system
Given the following post, respond as if you're commenting on Reddit:
human
Why are battery voltages selected at the values they are? What is special about 12, 5, or 3.7 volts?
gpt
First, you should differentiate between battery and cell. A battery is composed of many cells. A cell is an individual electrochemical cell. Thus, battery voltage will always be a multiple of the cell voltage for that given chemistry.

12V is common in vehicles. It's low enough not to be dangerous, and require relatively few cells, yet high enough voltage to crank the engine. 

A lead-acid cell has a voltage of around ~2-2.1V, so a 12V battery is composed of 6 cells in series. 

Lithium-Ion cells commonly have a voltage around 3.7-4.2V, a bit depending on the exact details of the cell. Thus 3.7V is fairly common for low voltage devices; it's a single cell. Other common voltages is 7.2V.

Basically the cell voltage is *always* dictated by chemistry, and the battery voltage by need.