[time-nuts] OT Gel Cell question

[Chuck Harris]

Neville,

Sorry, I don't agree.  There is plenty of ion exchange
capability in the "water" left over from a totally dead
lead acid battery to corrode the skin off of your hands.

I have never found one where the specific gravity of the
water reached 1.0.  Even a couple of drops of acid in
the water would be sufficient for significant current to
flow.

However, it still won't take any charge.  Not even a
milliamp at 15V.

The reason it won't charge is the lead sulfate covering the
lead plates is a very good insulator.

And, to quote my old college chemistry book:

- General Chemistry, 4th Edition, Nebergall, Schmidt and
   Holtzclaw, DC Heath and Company, 1972. p862:

"Lead sulfate PbSO4 is formed by ionic combinations, and
is insoluble in water but readily dissolved by solutions
containing an excess of alkali or acetate ions..."

I stand by my statements.

-Chuck Harris



Neville Michie wrote:
> To clear up the point,
> lead sulphate is very much more soluble in water than sulphuric acid,
> and when batteries get flat all the sulphuric acid is reacted leaving
> only water. That is why no current will flow when trying to charge them.
> It is all well documented, see:
> Vinal.G.W. (1945) Storage Batteries,
> John Wiley & Sons, Inc. New York Pp. 464
>
> The sulphate is more soluble at higher temperature, and the daily thermal cycling
> of an uncharged battery adds to the damage.
>
> There are many popular myths and partial truths abroad, largely, I guess,
> because the study of batteries is in few current engineering courses.
> Magic additives to restore dead batteries have been around for 100 years,
> but none of them are effective.
> The only trick I have seen was an old guy who heated car batteries in an oven,
> I never found out how long or how hot, he made a living reselling them
> with a money back six months guarantee.
> cheers,
> Neville Michie
>
> On 28/07/2014, at 1:59 PM, Chuck Harris wrote:
>
>> 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
>> discharged.
>>
>> 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.
>>
>> -Chuck Harris
>>
>> Neville Michie wrote:
>>>
>>> Hi,
>>>
>>> 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.
>>>
>>> cheers,
>>> Neville Michie
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