# Why are so many batteries 3.7V?

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.

• @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. Apr 5, 2021 at 5:32
• Discussion on roundness has been moved to chat where it may be continued if desired. Apr 5, 2021 at 23:42
• @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. Apr 6, 2021 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 ... . Apr 6, 2021 at 21:20

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.

• I think NiCad cells are (were?) closer to 1.2 volts. Apr 5, 2021 at 3:57
• Yeah, 1.5V is about what older zinc-carbon batteries produce. Apr 5, 2021 at 12:15
• @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. Apr 5, 2021 at 19:05
• 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. Apr 6, 2021 at 4:42
• 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) Apr 6, 2021 at 7:37

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.

• 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. Apr 5, 2021 at 12:43
• 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. ;-) Apr 5, 2021 at 20:41
• "cells" is correct for 1.5V batteries. Apr 5, 2021 at 22:24
• 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. Apr 6, 2021 at 15:17
• @PeterBennett But ${1}$ is a set, [5] is a list and $1!$ is a factorial. Why can't a cell be a battery? Apr 6, 2021 at 17:41

## 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.

• Historically, the ealiest common (and still available) "Dry Cell" was the carbon-Zinc cell. It is 1.5 volts and has largely been replaced by the more efficient alkaline cell. Apr 5, 2021 at 22:01
• @PeterBennett Good point! Edited. Apr 5, 2021 at 22:25