Batteries

All that's required is a salt and: battery.

List

https://www.eevblog.com/forum/projects/coulomb-counter-ic-for-coin-cell-products/ - not many options for measuring current out of a coin cell, without drawing a notable amount of current.

A bit of semantic nonsense, but technically a battery is made up of cells. https://www.eevblog.com/forum/projects/biggest-battery-capacity-for-22-x-22-mm-case/ - cell vs. battery (cell is a single, battery is a collection of cells)

3.5 or 3.6mah is the highest single 18650 cell you can buy. a couple amusing anecdotes.