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Old 29-01-2010, 05:40   #1
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AGM vs. Gel

I am sure this has been talked to death, but what are the pros, cons and costs differences in AGM vs Gel batteries. I am getting ready to buy a caliber 40. The listing shows AGM batteries. The survey says, NO, they are gel.

Bob
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Old 29-01-2010, 05:56   #2
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Gel batteries tend to have shorter lives than AGM if they are exposed to consistently high engine room temperatures. It can be MUCH shorter. Don't ask how I know this

P.
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Old 29-01-2010, 07:35   #3
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There are two schools of thought on this, but for cruising on the hook the gels are probably preferrable, as they have more tolerance for long-term operation at partial discharge. The gels are more susceptible to over charging, but a good regulation system can deal with that. My gel banks have needed replacing about every 7 years.
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Old 29-01-2010, 08:20   #4
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All batteries have a finite life that can be shortened by lack of care or common mistakes by even the best owners. Gel's weak spot is that a charge voltage that won't hurt other batteries (14.5v) is too high for gels. You also need to reduce the charge voltage when the battery is warmer than 75 degrees. Too many boats have battery chargers (or alternator regulators) that are not properly setup for gels or lack the temperature sensor to reduce the voltage. But if the charging was right, gels will last as long or longer than AGM's or most flooded batteries. (All batteries will have a shorter life in a hot engine room).

Battery failure away from major civilization is expensive and incredibly inconvenient. At around six years my peace of mind wants replacement. Sure I can get a few more years but I replace my rigging before it fails too. The six years assumes excellent care of the batteries. If the care was less than excellent - even briefly - then six years is too long. A one to two year life is quite possible. Unfortunately, I know of no reliable way to tell - until the battery starts to fail.

So I think the safest thing to do when buying a used cruising boat is to assume you'll decide to replace the batteries within a year for peace of mind - regardless of type or age. Even many otherwise careful previous owners don't really understand battery care. You'll have enough other things to worry about on your new boat.

You can spend the year trying to decide whether to replace with flooded, AGM, or Gel - and once you've made that choice - which brand and size. There are strongly held and reasonable arguments on this board for many different solutions.

Carl
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Old 01-02-2010, 13:27   #5
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For now, the great majority of my sailing is weekenders and long weekends.
So, whilst I'm in this mode I'm happy to live with the risk of battery failure as I won't be too put out. If I have a failure, it will likely be one of the 2 batteries in the bank, so I will have power, just less reserve. Then replace.
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Old 01-02-2010, 13:35   #6
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Agree 100% with CarlF. Obviously, he's the voice of experience.

For weekend sailing and most of the time at the dock on a charger, gels can be a good choice. I have a couple in my basement right now which spent 10 years on a sailboat in just those circumstances, were used for 18 months in a series of battery tests here, and have been pretty idle for about 18 months, though kept charged. Last week I checked their capacity with a sophisticated test instrument. One of the two checks very, very good...almost like new. The other tests at about 85% of capacity. And these are 13 years old!! There's a reason why gels are the choice for standby power supplies, UPS, telephone service, etc.

But, as CarlF says, you can't mistreat them as you can flooded batteries without paying a price.

Bill
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Old 01-02-2010, 13:42   #7
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A Diversion

FraidNot,

One hot afternoon in a dusty cow town two strings walk into a bar. They don't even make it across the floor before an irritated bartender shouts, "Hey, get outta here right now. We don't serve strings in this bar".

Back outside and very thirsty, one string gets an idea. He twists himself into a really nice knot at one end --- a bowline, actually--- and arranges the threads all 'round. Back into the bar he goes while his friend watches from the swinging doors.

Makes it all the way to the bar, orders a cool brew, and tosses it down. The bartender looks at him, tilts his head, then growls, "Say, ain't you a string?"

With great aplomb, the string pulls himself up to his full height and answers, "No, sir. I'm a frayed knot!"

Just to keep this in the nautical tradition, he learned that trick in the forecastle of a square rigger going 'round the Horn headed to Falmouth for Orders. :-)

Bill
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Old 01-02-2010, 14:48   #8
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Just to reiterate, abuse can mean only a one time charge at too high a voltage with a Gel Cell battery. For my money, they are just too fragile for a cruising boat. If your alternator, solar panel controller and/or windmill charge regulator screw up for even a short period of time and you are out of electron storage. Not something you want to discover in Fatu Hiva.

Personally, good old lead acid batteries are about half the cost and last just about as long so are the only battery I'd consider. If you've got the bucks and/or won't do the simple maintenance required of lead acid batteries, then go with AGM
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