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Axion Power International Inc. Message Board

  • kirktierney kirktierney Sep 27, 2011 2:58 PM Flag

    "Positive plate sulfation?"

    I see this term crop up from time to time. It needs clarification. I am going to refer to an Exide/US Army study (Catherino et al) on battery failure a few times, and will include the paper, which is definitive.

    a) First, what are people talking about?

    Definition 1. Sulfation is the name given to the general
    cause that brings about failure of lead–acid batteries. It is
    identified empirically by observing the effects of:
    • Loss of capacity.
    • Loss of voltage.
    • Increase in internal resistance.
    • A decrease in sulfuric acid concentration.

    Definition 2. Sulfation is the recrystallization of lead sulfate
    into a form that is no longer electroactive. That is, it no
    longer participates in the charge–discharge process.

    H. A. Catherino decries the former "loose" definition -- basically, his paper complains about many diverse things being called "sulfation", especially at the positive electrode. He prefers definition 2, which is specific. The positive electrode "sulfation issues" (definition 1) should be changed to their real names.

    b) A primary problem in the negative electrode is (definition 2) sulfation due to cycling. That cannot happen with PbC. The primary positive electrode issue is grid corrosion, for example, and also passification. Sulfation (definition 2) is not a big thing at the positive electrode; this can also occur, but fundamentally more as a result of age.

    c) Detailed Failure mechanisms in a VRLA/AGM battery and notes relating to PbC:

    o Loss of electrolyte. PbC is exceptional in this because of less gas generation and greater gas recombination.

    o Electrode sulfation (definition 2) in the negative electrodes. PbC Is immune to this.

    o Electrode sulfation (definition 2) in the positive electrodes. A calendar issue, not a cycle issue -- it is simply not showing up in the cycle tests in PbC. The known fix is "electronic ring" injection, breaking up the crystals. This charge controller trick works especially-strongly in PbC because its super-high charge acceptance in sub-millisecond rates enhances the ring injection.

    o Passivating lead oxide film formation at positive current collectors. Use advanced antimony grids that also resist grid corrosion in order to resist this. PbC does this.

    o Electrolyte stratification. This affects all stationary batteries. It is not an issue in automotive batteries due to shakes, rattles and rolls.

    o Hydration. This is caused by leaving the battery unused for very long times. It is a calendar/usage effect.

    o Positive grid corrosion. A main failure mechanism. Advanced structure / heavy duty antimony-enhanced grids are used in PbC (from East Penn, as I recall). Also, PbC has a much narrower pH swing through its cycle, so the effect is considerably lessened (so I am told).

    o Internal shorting. Cheap separators and high degrees of paste shedding can cause this. There are a number of partly-applicable industry solutions.

    o Agglomeration of finely divided lead in the negative electrodes. This requires decent construction of the expander to avoid it. 'Nuff said.

    o Electrolyte contamination. This is from refilling batteries, primarily, with impure water. PbC is never supposed to need this because of superior electrolyte conservation.

    o External damage to case and terminals. Okay, there's that.

    Just sayin', one needs to be accurate when discussing failure modes; electrochemists would consider my list a light-weight degree of specificity.


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