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Lead Acid Battery Desulfator

Discussion in 'Electronic Projects Design/Ideas/Reviews' started by wasrana, Aug 21, 2008.

  1. mvs sarma

    mvs sarma Well-Known Member

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    even if we add or use distilled water, the process of sulfasion happens. due to prolonged discharge, uneven charge cycles, so on. We have to study the electro-chemistry of the lead acid battery management and reasons for sulfasion.
     
    Last edited: Aug 28, 2008
  2. Sceadwian

    Sceadwian Banned

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    The reason for the sulfation is that when a lead acid cell is discharged the negative plates have lead sulfate on their surface, if current is fed into the battery after a short period of time it will disolve all the sulfate back into the acid solution. If all of the sulfate isn't dissolved back into the acid solution (by softly gassing the battery) it eventually passivates. The exact reason the lead sulfate passivates I can't remember, I read a really good explanation for it with a very long list of the chemical equations but I'm not a chemist and it was probably a year and a half ago. I'm going to guess that if the plates are still physically sound that removing the plates from the pack/cell and chemically treating them should be a way to return the cell to good working order, at the same time it's probably not a bad idea to replace the electrolyte solution. There may be additives out there that can assist an electrical desulfinatioin process into working better but I somehow doubt this is good for the long term survivability of the pack as you basically want as close to having nothing in the system by activated lead, sulfuric acid, and water, anything else is going to have consiquence of some sort.
     
    Last edited: Aug 28, 2008
  3. jpanhalt

    jpanhalt Well-Known Member Most Helpful Member

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    The simple explanation is that lead sulfate is formed initially as a metastable amorphous precipitate, which then slowly converts to a crystalline form that is stable.

    The amorphous form has a large surface area and can be electrochemically converted to lead readily.

    The crystalline form has a much lower surface to volume ratio and its electrochemical conversion back to lead is extremely slow. (See: http://en.wikipedia.org/wiki/Sulfation)

    Such dissociations between kinetics and thermodynamic equilibriums are common.

    One example is dissolving sugar in water. If one takes 50g of sugar and adds it to 50 mL of water, it will take a long time to dissolve. Once dissolved, however, it will not come back out of solution unless highly concentrated over a long time (e.g., crystallization of honey). Polysaccharides (e.g., agar) are particularly prone to such behavior.

    That is the simple explanation, but it is not the whole story. Another contributing factor seems to be the formation of mixed salts, such as "tri-basic lead sulfate" and "tetra-basic lead sulfate". These are solid forms that contain relatively stoichiometric mixtures of lead sulfate and lead oxide in different ratios and water molecules. The behavior of such salts is complex and of great current research interest in the production of long-lived batteries. (Here are two references those interested can use to find the primary literature: http://www.osti.gov/energycitations/product.biblio.jsp?osti_id=599910 and
    http://en.wikipedia.org/wiki/Lead_sulfate)

    When one considers that lead-acid batteries have been around for about 150 years, the complexity of the chemistry of sulfation is revealed by the recency of those studies.

    The bottom line seems to be the inexorable march of the "amorphous" lead sulfate that is initially formed (whatever its actual complex composition) to a much more stable crystalline lead sulfate.


    Maybe with more detailed knowledge of the chemistry, long-life lead-acid batteries will be developed. In the meantime, my BS detectors are triggered by claims of desulfation using pulses at some "resonate frequency" of lead sulfate or by adding secret rejuvenators to the dead cells.

    The Fountain of Youth has yet to be found -- either for old batteries or old people.

    John
     
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  4. dave

    Dave New Member

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  5. Sceadwian

    Sceadwian Banned

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    So what are the causes of the passivation of the soluble lead sulfate into the crystalline form? Sounds like what you have to do is start with really pure lead, really pure sulfuric and H20 and maintain that balance. I peaked inside one of the cells of a fork lift we just acquired at work and it has something that looks like hair floating in it. Sounds like the real problem is all the basic impurities that end up or start in the system in the first place.
    I know over charging or gently 'gassing' the cells will work, I think these de-sulfation circuits work by large current pulses to try to blast the crystals lose, but what causes the actual passivation, and is there anything that can be done to slow or change the hardening process that the various sulfates go through?
    The complexity of the chemistry involved is a testament to the advances in modern material sciences and chemistry that have occurred in just the past 5-10 years as far as understanding goes, and a testament to random chance and just doing what works as far as how practicality goes. It still has a LOT of uses, a well designed, fabricated, and engineered lead acid system will last 10+ years in a hard environment.
     
  6. jpanhalt

    jpanhalt Well-Known Member Most Helpful Member

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    Using the common model, imagine that at the surface of the amorphous lead sulfate there is a constant interchange between the solid and a layer of dissolved lead sulfate. As that process continues, some of the lead sulfate will start to grow as crystals on crystal nuclei that will inevitably form or may already be present.

    For example, take any concentrated solution, like sugar or more easily salt. You can cool it and nothing happens, but add one tiny flake of crystal or scratch the container, and crystal growth will begin.

    The rate of reduction of lead sulfate back to lead is controlled by the amount of exposed surface. The difference in surface area can be enormous between a crystal and amorphous solid. Thus, the amorphous form can be reduced back to lead much more rapidly. That change in reactivity is the passivation to which I think you refer.


    In general, pure solutions of compounds will form crystals more easily than impure ones will. I am not sure the tiny amount of impurities in batteries make a big difference.

    However, I suspect one aspect of current research may be to find additives that can be added to retard crystallization of lead sulfate. Hypothetically, if one could add something that formed a complex salt with lead sulfate (e.g., PbSO4 · XXX · H2O) that was slightly less soluble than lead sulfate and only precipitated as an amorphous form, you might inhibit the crystallization of lead sulfate, as the complex salt would come out of solution first.

    That makes sense to me, as it would reduce the amount of residual lead sulfate to a minimum.

    John
     
  7. BUNSDAD

    BUNSDAD New Member

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    Inverter

    Just hook almost any battery up to any IGBT inverter welder and you will desulfate. Set the voltage below 13V/battery and if you can't get it there add another battery done.
     
  8. Birdman Adam

    Birdman Adam New Member

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    Isn't this thread a little old?
     

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