Can a normal "C" battery for a portable stereo change polarity?

Question

I recently bought a portable stereo, a "boombox" sort of thing. It took 8 "C" batteries and after using it a few times, it stopped working with batteries, it would only work when plugged in. I bought a second portable stereo and it did the same thing. Both times, my husband tested the batteries and found that a few of them had reversed polarity! What made this happen?

Answer 1

This can happen if you put a partially used battery in with new ones or a battery with less capacity in with higher capacity batteries, like putting a carbon battery in with alkalines.  The used or smaller capacity battery will die first and the other batteries will put a reverse voltage across it.  This will give it  the appearance of a reverse charge, but it will only be a surface charge.  There's no real reverse charge on the battery.

Never mix types of batteries (alkalline or carbon, etc) or new and used batteries.