I regularly use rechargeable 1.2v AA NiMH batteries, and I'd like to be able to use my multimeter to test both how much charge they have when I've finished charging them, and how much charge they have left at any one time. However, being an electronics newbie and doing some research, I'm left confused as to just how directly voltage is correlated with a battery's level of charge, due to there being lots of conflicting voices surrounding the topic.
A highly-rated EE answer here claims that voltage is directly related to a battery's level of charge, stating:
NiMH cells start at about 1.5 V right when fully charged, drop to about 1.2 V most of their discharge life, and are pretty much empty at 900 mV
Then this answer here claims the exact opposite:
It is not possible to measure or guess the capacity of a battery with a single set of instantaneous measurements, like voltage, current, and temperature.
At best you can tell how much current is going into or out of the battery a what voltage. However, there is no way to infer capacity from that. If you can control the load, you can get some idea of the internal resistance, but even that would take at least two measurements separated in time and therefore can not be done instantly. And, battery voltage and internal resistance does not tell you capacity.
The FAQ at GreenBatteries.com also claims voltage is inaccurate:
...a NiMH (or NiCd) battery stays at about 1.2 volts until it is nearly completely discharged. This makes it almost impossible to know the amount of capacity left based on its voltage alone
So, I'm here for a final, definitive answer on the topic - are voltage and similar measurements accurate for assessing a battery's level of charge?
If they aren't, then what is?
N.B. In order to make this a definitive question for the topic, please ensure that any and all answers are well-sourced.