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.

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.

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
Friends of Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!

On 12/19/11 2:13 PM, William Sommerwerck wrote:
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.



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"

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.


** 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.

1. The reverse inserted cell will be charged by the others.

2. Pressure will build up inside it and it WILL explode and make one hell
of a mess.

Very good reason not to let kiddies play with alkaline or other high energy
cells.



.... Phil