Question

What is the practical unit of electric charge? Why is this unit used?

Answer 1

Although the SI unit of charge is the Coulomb (`C`) the practical unit of milliamp-hour (`mAh`) is used in many contexts.

Explanation:

The Coulomb, `C`, is the SI unit of charge, and is itself quite simple, being the charge that results from 1 Ampere of current flowing in 1 second. However, that's a lot of current over a very short time.

In low power applications, particularly those where batteries are used, it is more practical to work with a much lower current, over a much longer time. For that purpose one uses the milliamp-hour.
`1 mAh` is the amount of battery charge that is needed to supply `1mA` for `1h`. It's not hard to convert back to Coulombs:

`1mAh = 3.6C`

Typical mobile phone batteries have on the order of thousands of `mAh`.

For higher power applications, one sometimes uses Amp-hours (`Ah`) which are 1000x as big as `mAh`