AGM battery question

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ddurand

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I have your standard GM small block with the normal marine alternator and a basic 2 battery isolator. Any issue with having one battery (starting) a flooded cell and the other (house) an AGM?
 
The charging rate on different batterry type is going to be an issue, especially with your battery charger.

If you have one flooded battery, you re going to have to add water to it anyway so filling out a second one is no big deal
 
You can "get away" with treating an AGM like a flooded LA battery as they're similar, but it's not ideal as AGM batteries like to float at a higher voltage (among other things). I would personally recommend you don't mix battery types.
 
Actually, aside from the portable plug in the wall ones I have for cars, boats, etc I do not have a battery charger for the boat.

So the only charging is the normal marine (Delco) alternator on the Mercruiser.
 
That being the case, you're better off sticking with flooded lead acid batteries.

What are you expecting from an AGM over a lead-acid battery?
 
Don't know what isolator you currently have but the Yandina C100 can be setup for mixing flooded and AGMs.
 
I was thinking the AGM battery would better handle deep discharges.
 
How about if I replace the starting with an AGM, so both are AGM?
 
No flooded battery (that means AGM or lead-acid) should be discharged below 50%. If that happens, AGMs have a track record of better being able to recover from it, but it's not a good thing. What you need to increase capacity is more amp hours. That means either a larger-capacity house battery or adding a second house battery in parallel (easy to do if you have the space).

You could go with AGM for both starting and house, but I'm going to bet your alternator is set up for lead-acid and isn't going to be optimized for AGM.

One thing in your case that AGM does have over lead-acid is a much lower self-discharge rate. Meaning that if your boat has no shore-side battery charger AGM batteries will hold their charge better than lead-acid. However, this has nothing to do with getting more amp hours out of a house bank.
 
I concur with the other comments. Having both batteries of the same type would be best. And switching to AGM would seem to offer you some distinct advantages, even with an older alternator which was not specifically designed for AGM batteries.

Mike
 
So will will the effect be "but I'm going to bet your alternator is set up for lead-acid and isn't going to be optimized for AGM."
 
You won't get the full potential out of the AGM battery(s). They like to float higher than lead-acid. They also like, but don't need, a much higher charging amperage (with a high-power charger you can bring and AGM up faster than a lead-acid).

Also note that while again you're not going to increase your bank capacity with the same-sized AGM vs. lead-acid (they'll both have similar power/amp hours if the same size), you may run into an issue with your house battery if you switch to AGM and you don't fully charge it each time it's discharged. Look up cruising forums and see how many cruisers have switched to either Gel or back to L-A. Over time crud builds up on the plates, especially if at anchor and not bringing the bank fully back up (tough to do). L-A batteries can be equalized to compensate for this issue; AGM batteries (other than Lifelines) cannot. I fell into the "AGM batteries are better" trap about 2 years ago. At 18 months the AGM batteries were shot and I went back to L-A. Thankfully, the pro-rated warranty more than covered the switch.
 
Sounds like I should return the AGM and get a group 27 lead acid.
 
Why don't you return the AGM and get two Group 31 LA batteries, the extra battery box and the cables? It'll probably come out to the same price but you'll have a heck of a lot more amp hours in your house bank (if you have room for it that is). Otherwise, I'd still try to at least upgrade to a single group 31. It may even fit in the same box you have and you'll gain amp hours over a 27.

Don't get me wrong, AGM batteries are great. But in my opinion only if you have a special need like installing them on their side or having such poor access as to not be able to check LA water levels.
 
I will measure to see how large a battery will fit. Not much extra room and another battery besides my starting and house battery.
 
I agree with everything said about AGM batteries. I'm an all AGM boat. 13 batteries in various banks but I always have the proper Perkoff's exponent set when charging, be it solar, with 3 stage smart charges and off the alternators. I subscribe to Lifelines and I have been able to bring back my main (3)-8D house bank from 2 really hard long discharges. I'm on my 7 season with most of the Lifeline batteries.
One other comment a battery isolator rarely lets you fully charge either battery. They are basically a big diode and you loose the top .6 of a volt. This may seem like knothing but it really counts when charging AGM batteries.
Bill
 
quote:
Originally posted by ddurand

I was thinking the AGM battery would better handle deep discharges.

Click to expand...


A deep cycle and some dual-purpose batteries (i.e., not typical "starting" batteries) are better suited for house purposes, but FLA, gel, or AGM batteries of the same size pretty much all offer the same amp-hours.

If you have Group 27 batteries now, a Group 31 deep cycle battery may fit in the same box.

Given your only charge is from the alternator, I'm guessing you probably wouldn't normally cook-off any electrolyte from a flooded battery... so if access is an issue, a sealed FLA battery may be about the easiest to deal with. If access is easy, a standard deep cycle FLA would be least expensive. If self-discharge is an issue, yes, AGMs are maybe better, if you can fully charge the things after each use (but simply changing to AGM of the same size won't buy you more amp-hours).

-Chris
 
AGM batteries are pretty much a "drop in" replacement for flooded cell batteries. No changes are necessary. It is a good idea though, to have all the same type of battery so both batteries should be replaced.

If you actually have a "battery isolator", get rid of it and install a "battery combiner" instead. As Billylll posted, an "isolator" causes a voltage drop which is not good.
 
I know battery isolator has a 0.7V drop but this is only in the charging wire that goes to each battery. The sense wire goes directly to the starting battery and it sees the true voltage of the starting battery. It seems to work OK.

I see there is a combiner called StartNow that looks pretty slick.
 
I would recommend replacing the isolator with a BlueSea high current ACR. There is no charging voltage drop, you can combine the batteries for emergency starting with the remote switch or you can lock the 2nd battery out if needed. I have been using a few of the 250 amp continuous duty ACR's (500 amp intermittent) on my boat since 2007. They are almost as old as my batteries.
Bill
 
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