I have an old lead-acid car battery that got taken out of my car by AAA 5-10 years ago when it died during a cold winter. I stashed the battery away and forgot about it.
Recently I dusted it off and measured it with a multimeter. It registered -0.189 volts. Kind of weird.
I hooked up a battery charger to it and the battery charger generated an error signal: reversed polarity, even though the leads were hooked up correctly. So, apparently the charger kind of did the same thing as me: test the voltage. When it found out it was backwards it refused to charge it the right direction.
Why does the battery have a reverse charge? Does it just drift negative when it gets sulphonated or something? Or is it an effect of extreme cold?