I need to replace my 4D batteries on a 2008 309. I notice that a gel battery apparently has about three times the discharge/charge cycles as compared to the AGM type. But the AGM has about a third more cranking amps.
Does anybody have an opinion on my best replacement? I sail in warm weather (Florida) and am rarely more than two days from a shore power...mainly day sailing and weekend trips. I may eventually look at an air conditioner.
I appreciate your advice.
If you have the height in your battery compartment for wet cell 6V that is an excellent option. Gel's generally do last longer than AGM, HOWEVER, they require absolute precision in charging and are the easiest batteries to ruin if you charge them poorly. While I have seen Gel batteries last for a long time it is always on a boat that is fastidiously maintained and owned by someone who has a deep understanding of the charging system.
AGM's in contrast I see last considerably less long than deep cycle wet cells. This is probably attributable to the fact that AGM's really like to be topped up to 100% full after each use. They also do not like to be trickle charged, Lifeline specifically states not to, so both mooring sailed boats and boats at docks that leave the charger on 24/7 can suffer shorter life.
As for cycle life we should remember that those are LABORATORY cycles not real world.... They do one after the other and the batteries never get a chance to sulfate like they do in a boat. Charging is also usually temp compensated in these lab tests.
If I had to guess at it I would bet that 99% of the sailboats out there never even see 1/3 of the claimed cycles of any battery, wet, gel or AGM, before they are dead. Heck Lifeline claims 1000 cycles at 50% depth of discharge and I have yet to have any customer that I know of break 300 cycles before their AGM's were dead. I'm sure you can find a few but I have not..
My customer a few weeks ago was under 100 cycles and his AGM's were flat dead at less than 4 years old.. Even if you do 50 cycles/overnights to 50% per year, which is a lot for most coastal boaters, that is still only 250 cycles over 5 years. The vast majority of batteries on boats don't last much more than 5 years, though some do and I do have customers with 6-11 years on banks but these boats are all cycled rather lightly to perhaps 70-80% state of charge as the lowest depth and all are deep cycle wet cells including very heavy duty brands like Rolls..
Just looking at the calendar for this season we have approx 54 nights, I may be wrong on some weekends whether they were two or three nights, and we often take 3 day weekends as my wife does not work Mondays. I suspect this is more than most coastal boaters do in an average year? One of our friends only got 15 nights this year and on a good year they might do 20 nights on the hook. At that rate, based on Lifeline's "lab testing" for cycles, their batteries should be lasting 50 years....
Ignore "claimed" cycle life and buy the thickest plate batteries you can fit. 6V are always thicker than 12V.
