[time-nuts] OT Gel Cell question
cfharris at erols.com
Sun Jul 27 23:59:16 EDT 2014
A small disagreement on a couple of points....
Lead sulfate does not dissolve (in the normal battery chemistry),
and does not go all over the place. It forms at the lead and the
lead oxide plates, during discharge, and there it stays
(unless it breaks off) until you charge the cell. It is the
electrolytic cell action that allows the lead sulfate to be
converted back into lead metal, lead oxide, and sulfuric acid.
Everyone wishes lead sulfate could be dissolved safely, as this
could be a way of recovering batteries that have been overly
Lots of snake oil remedies have been created that tout to do just
that... things like lime juice, ETDA, adding more sulfuric acid...
AFAIK, none of them really work.
Shorting in a wet (flooded) lead acid battery happens because the
charging/discharging action causes the creation and destruction of
lead sulfate, and because the lead sulfate is less dense than the
lead and lead oxide it replaces, it flexes the plates. The flexing
causes some of the lead sulfate to break free of the plates, and
drop to the bottom of the cell. Because energy density is
important in a lead acid battery, the manufacturer wastes as little
space in the battery case as possible by putting the plates as
close to the bottom of the battery "jar" as it dares. This allows
the lead flakes to build up on the bottom until they reach the
level of the plates and short them out.
The gel cells, and glass mat cells short because the lead dendrites
that sometimes grow as a result of charging/discharging, pierce the
separator and short the plates directly.
Neville Michie wrote:
> Lead acid cells have lead supports carrying lead oxide and lead metal active material
> in an electrolyte of sulphuric acid.
> When they discharge, the sulphuric acid electrolyte is reacted with the oxides and metal
> to form lead sulphate and the concentration of the acid falls, that is why garages
> used to check batteries with a hydrometer to measure the electrolyte concentration.
> At the same time the terminal voltage drops and the internal resistance rises,
> when the concentration of the electrolyte gets very low, the lead sulphate becomes
> soluble and will re-deposit all over the battery. With gel cells the electrolyte can
> be completely absorbed making the battery resistance infinitely high.
> If you can get some current to flow, you may be lucky enough to get the battery to
> reform some electrolyte, conduct some more, and eventually charge.
> However, when flat the lead sulphate dissolves and redeposits all over the battery,
> and when recharged will convert back to lead and lead oxide, often most inconveniently
> bridging the plates to a short circuit.
> The lesson is to not let the battery ever get flat.
> Lead acid batteries have some very good features.
> The terminal voltage rises as the concentration of the acid increases. So a constant voltage will
> charge a cell, and current stops flowing when the electrolyte reaches its proper concentration.
> The catch is, when you have a battery of several cells, if one cell gets weak, the others will be overcharged
> causing gassing and over concentration of the electrolyte.
> There is a judicious voltage that causes an acceptably low rate of gassing
> (the oxygen hydrogen catalytically recombining) that will keep the charges of cells equalised.
> But it only takes one total discharge event to cause enough leakage in one cell
> to bring about failure.
> Lead acid batteries are also environmentally excellent.
> They consist of nothing but pure lead and sulphuric acid and water.
> Sulphuric acid is not volatile so you can make batteries out of old batteries
> forever, recycling the acid, lead and water.
> If made on a large scale they are also very efficient (99.9% +) electrically.
> Neville Michie
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