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Old 05-18-2019, 09:42 AM   #1
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AGM batteries and charger mode question

Greetings all!

I just purchased 3 new AGM batteries to replace FLA ones. I've got a Xantrex TC4012 charger. I've put the charger in AGM mode. My charger also has two different charging modes:

1. Three-Stage (Bulk, Absorption and Float)
2. Two-Stage (Bulk and Absorption)

I seem to remember reading somewhere that AGM's can be over-charged, but the interwebs shows conflicting information, and Nigel's boat bible is at the marina in the boat.

Any reason to put the charger in two-stage?

Also, my engines are 2005 Mercruiser 5.0MPI's. Should I be looking into alternator in are the ones on the engines intelligent enough to charge AGM's responsibly? Or do I need to look into something like an ACR or some other device?

Thanks for your comments!

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Old 05-18-2019, 10:29 AM   #2
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I would stick with the three stage charger, just make sure the float voltage follows the manufacturer's guidelines for float voltage. IE: Lifeline specs the float charge for their AGM at 2.2 to 2.23 volts per cell. As far as the alternators B/A is spec'ed at 2.37 to 2.4 volts. You gotta measure it to know what to do. Bulk charging is constant current so as the battery gets charged the voltage will increase and shouldn't exceed the B/A spec.

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Old 05-18-2019, 10:30 AM   #3
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Lot's to talk about in your post.

AGMs have a little different charging profile (voltage and time parameters) than FLAs so most battery chargers have a select switch to set the battery type.

The 2 or 3 step charging modes are a different setting and I suspect the two step just switches the charger off when it meets the absorption criterion. For AGMs I don't see why you would want to do that. Just keep it in 3 step so it will float at a low 13.5V or so value to keep the batteries fully charged.

The alternators in most marine engines are regulated to produce one single voltage, about 13.5 like the float mode on the charger. As a result they don't do a good job of fully charging. But if you are back at the dock after a couple of days on the hook, the shore power charger will charge up the batteries completely, so no problem.

Only if you stay on the hook for a week and cruise to another place every couple of days would I worry about alternator regulation. Some say you can modify the existing alternators to accept external regulation. This can be done, but you are asking a lightweight alternator to perform as a heavy duty one and you may burn it out prematurely.

I would much prefer getting a heavy alternator like the Leece-Neville with external regulator terminals or a more expensive high output Balmar alternator and regulator.

Finally, the OEM alternators won't hurt your new batteries but they won't charge them up fully either. An ACR isn't the right device to deal with this issue, a new alternator/regulator is. Sterling does make a device that can help and it doesn't require external regulator terminals, but I would first see how what you have works first before doing anything.


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Old 05-18-2019, 10:51 AM   #4
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Thanks for the replies!

Ka_sea_ta, I'll have to go investigate and measure the alternator's output. Thanks for the tip.

djmarchand, kind of what I was thinking, leave it in thew three-stage mode. I've set it up to be in the "AGM" mode on the charger.

Right now our main cruising mode is weekends, with 2-3 days max away from power. One thing I forgot to add, I have a generator I can use to power the charger. My next assignment is figuring out if my generator outputs TSW (true sine wave) or MSW (modified square wave) as my charger recommends TSW.

Thanks again!
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Old 05-18-2019, 11:08 AM   #5
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Three stage, good.

Compare charger output voltages on the various modes to the battery maker's recommended bulk/absoprtion and float voltages. The AGM mode might be best, but it could also be the FLA or even the Gel mode could be better depending on your specific batteries. Odyssey tolerates higher voltages in each stage relative to Lifeline, for example.

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Old 05-18-2019, 11:11 AM   #6
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I donít know much about electrical systems and you have already been given advice that is better than I could give. I would suggest that you check to see if your charger has a ďcustomĒ setting beyond simply flooded and AGM. If it does, then set the 3 stage charge profile to that recommended by your battery manufacturer.

I have two different types of AGM battery banks on my boats by two different manufacturers. While both are AGM, they have significantly different recommended charge profiles. My point is just that if your charger only gives you two choices, flooded or AGM, then just pick AGM and donít worry about it. However, if it gives you a custom option, then match it to your batteries.


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