How can I try to revive this AGM deep-cycle battery

[Sunder]

Not from 9.69v.

What has happened is that during the month of low charge, sulfate crystals formed on the plates. After long enough, these crystals grew large enough to breach the separator and short two of the cells.

Battery is now a dangerous paperweight.

 

[dogman dan]

Once those sulphite crystals grow to the point they touch, it's a short circuit in the battery.

Some say you can bust those crystals up with pulses of charging power. But the way you used the battery is still a problem. It should be recharged as often as possible, not drained slowly for a month or 10 days, or whatever. Once not full, the crystals really start to grow.

 

[nutnspecial]

Yes slow deep drains = early death .

That's a nice article OP. In that section it doesn't actually specifically say or allude that <10.5v renders lead destroyed/useless. The more time spent below full charge, the faster cells will decay. The lower that SOC, the faster and more severely they decay.

From the tests you describe your battery is pretty bad. Make sure it is at least fully conditioned/charged. A proper charge (from a proper charger) can be indicated by the charger kicking into float phase of ~13.5v.

I would do some googling to learn about sulfating/desulfating. There are 100 customer reviews on this product that might offer something of use to you. https://www.amazon.com/BatteryMINDer-Charger-Desulfator-Batteries-Model-2012-AGM/dp/B005EKY20K/ref=sr_1_3?ie=UTF8&qid=1476701431&sr=8-3&keywords=desulfation#customerReviews
And here's some other products. I always scan for reviews, they can be very informative. https://www.amazon.com/s/ref=nb_ss_gw?url=search-alias%3Daps&field-keywords=desulfation&x=0&y=0&tag=askcom05-20
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I wonder what the chances are that alot of online reviews are written by an advanced AI. . . ? I def think we have the tech, and it would be an interesting and non-threatning first test in the mass control of information.
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In the future maybe you'll find an alarm/cutoff useful. For longevity I don't like my batteries resting below 11v for any length of time. If you don't have AC why not just get a little solar panel to keep the thing topped off? A small one prob wouldn't cause harm even without any charge controller.

 

[DRMousseau]

I really hate those glass-mat and gels. But for applications where the safety of such a sealed battery is necessary,.. there's little choice. But my preference in lead/acid batteries has always been for liquid open-vented cells. May require more care and caution,... but they have a longer life by design.

Those replies above are correct, that the manner of your use is pretty harmful to ANY lead/acid battery. Unlike glass-mats and gels, liquid cells have a space beneath, in the bottom, to collect shed sulfides and contaminants. Deep-cycle types have a larger space and heavier plates to allow for desulfation/equalization cycles they are subjected to as well as that which jars loose from vibrations. When that space becomes full, a dead cell will result from shorts in this area. It's the most common cause of dead cells in auto batteries. A battery that's maintained and used properly will last many years and often beyond what many expect. Your glass-mats and gels lack this quality, and instead keep the cells plates completely "wet" by design, a problem of liquid cells when electrolytic levels become low exposing the tops to oxidation and sulfating, and reducing effective capacity. But even this condition can be recovered in liquid cells. Glass-mats and gels jus don't take kindly to overcharging like liquid cell batteries.

Unloaded voltage readings are pretty deceptive too. How many times have I heard, "But I jus charged it real good and it reads 13.5!!!".
Yes, now,... turn on your lights and read it again. What,... only 10.5??? Looks like you have a weak or possibly shorted cell. And if it doesn't recover by overcharging, then you'll need a new battery. Open cell liquid batteries also allow for SP readings. This too helps in quickly determining dead cells and battery condition.

Test your glass-mat in the same way,... with a light load. If there's a significant voltage drop, then you likely have poor plate surfaces and there's jus no way to recover them. Even under the best of use and conditions,... liquid batteries will outlive gels and glass-mats.

 

[Jon NCal]

You may want to experiment by adding an measured amount of distilled water and then charging and testing. However, there is a possibility of a leak.

 

[gogo]

I got some 'high quality' 12V 20Ah AGM batteries in a cordless mower that had lasted 8 years. I was still using them for the small amount of capacity that survived, and then put them on an overnight reconditioning cycle just to see what would happen. I got about half the run time (2.5 min. vs. 5 min.) after reconditioning.

I don't think de-sulfation works for anything other than flooded lead-acid and it may even hurt AGM's to knock crystals off when they can't fall away.

 

[DRMousseau]

I got some 'high quality' 12V 20Ah AGM batteries in a cordless mower that had lasted 8 years. I was still using them for the small amount of capacity that survived, and then put them on an overnight reconditioning cycle just to see what would happen. I got about half the run time (2.5 min. vs. 5 min.) after reconditioning.

I don't think de-sulfation works for anything other than flooded lead-acid and it may even hurt AGM's to knock crystals off when they can't fall away.

That's a LONG time for Glass-mats!!! But well cared for, and used properly,.. such long life CAN be expected of them. Quality AGMs CAN be better than many cheap flooded cell batteries, and good ones can hold a static charge over time quite well. Good ones are also designed to better resist sulfate and degrading. But they should never be discharged below 50% of capacity, about 12.2V, to achieve this long life.

Overcharging and "fast-charging" ANY battery is quit damaging. But flooded-cells can actually benefit from this damage, and are specifically designed to do so with special charging systems, heavy plates, and that extra collection space in the bottom of the cell. Overcharging, fast-charging and special cycling (balancing/de-sulfation) erodes the plates surface. The oxidized, sulfated surface is shed and washed to that bottom space, AND the new condition of plates becomes clean, pitted and rough. This significantly increases plate surface area, and actually increases capacity!!! New flooded cell automotive and deep-cycle batteries see a significant improvement and increasing capacity early in life, due to these charging cycles, before slowly deteriorating over the remaining lifetime.

Glass-mats and gels using these "deep-cycle" charging techniques, will form an oxidation barrier on the matting or in the gel from this shedding action, with an increase of sulfate buildup on this barrier, actually isolating the cell surface, GREATLY reducing capacity, and can actually destroy them, as other above will testify to.