Last night I pulled the battery holder out of a toy RC car to test the voltage with my meter. The pack has 4 AA batteries. The combined voltage at the terminals of the pack was well under 1 volt, and then I tested each battery individually. These are ordinary AA alkalines (not rechargables). They were in good condition, with no leakage.
The first two batteries showed about 1.2V; the third had a small negative voltage, about -0.2, which I thought was pretty interesting, and the last showed about -1.2v. I have never come across a negative voltage on a battery.
Now before you ask, yes, I double-checked, and then triple-checked, that my leads were plugged in correctly. And I was not holding the batteries upside down. I had all four right in front of me, all pointing the same way. Two showed a positive voltage, one slightly negative, and one -1.2v. I did this repeatedly,
So my question is, what the heck? How does a battery get a negative voltage on it? The pack had been in the RC car for a couple of weeks, with the car switched on. I think, but cannot prove now, that they were all inserted the right way in the holder. But even if one or two were in backward, how could this happen?
Now the batteries are Canadian, and my meter was made in the USA, but I don't think that explains it. [That's just a joke.]
Answer
Batteries when are fully discharged they can reverse their polarity. Sometimes you can carefully discharge this reverse voltage on a single cell and the battery will then successfully charge back up. Other times the cell is ruined and needs to be replaced.
I used to see this on the large batteries used on aircraft.
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