Desulfating an AGM Battery...

Discussion in 'General Technical Questions and Answers' started by AI4IJ, Feb 20, 2007.

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  1. AI4IJ

    AI4IJ Ham Member QRZ Page

    Hi guys,

    I just got one of those "Smart Chargers" (the Husky model from Home Depot); and, among many other features, it includes the option of "Reconditioning" lead acid batteries.

    By "reconditioning," they really mean desulphating the lead plates of the battery.

    They describe a procedure to analyze the battery to determine whether, or not, it is suitable for desulphating - a code "F3" shows in the display for a battery that is suitable (F3 = open cell).  The instructions say that the process takes 24 hours and stops automatically.

    Now, I read a couple of articles about the process; and, it seems like a reasonable approach.  But, what isn't clear is just how long you have to go through the desulphating process.   The author of one of the articles I read said that he had to go through the process for as long as a month to achieve satisfactory results ( a bit too much to be practical, in my opinion).  But, the directions for the charger say it only takes 24 hours and they don't mention repeating the process, if the first cycle didn't seem to be effective.

    So, this leaves me with a few questions:

    How long should this take?

    How can I tell when "enough is enough?"

    Can I damage the battery by doing this too much or too often?

    What percentage of the original capacity is typically recovered after the battery has been completely desulphated (or, as much as is possible)?

    Do larger batteries take longer?  Or, is the length of the process more tied to the degree of sulphation?  Or, both?

    Is there a difference in how effective this process is on AGM batteries, as opposed to gel or flooded lead acid batteries? The charger has the ability to select the type of lead acid battery (Wet, Gel, AGM).

    I would appreciate your sharing any experiences you may have had with desulphating batteries.  

    Thanks, and 73
  2. AB1GA

    AB1GA Ham Member QRZ Page

    The amount of time it takes to fully desulphate a battery depends on the thickness of the lead sulphate layer and the physical structure of the plates.

    Desulphators typically apply a higher charging voltage than usual to the battery in an attempt to "punch through" this dielectric layer. Some reconditioners use high voltage pulses, some apply a high frequency (a couple of MHz) ac to the battery.

    As the sulphate layer breaks down, the internal resistance goes down, and charging current goes up. The desulphator has to be smart enough to know to crank down the voltage or the battery could be damaged. This is not a big risk, I think, especially in commercial desulphators like in your charger.

    The lead sulphate layer forms continually as long as the battery is being stored. The best way to prevent sulphation is to keep the battery on "float" charge all the time. (This is for wet cells, I'm not up on AGM batteries, so I can't help you there.) The longer the storage, the thicker the layer. If the layer gets too thick, then the desulfator can't apply a high enough voltage to break through it, and the battery is unretrievable. I think that's what the "F3" code is all about: the charger applied a test voltage and decided it could recover the battery.

    The recovery time depends on the type of battery, with the higher capacity/deep discharge batteries taking longer. I think this is because the plates of deep discharge batteries have more nooks and crannies for the sulphate crystals to hide in, and it takes longer to break them up and force them to redissolve. I have also heard that car batteries could take up to a week, and deep discharge, high capacity batteries could take a month. I suspect the one hour reconditioning is because of a safety timer in the charger to prevent over-treatment of smaller batteries.

    Sulphation reduces battery capacity, so I'd keep reconditioning the battery every charge cycle until the capacity stops rising. That's when you decide whether the battery is worth keeping.
  3. AI4IJ

    AI4IJ Ham Member QRZ Page

    Thanks, Dale!  That was great information!  

    Anyone else have any experiences to share?

    I'm currently trying to recondition a 17AH AGM battery that is about 7 years old.  24 hours of reconditioning didn't seem to make much of a difference; so, I'm doing it, again.  When I apply a 1.8 amp load on the fully charged battery, the voltage at the terminals drops to 12.2 from 12.7.  But, when I attach the same load to a brand new fully charged 17AH AGM, the voltage drops to 12.9 from 13.2.  Obviously, the battery isn't anywhere near reconditioned, yet.

  4. KA9VQF

    KA9VQF Ham Member QRZ Page

    A very long time ago I did this all the time. I’d take a battery that wasn’t holding a charge very well if at all and reverse charge it.

    We had a real heavy duty battery charger that had a really huge rectifier tube in it. When you first started charging the battery backward the tube would light up very brilliantly violet.

    After the battery had obtained enough electricity that it was fully charged, backward, we had a bank of headlights that we would use to fully discharge it with then recharge it the other way again.

    Usually once was enough to bring otherwise useless batteries back to use. Many times they were good as new for a year or more.

    Generally afterward we would drain the electrolyte out of the battery and strain it through a milk filter or replace it with new.

    I even tried it on a sealed lead acid battery. It didn’t work out. That was the only battery I ever witnessed blowing up. I’d seen a few that someone else had blown up, and I cleaned up the mess, but that was the only one I was present for.
  5. M3WLY

    M3WLY Ham Member QRZ Page

    We had a 12v Motorcycle battery that was used for starting model aeroplanes.

