Electronic – Is voltage an accurate metric for testing the charge of an NiMH battery

batteriesbattery-chargingmultimetervoltagevoltage measurement

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

Elsewhere on the internet, opinion is divided at best.

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.

Best Answer

The answer is, of course, neither extreme, but somewhere between the two.

You can tell the state of charge of a cell from voltage

If a cell measures 1.5v off load, it is fully charged. You should not charge this cell further. If it's 1.5v on charge, on a low current, then it's pretty much fully charged.

If a cell measures 0.9v off load, it is fully discharged. You should not discharge this cell further. In fact, you should have stopped discharging when it was 1v, on load.

By all means use a multimeter to measure the terminal voltage of a cell that's finished charging. It will tell you there's no point in charging the cell any more. It won't tell you whether the cell now contains 100%, or 80% of its original capacity, only a discharge test will tell you that.

You cannot tell the state of charge of a cell from voltage

If a cell measures 1.2v on or off load, you cannot tell where its state of charge is from 10% to 90%. If you stop or start charging or discharging, then you find the terminal voltage can wander 10s of mV in the first few seconds, then more 10s of mV over the few minutes, in a way that's not modelled by the terminal voltage being dependent on charge alone. With the flatness of the discharge curve, that dependence on history pretty much destroys any predictive power that measuring voltage alone might try to claim.

This is why practical battery metering systems use a gas-gauge IC (coulomb counter) to track the state of charge directly.