Negative voltage on AA battery

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.

It happens if one cell is somehow "weaker" and gets charged by the other cells.

http://en.wikipedia.org/wiki/Rechargeable_battery#Reverse_charging

Alkaline and other batteries can switch polarity in a series configuration. The battery doesnt actually have a negative charge, the positive terminal became the negative end and will meter -V when tested normally. A common occurrence, although rare that someone volt checks 'dead' batteries. HOW IT HAPPENS: a single cell depletes before the other batteries drop below half power and is deep cycled to 0.00v. This zero voltage state makes the + & - field unstable. Most batteries,on their own, will rebound to a low voltage when drained too far. But at that critical moment the positive tip is against a negative end of a charged battery and the electromagnetic field re-stabalizes backwards and starts taking positive charge from the negative terminal.