Got a cell voltage and would like to know how much charge is left? batsoc to the rescue!

pip install batsoc

Measuring state of charge based on voltage alone is not terribly accurate. The measured voltage depends on multiple factors besides the state of charge: temperature and load affect matters as well.

from batsoc import soc, BatteryType
soc(BatteryType.ALKALINE, 1.3)

will output 0.633 indicating 63.3% state of charge.

Any generic 1.5V alkaline battery under low load. The curve is defined for 0.8 - 1.6 volts.

Any generic lithium-ion battery with a nominal voltage of 3.7 or 3.8. The curve is defined for 3.0 - 4.2 volts.

A 3V lithium coin cell battery under very low load. The curve is defined for 2.0 - 3.0 volts.