I've experimented a little bit with some of my old, rechargeable batteries. (They "slept" around 5 years in a cabinet. I've long forgot their previous charged status.) What I did:
- I've measured their voltage. It was okay, maybe a little bit lower as their nominal voltage.
- I recharged them: I gave them around 12 hours in a battery charger. Unfortunately, this battery charger didn't give me any information, with the exception of a led showing, some type of charging actually happens.
- I've measured their voltage again. This time, it was better.
Earlier I experienced that completely (or near completely) exhausted batteries give their nominal voltage measured by a multimeter, but it doesn't mean, that they could produce usable power on load.
My idea would be, that if measuring the voltage doesn't show anything about their power, maybe measuring their short circuit current would. Yes, I know it harms the battery, but I think, for the some seconds while I read the current on the display of my multimeter, it is practically negligible. And knowing the voltage, and their short circuit current will show, if they are really usable or not.
One of my friends (professional electric engineer) said, it is a bad idea, but he didn't explain clearly, why.
So, why is it a bad idea? How could I test a battery, if I have only a multimeter?
(P.s. they are NiMH batteries, if it matters.)
Best Answer
It's generally a bad idea, especially if you don't know with 100% certainty a lower bound on the internal resistance of the battery. As a secondary concern it may damage the battery.
That said, I do this myself briefly with alkaline batteries because I know that the cell or battery can produce only a few amperes into the 10A multimeter input. I would not do it with any other kind of battery, including NiMH.
Some battery types can produce much larger currents- enough to damage themselves (heat up, vent electrolyte, explode, even), melt the test leads causing severe burns etc.
The proper way would be to apply a reasonable load (reasonable depends on the battery chemistry, size, design etc) and measure the voltage. NiMH is a bit thorny since it's difficult to determine the charge condition from the terminal voltage. It's better to discharge with a moderate load to a known discharge condition (perhaps 1V per cell or a bit less) then charge from there. Overcharging NiMH batteries causes irreversible cumulative damage.
Once the battery has charged properly you can discharge it into a reasonable load and measure how long it lasts (taking care not to discharge it too much, which can also cause permanent damage).