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Old 12-08-2018, 10:58   #1
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Sizing battery charger for AGM and Generator On-the-hook

Yet another battery charger thread. Sorry.

It's easy to find threads that recommend sizing a charger for AGMs at 20% of C, or a 60 A charger for a 300 Ah bank. It's also easy to find threads that recommend sizing the charger as large as practicable if it's expected to be using a generator to power the charger while cruising or mooring. Obviously, he wants to run the generator as little as possible to keep the battery recharged. (And, this is a different question, but is it realistic to us a generator to bring a bank up to absorption, say, and then turn off the generator and use solar panels for absorption and float?)

Is there any consensus on reconciling these advices? What I'm really asking is this: Will AGM batteries safely take as much current as you throw at them in bulk phase, or should the charge current be limited, say, to 20% of C. I think for a minute I confused AGM rates with lithium battery rates, where I believe I've read that a lithium battery will take up to 4X C in the bulk stage.

In this specific instance, I'm working for a cost-sensitive owner. I've told him about the Iota chargers because others on this forum have said they have them and are satisfied. Personally, I like the Sterling Ultracharge Pro 60 A. He has West Marine AGMs: 3x105 Ah house bank and another 105 AH starter battery. I'm planning to put the charger and alternator outputs on the house bank with an ACR/VSR to the starter battery. When I brought this up, he indicated that he liked Xantrex chargers; apologies to the Xantrex people here, but I believe I've read a number of anecdotal experiences of people having problems with Xantrex. If anyone here really likes Xantrex, I'd like to here about your experience.

Thanks, and I'm really, really sorry for yet another charger thread, but I'm not finding info on the max current that AGMs can safely take in bulk.
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Old 12-08-2018, 12:43   #2
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Re: Sizing battery charger for AGM and Generator On-the-hook

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Is there any consensus on reconciling these advices? What I'm really asking is this: Will AGM batteries safely take as much current as you throw at them in bulk phase, or should the charge current be limited, say, to 20% of C. I think for a minute I confused AGM rates with lithium battery rates, where I believe I've read that a lithium battery will take up to 4X C in the bulk stage.

Personally, I like the Sterling Ultracharge Pro 60 A. He has West Marine AGMs: 3x105 Ah house bank and another 105 AH starter battery.

I don't think the "advices" are contradictory. Lifeline (for example) suggests a minimum of 2C if possible but more is OK, and their follow-on wording makes less sound not exactly horrible, just more is better for longest life. Odyssey suggests something higher, maybe 4C or some such. I think that usually means more charger is better, and then the batteries can decide how much current they want.

You can probably look up charging recommendations fo rthe WM AGMs (Maine Sail in another thread has said they're made by East Penn, so they'd probably be Deka Intimidator batteries.

Real world prices for the ProMariner Pronautic 1260 (same as that Sterling model) are significantly more friendly than MSRPs, if that helps.

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Old 14-08-2018, 20:23   #3
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Re: Sizing battery charger for AGM and Generator On-the-hook

.2C

.4C

very different from 2C and 4C

These are minimums for longevity.

Bigger is always better, up to .8C anyway, for lead.

Choose ones that let you de-rate if you want to.
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Old 15-08-2018, 04:08   #4
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Re: Sizing battery charger for AGM and Generator On-the-hook

Ummm, yes, sorry, good catch.

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Old 15-08-2018, 04:47   #5
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Re: Sizing battery charger for AGM and Generator On-the-hook

Bigger isn't always better but for genset powered battery chargers for AGM or other high acceptance battery banks it is... don't forget to set up battery post
temperature compensation...
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Old 15-08-2018, 08:47   #6
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Sizing battery charger for AGM and Generator On-the-hook

You cannot have too much charger, a lead acid battery itself will limit how much power it accepts to safe levels, you can throw a 1000 amp charger to a single battery, it will only accept what it can, if the voltage is correct.
How big is the generator? I can charge 185 amps and that is about all that my 30 amp generator can make anyway, itís loaded up pretty good, so for me 185 amps is about right. I stay in bulk if pretty deeply discharged for 20 min. at 185 amps. Iím a 660 AH AGM bank.
The best use in my opinion for generator charging is early in the morning to get the bank through the bulk phase, then shut down and let Solar finish it off, this way you can get to truly 100% charged.
However I hear all the time people running their Hondaís late in the day trying to get charged, and this is very inefficient as the banks charge acceptance rate is low now, so your just dribbling in the charge.
Best use it to hammer in a lot of amps with the generator in the early morning cause it can, and that is good for the bank, then let Solar do the dribbling in.
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Old 15-08-2018, 09:08   #7
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Re: Sizing battery charger for AGM and Generator On-the-hook

Yes.

