Subject says it all. I pulled and tested the two name brand AA cells in my dead MagLite and found one cell reading weak and the other with the poles reversed-- negative at the top nipple, positive at the round base. Rechecked with another meter-- same result.
What's that all about?
It's called "cell reversal". It's most-common with rechargeable devices using nicad or NiMH cells, but I also saw it in an HP calculator that used alkaline N cells.
Here's what happens... One of the cells has significantly lower capacity than the others. When it hits zero volts, the other cells are pumping current through it /in reverse/. This causes the cell to charge up backwards, with reversed polarity.
At a guess... "overdischarge". The two cells in the battery were of slightly different capacity. One of them ran down to zero before the other, and the remaining somewhat-live cell continued to push current through the "dead" cell.
This would have been equivalent to sticking the "dead" cell into a battery charger, backwards.
With some electrochemistry types, this can lead to at least a partial reversal of the roles played by the cathode and the anode, and a reversal of the voltages on the two terminals of the battery.
I've read that this is an important reason why you should not run a NiCd or NiMH battery down below a voltage of 1 volt per cell. If you do, there's a fair chance that you'll push one or more of the weakest cells into overdischarge, and this can seriously weaken or damage them.
With alkaline cells, once they're discharged, you really should take 'em out promptly... they have a much greater tendency to leak than they do when they're new.
--
Dave Platt AE6EO
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Thanks William and Dave for the explanation.
I had saved the two alkaline batteries that I pulled about 10 days ago and just retested them. One was still weak, the other (presumably the one with the reversed polarity) is stone cold dead.
"Dave Platt"
** There are warnings printed on most Alkaline cells against attempting to recharge, improper insertion and disposal in a fire.The funny thing is that with a device using 4 or more such cells, the simple mistake of inserting one cell backwards will result in breaking all the above rules.
Very good reason not to let kiddies play with alkaline or other high energy cells.
... Phil
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