[EVDL] Cell reversal

Many moons ago there was a good post about this from a
gentleman that worked in the battery industry.

In essence his statement was that when a cell goes flat,
it does not reverse immediately.
This had to do with the difference in efficiency of the
positive and negative plate discharge - I forget which
was more efficient, but the result was that a cell would
first go to zero volts for a number of AmpHours, before
the other plate was also completely stripped from its
charge and also turned into the opposite plate composition.

I am not a chemist and have not looked up the processes
involved, but the important message I got from reading his
post (besides that you destroy your cell in short order
if you reverse it a few times) is that the cell does not
reverse immediately, it goes from +2 to 0V and after a
while to -2V.

Note that in this process, there can be a period where the
concentration of the electrolyte is so low that the
resistance of the cell seriously increases, which can
lead to the *loaded* voltage jump up very high and the
cell getting very warm, even cooking/exploding!

The above mentioned +2, zero and -2 Volts are the open,
unloaded cell voltages.

All the above assumed Lead-Acid battery cells.

Regards,

Cor van de Water
Systems Architect
Proxim Wireless Corporation http://www.proxim.com
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-----Original Message-----
From: [email protected] [mailto:[email protected]] On Behalf Of [email protected]
Sent: Monday, September 17, 2007 11:58 AM
To: [email protected]
Subject: [EVDL] Cell reversal

Can someone explain "cell reversal:" Causes, symptoms (how to recognize) and results. I hear it mentioned frequently, but don't understand.
Roger Daisley
Pullman, WA
http://www.96-volt.com

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> You may have to let the pack rest a bit before proceeding.
>

While this is ok on the rare occasion, do NOT drain your pack to this
point on a frequent basis.
Not unless you like murdering your pack and buying a new one every year
(or sooner).


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Yes, and in series packs it seems possible to have a cell damaged even when
the average voltage is still at 10.5V. Once a cell goes dead, or reverses,
your charger may not cut out... since the charger usually uses pack voltage
to cut back.

You can usually get a good clue when this condition happens. Your range will
drop significantly, and you may see the batteries continue to "charge" even
once the charger is cut out.

At least that's what I seemed to have seen w/ mine.
----- Original Message -----
From: "storm connors" <[email protected]>
To: <[email protected]>; "Electric Vehicle Discussion List"
<[email protected]>
Sent: Wednesday, September 19, 2007 7:54 AM
Subject: Re: [EVDL] Cell reversal


> If you have a series string of cells, one cell is always at a lower
voltage
> than the others. If you keep drawing power from the pack each of the
cell's
> voltage is reduced. Eventually the weakest cell will be reduced below 0
> volts. At this point, the negative terminal becomes positive and vice
versa.
> Lead acid batteries will be permanently damaged. Be careful with them not
to
> let the voltage of the 12 volt battery go below 10.5v under load. As your
> pack gets discharged, watch your voltmeter. Use your right foot gently to
> keep the amps low enough not to drop the voltage of the pack below the
> minimum. You may have to let the pack rest a bit before proceeding.

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