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AGM restoration ideas

Packscrapper

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May 21, 2022
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These batteries aren't used for off grid, but for a Ryobi riding mower. Nonetheless, I figure my solar brethren would be the best to give some ideas. It is also why I am open to experiment with it since consequence of doing so is much lower.

Backstory, these are Leoch 12V 75AH AGM (x4 = 48v nominal). When the mower was delivered new I suspected one or more of the batteries was degraded based on the voltage sag I would see after about 10 minutes of use, but it always finished the lawn with voltage indicating 40% SOC remaining, so I didn't make a fuss. It has always been on the factory provided charger when not in use as the manual dictates.

Fast forward into the second season, just before warranty is up on the batteries the voltage sags so low it goes into limp after about 10 minutes. Loaded, the series is around 42v, unloaded it rebounds to around 51V.

I load test each battery, find 1 is good, 1 is ok/marginal, 2 are bad. Resting voltage is fine on each, no evidence of mentionable self discharge.

I have a set a new of batteries being shipped under warranty but they are backordered with no ETA. Could be weeks or months.

Getting desperate I pried the glued down plastic cell plates off to expose the rubber caps sealing the cell ports for the marginal battery and one of the bad batteries. I wanted to scope the inside for evidence of sulfation or dry mats. Batteries where reading around 12.5v (not charged and had been sitting for several days since getting limp mode).

The marginal battery has virtually no pressure under the rubber caps.

The bad battery has quite a bit of pressure in each of the middle cells (cell 2 and 5). The rest were like the margin battery. On the one of the pressurized cells pulling the rubber back a hair a couple times had it fully vented.

On the other cell, slightly pulling back the rubber it starts to vent like crazy, spitting bubbles and pooling liquid around the port so I have to put the seal back on.

I come back and try venting it a bit again and the same result. It isn't gas only, it is acting like venting a hot radiator, where the pressure drop is allowing it boil. Mind you these are low pressure soft rubber caps.

I couldn't imagine an AGM being wet enough in a cell to spit fluid out of a resting battery.

I put a scope down the ports on both batteries everything looks good, no sulfation that I can see. Mats look wet (hard to to tell because the mats shine either way I am sure).

The only port I haven't been able to scope is the one that keeps staying under pressure at rest. If I just pull the cap it will surely let out electrolyte and not just gas.

Anyone have any suggestions on what this sounds like? Any insights on what could be causing a AGM cell to seem so wet and pressurized at rest?

I'm trying for a hail mary. I had already tried charging each individually with my 12v charger with a EQ/desulfate cycle. This is where my theory that maybe some of the cells in the battery were not properly filled by factory or I had some off gassing (the factory charger seems a little aggressive).

I think my only recourse is probably to charge it and put around 10ML of distilled at a time and retest and hope I had some dry cells, but I figured I would get a second opinion before I do anything. My theory on the dry cells is only because fresh off a charger, albeit with a surface charge, the bad ones load test pretty well. It's only after load has been applied for several minutes that IR goes way up and amp delivery falls off.

If these were flooded it would be so much easier to identify the issue, argh.
 
I would hit them all at 15.5 volts for an hour per day (after the full charge is complete)
Do this for a week and see if they improve as you go. Otherwise they are shot. I don't know the voltage profile but I doubt that factory charger on 24/7/365 was a good idea. Only one year service is terrible.

And of course I would replace with LFP.
 
Update, after slightly pulling the cap on that cell holding a lot of pressure back a few more times I got it vented. Scoped it and it looks a bit of sulfation present, but nothing extreme.

Leaning towards adding a tiny amount of distilled water to each cell to see. Maybe hook it up to bench supply and push high voltage, low amps into just to see if I can get some desulfation better than my charger (unknown exactly the process it uses).

Thoughts?
 
I have doubts adding water will help unless the cell is noticeably dryer than the others. Even then the matting will slow the movement of the water and acid for mixing. Have read of some success but it is rare for people to actually open an AGM. I don't think there is any harm in trying. Go easy on the water. Best of luck.
 
I would hit them all at 15.5 volts for an hour per day (after the full charge is complete)
Do this for a week and see if they improve as you go. Otherwise they are shot. I don't know the voltage profile but I doubt that factory charger on 24/7/365 was a good idea. Only one year service is terrible.

And of course I would replace with LFP.
Yea, I had reservations about that chargers profile. It was adamant it should be left on the charger at all times, presumably it would go to float/trickle after bulk charge.

Oh yeah, I am going LFP if I buy new. I can get some great Lithium cobalt gently used packs for a song and just need a 60+ amp BMS (amp clamped the mower and max peak is 59 amps, about 40 continuous under normal cutting conditions). Price is hard to beat even though LFP will be better in the long run.

I will get out my bench supply and hit with higher voltage and low amps to start.

This is why I hate AGM, the no maintenance means you don't have the ability to easily diagnose and repair like flooded. If it were a flooded I could tell exactly what was happening pretty easily.
 
I have doubts adding water will help unless the cell is noticeably dryer than the others. Even then the matting will slow the movement of the water and acid for mixing. Have read of some success but it is rare for people to actually open an AGM. I don't think there is any harm in trying. Go easy on the water. Best of luck.
Yea, it's hard to find much in the way of people experimenting with AGM because you aren't suppose to open them. In my case I had already tried desulf, they were pretty new and had maybe 30 cycles with the lowest SOC being 40%. Each was perfectly in balance. Voltage indicate 100% SOC across each, but SOH on 2 are poor, 1 decent, 1 good based on load test and IR test. Go figure.

I will start with a more controlled desulf and then add water from there in small amounts (10ML) and and log the results.
 
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