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.)