:55
  • @Nelson Re-read your sentence... You said "Li-Ions actually has [...] memory degradation". – DKNguyen Apr 30 '21 at 03:00
  • @DKNguyen If you define "memory effect" as simply "loss of capacity" (which is what most people I talk to understood it as), then my use of it is appropriate. If you specifically define "memory effect" as "the damaging of a battery due to recharging before deep discharging", then no, Li Ion does not have that. The Li Ion battery still will lose max capacity due to time and use, but then the counter of "I thought it doesn't have a memory effect?" tells me they don't know what the memory effect actually is. – Nelson Apr 30 '21 at 03:08
  • If you deep discharge a Li Ion battery (drain it to 0% before recharging), that is actually bad for the battery, but the safety circuit prevents immediate damage. If you deep discharge it, then not use it for a couple months, resulting in natural charge loss over time, then the battery is permanently damaged and the safety circuit will prevent it from charging again. The exact maintenance needed for both types of batteries are drastically different, and you don't do the same thing to keep the battery in good health. – Nelson Apr 30 '21 at 03:11
  • @Nelson Let's not continue to use the incorrect term while simultaneously trying to correct the term. That just causes confusion. – DKNguyen Apr 30 '21 at 03:41
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To be slightly pedantic, the common AA, AAA, C and D "batteries" should really be called "Cells". The voltage of an individual cell is determined by its chemistry.

The common 9 Volt batteries are really batteries - a number of cells connected in series. The 9 volt battery is made of six 1.5 volt alkaline cells.

A nominal 12 volt car battery is made up of six 2.2 volt cells, and will be about 13.2 volts fully charged.

Peter Bennett
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    To be slightly pedantic :-) : Zinc - carbon is about 1.5V when new. Alkaline is a whisker over 1.65V. With age but with no use this drops to about 1.6V in a year or so. – Russell McMahon Apr 05 '21 at 12:43
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    To be slightly pedantic, I remember when C and D batteries were called cells. In fact, I almost wrote that as I typed the previous sentence. ;-) – computercarguy Apr 05 '21 at 20:41
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    "cells" is correct for 1.5V batteries. – Harper - Reinstate Monica Apr 05 '21 at 22:24
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    So there would be no such thing as a battery composed of only 1 cell? Wiktionary and Wikipedia do not seem to agree with that, they both define a battery as having one or more cells. – Didier L Apr 06 '21 at 15:17
  • @DidierL: my dictionary says: "Battery: A combination of galvanic cells for generating an electric current". One cell is hardly a combination. I will admit that many people do call a single cell a battery, but that doesn't make it right. – Peter Bennett Apr 06 '21 at 15:57
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    @PeterBennett But ${1}$ is a set, [5] is a list and $1!$ is a factorial. Why can't a cell be a battery? – wizzwizz4 Apr 06 '21 at 17:41
  • @Peter Bennett - That's purely down to how we define battery, either as a 'number of cells' or as a 'plurality of cells'. We are entirely free to agree (or indeed disagree) about which definition we like. Since the former is in common usage I'd be inclined to go with that. – Frog Apr 07 '21 at 04:05
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Because of battery tech changes

Historically, the popular battery that owned the market was the "carbon-zinc" cell... in size AAA, AA, C, or D. Think garden variety Ray-O-Vac. These are 1.5V - that's decided by the chemicals in the battery, chosen for safety and low cost.

Appliances typically use them in series strings of 2, 3, 4, 6 or 8 cells - giving 3, 4.5, 6, 9 or 12 volts. The "9V" batteries have 6 tiny cells inside a package, and the "Lantern" batteries have 4 cells inside a package.

Since then, companies developed the "alkaline cell" chemistry - think Duracell/Energizer. The chemicals decide the voltage, so the chemistry had to be carefully chosen to a) be safe, b) be a similar 1.5V voltage, and c) be affordable. It provides much more energy in the same volume, and of course they used the same AAA, AA, C, D form-factors.

Note that carbon-zinc and alkaline are "primary batteries" meaning use once, and throw them away.

However, as economics changed, lithium battery technology has become popular. Those have a nominal voltage of 3.7 volts per cell. This was an all-out quest for maximum energy density: they were not willing to compromise to match up to an existing popular voltage such as 1.5V.

Lithiums are more expensive and are not used for everything. Many applications are a better fit for the less expensive and extremely low self-discharge rates of alkaline batteries, like TV remotes and smoke alarms.

Further, types like nickel-cadmium (1.2V) play well with the "solar garden light" application for a variety of reasons.

Power tool battery packs had largely settled on NiCd's big brother nickel-metal-hydride (also 1.2V)... however they are now migrating to lithium.

Lithium is not a "drop in" replacement like alkaline or NiMH. Lithium is very sensitive about charge and discharge management, and can catch fire if abused. As such, it requires special protective circuitry, and that must be accounted for either in the battery module itself, or in the charger circuit.

Harper - Reinstate Monica
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