AGM Battery Charging Voltage


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Feb 21, 2023
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What is the maximum voltage you can charge an AGM battery? I've seen most modern smart chargers say 14.3 to 14.8 volts for sealed AGM batteries.

I've got three of these X2s from Batteries Plus (very good batteries) in my boat and they all seem to be going to hell at the same time. I thought it was on board charger, but that seems to be working after some testing.

I have about 5 different chargers and I was able to get them charged up to 13.8V using an old school Schumacher analog charger, then Optimate smart charger, then back to my Bass Pro onboard charger. But they won't stand up to a load.

I'd like to blast them with about 16.5V at 2.5-3 amps for awhile to blast away any sulfates. But don't want to do any damage or vent any water. I know that voltage works for flooded lead acid batteries with caps for venting.

That 20 AMP onboard charger hits 15.8V, but not sure that's sustained for any length of time.


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I have Noco Genius 10 and a Genius 5 chargers that has a repair mode. Basically runs through a cycle to remove sulflates from the plates. From my understanding it does this by pulsing the voltage. I'm not sure just slamming higher voltage continuously into the battery would be a good idea. Might be worth getting a similar charger that offers a repair mode. They are good chargers, I've got a few of them. I think even the 2 amp has a repair mode. But for that size battery, I'd atleast get the 5 amp. My onboard boat charger is also a Noco Genius.
Yeah the Optimate and On Board charger use the pulsing mode. I can't recall how long these batteries have been in service, but the starter battery is probably 4 to 5 years, but the other two are probably 6 to 8 years old. Pretty good life overall. I think they fell victim to global warming this summer. Many days over 115 and a new record of 53+ days over 110. That's the only reason I can think that they just all went at the same time.

Last time I bought these X2s they were $260-280. Now they are $500ish.
I had one of those NOCO GB70 2000A jump boxes. Tested it a couple times and it worked fine. Shortly thereafter the battery pack swelled up and burst the case open. They wouldn't warranty it. Not sure I'd buy another product from them.
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I just bought this PZP charger for about $90 on Amazon. You can select amps and volts by dialing them up or down. I figured it would be fun to try it out.

I see they have a Force option listed on their recommended settings that shows higher volts for recovery. I might try that before I give up. Gonna leave them.on the conditioner a while longer.

If it was a flooded lead acid battery, I'd blast it long and hard for about 5 or 6 cycles. We've got batteries at the farm that are probably 40+ years old. Now they just make them disposable and charge accordingly for the waste.


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I had one of those NOCO 2000 jump boxes. Tested it a couple times and it worked fine. Shortly thereafter the battery pack swelled up and burst the case open. They wouldn't warranty it. Not sure I'd buy another product from them.
That sucks. I've got a NOCO 2500a jump box by them. I've used it a bunch and never had a problem with it. Used it yesterday on my son's motorcycle actually, lol. But I understand, I'd probably be the same way if I had a bad experience with them. I have some generic made in china one that's in my wife's car. I use to carry it in my old truck. That thing is probably 6 years old and still works pretty well. I've used it to jump a few different HD diesels, my tractor, you name it. Pretty impressed with it for some random Amazon Prime day item. lol

Honestly if you got 5 years out of them, that isn't terrible. Batteries don't last now. I'm happy if I get 4-5 years out of one. I pretty much stopped buying high end ones. They don't seem any better. I just get Duracell ones at Sam's club relatively cheap. Or for small batteries I just get Mighty max ones off amazon. Optima, oddessey, ect, seem to go out just as fast now. But yeah with an AGM you don't want pump to much in them and get them boiling. Who knows, maybe that's why they all went bad at once. That might be exactly what happened. Maybe the heat in combination with charging cooked off to much of the electrolyte.
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I've been using the Audew 2000A jump packs that Project Farm tested and reviewed on YouTube. About $100 each and I have four of them. I actually jumped my boat with it on the water after it wouldn't turn over after a few starts. When I first got it, I didn't think it would roll my Duramax. My batteries were getting weak, so I let them go. Just so I could test the pack. I ended up jumping it 3 times ant it was still showing 50%. The NOCO GB 70 did too, just didn't last and was bulky with no case.
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I'm on cycle 3 of charging 16.5V at 3A for 10 minutes. No heat build up. A few degrees above ambient. Only slight improvement so far. Probably do a few more and admit defeat.

Would like to do about 3 hours and see if it blows.
Did a couple more 16.5V charges at 5 amps for about 30 minutes. Battery side wall is getting up to about 125° F, so seems to handle the extra volts and amps ok. I'm sure heat is more than that when running.

Haven't load tested it yet. Gonna do one more longer cycle and see if it blasts that sulfate off the plates. Might be pissing in the wind, but I like tinkering.
I tried to rescue this turd, but it just wasn't gonna happen. Pulled the "maintenance free" plastic cover off the battery, which was held in place securely by adhesive. Turns out you can maintain it and the caps were just soft plastic caps held in place by the cover.

Every cell was low on electrolytes and the specific gravity was way off. Added some electrolytes and charged a few more times with a couple chargers. Battery would sustain 13+ Volts, but it just wouldn't handle a load. Recovered about 150 CCAs, but still 600 short of spec.

I guess it was purchased about 6 years ago in 2017, so pretty good life. Had to cough up $300 for another one. Probably another $750 for the trolling motor batteries.


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