Trying to get to 100% finish charge, burning dino juice for just that purpose is silly.

An ammeter set up to show bank acceptance rate is helpful, shut down once it gets low.

An AH counting meter and/or SoC gauge even more so, help make wiser decisions based on objective data.
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Old 15-08-2018, 09:42   #8
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Re: Sizing battery charger for AGM and Generator On-the-hook

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.2C
.2C

.4C
very different from 2C and 4C
These are minimums for longevity.
Bigger is always better, up to .8C anyway, for lead.
Choose ones that let you de-rate if you want to.

Ok, John, I'll bite.

225ah 12vdc x .8 = 180ah alternator or charger. Yikes. When would I use that?
(I was debating if a 40a charger would be enough just because I didn't want to find out later I needed more. I think it will be fine, particularly if the small bank is charged by Alternator up to .8C-.85C, and also including some small house loads of 8amps.)
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Old 15-08-2018, 10:38   #9
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Sizing battery charger for AGM and Generator On-the-hook

An AGM battery can initially accept a phenomenal amount of current.
A Lifeline battery can accept up to 5C, but itís only going to do so for a very short period of time before the battery will accept less.
For me, that would be a 3,300 amp charger, so none of us realistically can do a 5C Charger so we accept what we can have, but if possible I think you should strive for .2C at least.
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Old 15-08-2018, 17:15   #10
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Re: Sizing battery charger for AGM and Generator On-the-hook

A large charger serves us well when we run the AC watermaker through the inverter (We could run it right from the genny but sometimes we choose to run it through the inverter) so it has other uses besides charging the batts. Having a large charger allows us to keep pace with the massive power requirements of the watermaker. I assume this is also true for other high amp draws like washers and dryers (which we don't have) and air conditioning units. Victron (and probably others) has chargers that you can gang, so you can start with a 100 A charger and then get another one later on and because they talk to each other, they work together to optimally charge.
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Old 15-08-2018, 17:26   #11
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Sizing battery charger for AGM and Generator On-the-hook

I believe pretty much any charger will work with any other.
At least my Sterling, the Magnum inverter / Charger and the Solar Outback 80 and the Balmar 614 for the alternator all will charge together, none of them fight each other or whatever. Amazingly they all seem to pretty much share the load although occasionally one will back off more than the others, that I feel pretty sure is due to the temperature regulation feature, itís sensing a higher temp than another charger is.

I will make water if we are motoring using the inverter to power the Watermaker, the alternator and Solar will keep up and even charge a little, but the Watermaker takes quite a lot of amps.
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Old 15-08-2018, 18:01   #12
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Re: Sizing battery charger for AGM and Generator On-the-hook

The .2-.4C rates, again are only minimum / required if you want to maximize longevity for your expensive AGM bank.

IMO another strike against them, get AGM only if you really need it, if you want best value $/AH/year, stick to FLA.

There .2C is a maximum.
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Old 15-08-2018, 18:06   #13
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Re: Sizing battery charger for AGM and Generator On-the-hook

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There .2C is a maximum.


Reference? Cause I donít believe that is correct, a battery will determine through its acceptance rate how much charge it will take.
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Old 15-08-2018, 20:31   #14
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Re: Sizing battery charger for AGM and Generator On-the-hook

FLA is a low-CAR chemistry, even deeply depleted cannot accept much more than .15C, and even that falls off sharply after CV transition.

That's why besides fitting sideways, minimizing charger runtime is a valid compelling reason to go to high-CAR AGM.

Upside to FLA is you don't need a huge charger to maximise longevity, and they inherently live longer, more robust anyway. And cheaper.

Now maybe the pricey Rolls / Surette units are somehow tweaked to accept higher rates, dunno.
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Old 15-08-2018, 20:33   #15
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Re: Sizing battery charger for AGM and Generator On-the-hook

And I meant maximum it will accept.

Of course you can put a 500A charge source on a 60A battery, just a waste of money.

And guard against overheating, keep water topped up etc.
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