    From 14 years of none use, we put it on the OptimateIII (a cool charger thing) it used to desulfate for 2 or 3 hours? (if it needed doing) then charge the battery and say if the battery was bad or good (then while it said that it would supply a float charge)

    We had the thing on, resetting it once it said bad, and it desulfated for 2 weeks! then the light came green! The battery was still broke, but it could spin the starter motor on the prop and start the plane before setting off.

    Think we disposed of the battery in the end, it was more of a... see if this super cool battery charger worked, and it did.
    So putting it on a battery thats not totaly knackard, and even putting it on your bike / car / ham radio portable battery, is a good thing.
  6. K9VQ

    K9VQ Ham Member QRZ Page

    I remember years ago when I was in the Air Force, we had a procedure to restore the batteries used with the no-break power system in Minuteman missile silos. These batteries where huge, eight cell 16 volt, 8,000 Amp hours batteries that weighed 1,600 pounds.

    When the batteries were checked out and found bad they were replaced, then sent back to the main support base's battery shop. They were then inspected and cleaned. Two suspect batteries were hooked up in series and charged. Then they were discharged with 100 Amp load bank until the batteries got down to 28 volts for the pair. Then they were put immediately back on the charger and recharged. This process was continued several time, calculating the time in hour and minutes it took them to read 28 volts. The reading were calculated, hopefully showing an increase of battery life and return to specs. This did not work in all cases, but did restore them about 75% of the time.

    This was done because these batteries were very expensive and almost as expensive to dispose of them.

    My specs might not be totally accurate, because this was almost 30 years ago, but this is the general procedure we used to do.

    Another thing, If a ham could get a hold of one of these batteries. I bet he could run all his gear eight hours a day for a year, before he had to charge the battery. [​IMG]
  7. AB1GA

    AB1GA Ham Member QRZ Page

    It had better last that long! If I had to move a battery that weighed 1600 pounds to and from the charger, I wouldn't have the strength or courage to do it again for at least a year, discharged or not! [​IMG]
  8. N9MXX

    N9MXX Ham Member QRZ Page

    Someone correct me if I am wrong, but I am pretty sure there is no way to desulfate an AGM battery.

    Simply put the desulfating process is dependent on the electrolight boiling to remove the sulfate from the plates.

    An AGM, Absorbed Glass Matt, has no electrolight to boil and really the plates aren't so much there in a typical lead acid sense either. I would not even try it, I think you will find catastrophic results, and possibly injure yourself or someone else.

    My thoughts, and please correct me if I am wrong.

  9. K9VQ

    K9VQ Ham Member QRZ Page


    The last this you ever want to do, is get a battery to the point of boiling the electrolyte. You will destroy the battery and worst case scenario, it will explode.

    The desulfation procedure I mentioned in my last post about my experience with Minuteman Missile silo batteries, was with electrolyte temperature being monitored. We had calibrated thermometers in the cells and they could not exceed 115deg F during charging and discharging. If the temperature was approaching 115, then we had to reduce the charge currant or reduce the load bank settings. If a battery cell did exceed 115 F for some reason, the restoration procedure was stopped and the battery was condemned.

    The charging and discharging procedure we performed, did reduce the lead sulfide from the plates. Basically, the process, chemically converted the sulfide back to electrolyte. One thing I do remember, if a sulfated battery was in a low charge state for very long, it could never be restored within specs.

    For an AGM battery, I have no idea if any kind of restoration process would work. I'm pretty sure, once they lose capacity, there is not much you can do.
  10. AB1GA

    AB1GA Ham Member QRZ Page

    Sulphation is reduced by redissolving the sulphate crystals in the electrolyte. The lead sulphate crystals will dissolve if the electrolyte temperature is increased or if the battery is fully charged.

    Traditionally, you could use both approaches at the same time by applying a higher voltage. The sulphate crystals increase internal resistance, so you need to apply a higher voltage to force charge into the cells. The high internal resistance also caused the temperature of the cells to increase, helping dissolve the crystals.

    SLAs and AGMs have a major problem with this approach. If you apply excessive voltage, gas is generated faster than usual and the electrolyte level drops. In a maintainable wet cell this gas is vented, and you can top off the electrolyte if necessary. Not so with SLAs and AGMs. The gas pressure will build and ultimately the cell will vent, perhaps catastrophically. Care is needed.

    At least one vendor makes a product (BatteryMinder)which it claims can desulphate SLAs and AGMs. It does so by using a pulsed charge at RF frequencies. This is supposed to keep the charging power low while maintaining a high enough voltage. The RF part sounds weird, but I remember an article in Home Power magazine which described just such an RF pulser. It seems those lead sulphate cells resonate in the RF region and break up into smaller pieces which dissolve more readily.

    AI4IJ, your charger may be able to desulphate conventional batteries but not SLAs or AGMs. The only way to be sure is to see if it will accept the settings. It may be that if you set it to AGM, the desulphate function disappears.

    MXX, thanks for the warning, I thought my charger desulphated all kinds of batteries, but now I have to go downstairs and check! [​IMG]
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