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  #1  
Old 03-10-2008
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AGM or GEL batteries?

Hi all,

I know there are several other threads more or less related and that most of your wisdom on this topic is already in some of them, but I would like your advise on this specific choice:
AGM or Gel batteries for the house bank?

To put everything in context, I'm refitting an old 40 ft steel yacht, with the purpose to do some cruising in western europe. I have one gel battery for the house bank now (100 Ah) and want to add two more to reach 300 Ah total. I just ordered a good charger and now I want to be sure to have good batteries as well. Since I have to add two batteries anyway, I might as well buy all three of them new. Also because the one I have is over a year old which might not work well with new batteries connected to it.

Hope to hear from you!
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Get AGM batteries. Gel have basically the worst features of any battery and the most vulnerable to overcharging...
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Old 03-10-2008
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A quick google found this which looks like a wealth of information on the subject -
Comparing marine batteries (Gel, Absorbed Glass Mat (AGM), flooded lead acid)

I'm seriously considering switching to AGM's myself and I will read everything I can find - I use two 4D's and two 8D's so either way it's a sizable investment for a poor (and necessarily frugal) Scotsman.
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AGM for the reason SD already posted (overcharge is safer).
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AGMs look good when you check their specs out and listen to the marketing. If you search the web a bit you'll find a lot of cases of early AGM failures on cruising boats. Some from under charging -- they don't like it. But others seem like just premature failures. If you are using these for cruising, I'd at least check out some of these failures before I'd jump into AGMs. Gels seem to have a much better reputation in practice for long service life. Yes, they can be ruined by charging at too high voltage. That's what the regulator is for.

Paul L
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Neither the AGM or Gel cell is bulletproof but AGM's generally stand up a bit better to abuse. If you get AGM's you must have a good 3 stage charger AND bring them to a 100% charge every couple of weeks at a minimum. they do NOT do well if you never get the last 15% or so of charge into them so you must plug into dockside or have some passive system that can really top them up. If this works for you AGM's are a GREAT choice for FULL TIME cruising where the economics of their higher initial price make sense due to increased cycling and lower fuel costs to recharge. The VonWenztel site that Larry and Sue cite above makes a pretty good case of this.
Flooded batteries make more economic sense if you are not full time cruising and don't require the additional benefits of AGM's (non maintenance etc.).
If you plan on extended cruising you may wish to consider the new Odyssey batteries we've been talking about recently here as they clearly have huge implications for the full time cruiser despite their expense.
WAY Cool New Battery Technology
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I have 4-6v Prevailer house and 1-12v generator gel cells that will be starting their 11th year. After installing them in 1997, I practically never look at them. At the end of last season I thought that they were starting to show signs of weakening. After winterizing in the fall, I charged them overnite on land. Today I was at the boat and checked them. 12.62v after sitting all winter. They have never been deeply discharged, I don't know if that has anything to do with their longevity.
Marc
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I second pretty much everything Cam said. However, I will add that there are a LOT of positives about AGM's that make them better for sailors/cruisers than wets, including: Can be mounted on sides, no water needed, quicker recharge rates, etc.

Also, like wet cells, AGM's are not created equal. You get what you pay for. The comments about some AGM failures are true; I have heard some of those too. But I have also spoken with many full time cruisers that have had them trouble free and would never go back. I will never use wets again, I doubt Cam would, and well as many other people.

- CD
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Quote:
Originally Posted by camaraderie View Post
Neither the AGM or Gel cell is bulletproof but AGM's generally stand up a bit better to abuse. If you get AGM's you must have a good 3 stage charger AND bring them to a 100% charge every couple of weeks at a minimum. they do NOT do well if you never get the last 15% or so of charge into them so you must plug into dockside or have some passive system that can really top them up. ....
This is the rub. Most full time cruisers cannot reasonably get a their batts back to 100% every couple of weeks. It is just too much time on a generator and solar generally won't do it. To compensate, you will probably have to equalize the AGMs once a month or so. This has its own risks. As in everything else on boat, it is all a trade-off. As far as getting what you pay for. I think you'll find that the early failure AGMs anecdotes are not limited to cheap batts.

Paul L
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If your charging system is properly designed and sized, there shouldn't be too much of a problem getting AGM's up to 100% once every two weeks IMHO.
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