= Available battery voltages
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Only certain battery voltages exist, because this voltage depends intrinscally on the battery's chemical composition.
https://learn.sparkfun.com/tutorials/battery-technologies/all (<CC BY-SA>) has a very good summary list, reordered from lowest to highest voltage:
|| Battery Shape
|| Chemistry
|| Nominal Voltage
|| Rechargeable?
| AA, AAA, C, D (Rechargeable)
| NiMH or NiCd
| 1.2V
| Yes
| AA, AAA, C, and D
| Alkaline or Zinc-carbon
| 1.5V
| No
| Coin Cell
| Lithium
| 3V
| No
| Silver Flat Pack
| Lithium Polymer (LiPo)
| 3.7V
| Yes
| <Nine-volt battery>[9V]
| Alkaline or Zinc-carbon
| 9V
| No
| Car Battery
| Six-cell lead acid
| 12.6V
| Yes
Bibliography:
* https://www.quora.com/Why-cant-I-buy-a-5v-battery
* https://electronics.stackexchange.com/questions/219457/why-rechargeable-batteries-use-1-2v