[time-nuts] OT Gel Cell question

[Chuck Harris]

Basically, it is a matter of thickness, and the quality of an insulator.

All lead acid cells work essentially the same:  A lead oxide electrode
immersed in sulfuric acid converts to lead sulfate when power is drawn from
the cell.  Lead sulfate is a very good insulator when it gets thick enough.

So, as the cell discharges, a lead oxide layer builds on the plates.  When
the lead sulfate gets thick enough, it insulates the remaining lead oxide
from the current flow, and the cell is discharged.

When it comes time to recharge the cell, you apply voltage, and if the
sulfate layer is still thin enough, current will be drawn, and electrolysis
action will commence, and the lead sulfate will be turned back into lead
oxide, and sulfuric acid.

If you let the dead cell sit around before recharging it, the lead sulfate
will have a chance to convert even more of the lead oxide to sulfate, and
the layer will grow thicker, and thicker, and become a better and better
electrical insulator.... To the point where the lead plates are completely
isolated and no current can pass.... The cell is ruined.

One way you can sometimes resurrect a cell in this ruined condition is
to apply a high voltage, with a low current limit to the cell.  This will
get some current flowing, and will convert, albeit slowly, some sulfate
back to lead oxide and sulfuric acid.

Such repairs take weeks, or months, to work, and have a great risk of
forming current hot spots which will physically destroy the plates.

Others say that applying a stream of short, high voltage pulses, to the
cell will break down the sulfate's insulation ability, and let the
electrolysis action commence.

Either method will work, somewhat.  And only on a cell that isn't too
far gone in the first place... eg. a cell that has only been dead for
a week or two.

-Chuck Harris

Robert Roehrig wrote:
> Anyone know why Gel Cell batteries go HI-Z if discharged below a certain level?
> Also is there a way to rejuvenate a "dead" one?
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