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Go Back   SailNet Community > On Board > Gear & Maintenance
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  #11  
Old 07-31-2007
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Agree with Wind Magic -- a topend AGM might live longer by the calendar than a bottom end deep-cycle wet, like the T50 golf cart. But I'd put L16 flooded batts against any gel or AGM in the world. And my batteries -- GNB 3700s, a dozen at 800 lbs each, 1/4" thick plates -- can accept charging current at a higher rate than any AGM. Too big to hurt. (That's conservatively 10,000 amp hours at 24v & 100-hr discharge rate.)

I would agree that AGMs offer rapid charge/deep discharge cycles; they have lots of surface area inside. So they are great for forklifts, floor polishers and such. But the surface area comes at the cost of electrode thickness; the mats will erode with charging & discharging and you may see excellent performance right up until the day they don't work at all. That's not a bad trait, really. At least, if you are in a place you can find new ones. Flooded lead acids have a longer but more insidious taper.

Wet cells remain tops for conversion efficiency and long-term, predictable performance. Cheap wet cells stink; good ones are unmatched for durability. On a cost-per-kWh-lifetime basis, good wets win by a ridiculous margin. That said, on a sailboat I'd plump for sealed AGM. To much pitching, vibration, temperature variation, etc. Not a good environment for flooded cells. (Neither is a car, really.)

The starter battery is an interesting issue. Those offer very rapid shallow-cycling but hate deep-cycling. Your basic car battery. Surprised all boats don't have a dedicated small one lashed to the engine & charge off a small alternator; or at least a diode isolater from the rest of the deep-cycle batts.
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Old 07-31-2007
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I have to say I disagree with the advice to use wet cells if you are a weekender/vacationer and AGMs for cruising. I would recommend AGMs for both types of sailing but, if anything, the many advantages of AGMs benefit weekenders and vacationers even more so than cruisers.

One of the biggest advantages, besides no maintenance, is that AGMs have a much slower self discharge rate and a faster charge rate. Perfect for people who leave their boats for weeks or months at a time, and only have limited opportunity to charge. If you're on a mooring, with no shore power charging, they are fantastic.

AGMs, despite the extra cost, are one of the best investments we made on our boats. I'm not talking about discharges-per-dollar, bang-for-the-buck, etc. I'm talking about removing one more hassle and worry from your maintenance list, and getting better performance to boot. Unless you enjoy monitoring electrolytes, adding distilled water, worrying about off-gassing, removing heavy batteries for winter storage, etc....

Forget gel cells. They are finicky beasts that cost the same as AGMs but require a very particular charging profile, necessitating at a minimum that the alternator's voltage regulator be modified. A previous owner installed gel cells on our boat without modifying the voltage regulator and they were defunct withing two years. AGMs can be swapped out as replacements for wet cells without any modification to the boat's electrical system (although a smart regulator would get you even better performance).
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Old 07-31-2007
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I'd agree that Gels aren't really the right way to go in any case.

If you have fairly high loads, then AGMs make far more sense than wet cells, since they have a much higher bulk charge acceptance rate, and will effectively maximize your charging capabilities. If you have fairly low loads and fairly low charging capacity, it doesn't really matter too much.
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Old 07-31-2007
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I just bought 4 6v Interstate U2200 batteries for $83.00 each. That is less then 1/2 of a AGM.
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Old 07-31-2007
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Free...Right...and you get less than half the life cycles and have to fill 'em with water AND under heavy amp load they last about 1/2 the time to discharge as an AGM even though the 20 hr rating for amp hours is the same.
And here's what Interstate (your company) says about THEIR AGM's....
Lasts up to 2 times longer.
Can sit unused 3 times longer.
Over 15 times more resistant to vibration.
It's nonspillable and can mount in almost any position.
It provides constant performance quality keeping your battery running at the same level even as it's being discharged.
So...your u2200 flooded may indeed be the MOST economical and best choice for you depending on how you use your boat...BUT AGM's ARE better batteries in several ways AND may be more economical if you use your boat heavily.

Here is a cost comparison vs. days aboard per year (assumes one cycle per day) using several popular brands:



Note that the ZERO line represents the base cost effectiveness of LIFELINE AGM's in the 8D format. Anything above the zero line represents a % increase in lifetime costs...below the line represents better cost effectiveness.

The background and inputs to this graph are QUITE complex and are available on the link I gave in post #9 above. The formula is driven by spreadsheet inputs on everything from battery cost to engine run time and alternator capacity AND you can download the speadsheet and put in YOUR OWN BOAT's parameters and any manufacturer's battery specs and costs to determine what is most cost effective FOR YOU.
Example...even if you live aboard full time...wet cells may be more cost effective than AGM's if you have a large bank and a small alternator.
We owe Mr. VonWentzel a debt of gratitude for his incredible work on this spreadsheet!

Last edited by camaraderie; 07-31-2007 at 07:18 PM.
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Old 07-31-2007
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I've had the same gel cells for 10 years. I installed them when I got the boat, and havn't looked at them since. Talk about maintainance free. If I leave the boat for the winter, they hardly loose any charge. I have 4, 6-volt batteries for starting and house, and 1, 12 volt for starting the generator. Whatever they cost originally, was worth it to me. I never even think about the batteries.
Marc
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Old 07-31-2007
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can I get a recommendation for AGM brands? You can PM me if you like, thanks.
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Old 07-31-2007
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BF-

Deka or LifeLine. Lifeline is a bit more expensive... but will probably last longer...
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You know what the first rule of sailing is? ...Love. You can learn all the math in the 'verse, but you take
a boat to the sea you don't love, she'll shake you off just as sure as the turning of the worlds. Love keeps
her going when she oughta fall down, tells you she's hurting 'fore she keens. Makes her a home.

—Cpt. Mal Reynolds, Serenity (edited)

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  #19  
Old 07-31-2007
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Yup...those are fine...one thing one might take away from the graph above is that West Marine is not a real good place to buy batteries of any type!
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Old 08-01-2007
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As I recall, Cam, aren't the West Marine's simply Dekas?

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