Originally Posted by Capt RonB
The choice likely comes down to maintenance issues. If your batteries are in a place that's difficult to check the water or if your boat gets intermittent use and you're not there to keep a check on things every day, then AGM would be a better choice. They do require proper charging ... but if you follow the manufacturer's recommended cycling, voltages, etc they will likely last longer than the wet cells. I got 10 years plus out of my gel cells and they were still going strong when I sold the boat.
Wet cells are quite a bit less expensive and may last quite a long time but they will require frequent water replenishment and constant terminal watchfulness. This is because they outgas or 'boil' away electrolytes and the stuff that they emit is very corrosive hence terminals tend to corrode. So you need good access to the battery tops, good ventilation of the battery compartment and lots of WD40. Distilled water is preferred for topping up wet cell batteries.
One other consideration might be charging time. If you have enough charging current
(amps) the AGMs have a higher acceptance rate and will charge quicker. Wet cells seldom accept more than 25% of capacity and that only for a short time. AGMs will likely accept up to 50% of capacity initially if you can provide that much.
Also the AGMs are much more tolerant of bumping, rocking & rolling, etc. than wet cells.
If you can stand the initial purchase price
, I prefer AGMs. Over the long haul, they'll likely end up the more economical choice.
OTOH, If your boat is an interim step to something bigger in five years, then I'd probably go for the cheaper alternative.
Please keep in mind that AGM batteries are not the same as GEL batteries. The life you got out of your GEL's is not atypical of well cared for properly charged GEL cells. This however would be a non-typical life for an AGM battery, in my experience.
Properly charged flooded batteries are very easy to care for and arguably more accepting/tolerant of improper use/charging etc.. where GEL and AGM are not very tollerant. If you know you may not be the best about getting the banks back to 100% regularly, or your charging voltages can't be adjusted, or you don't have the proper current to feed to them, then AGM may not be the best choice despite their many other benefits.
piles of banks of flooded cell batteries with zero corrosion
simply because the terminals were properly treated with a good terminal grease and the battery compartment has adequate ventilation. Terminal corrosion
should be a non-issue with properly charged, installed and cared for flooded batteries.
Last Friday I was on-board a boat I re-wired nearly 5 years ago. It had recently been sold and my name given to the new owner. The "issue" turned out to be a loose nut on the battery terminal. The boat yard had negated a lock washer and the nut came loose....
The flooded cell batteries looked as good as the day I installed them, zero corrosion, and they still tested at about 95% of new capacity using a Midtronics analyzer. This is not bad considering the boat only has a stock alternator
and a 15W solar
panel.... The entire bank cost well under $300.00.....
This spring I replaced four 6V Lifeline batteries (old ones are still sitting in my shop for core) at four and a half years old. The previous bank of 4 GC2 6V wets had lasted 9.5 years on the same boat under the same owner with the same charging system. In both cases charging parameters had been set up to manufacturers specifications.
The flooded bank cost one quarter the price of the AGM's and outlasted it by more than double the service
life. He still chose to stick with AGM, because money
is a non-issue to him, and he wants the acceptance rate AGM offers. He was still not happy with the life and now has finally agreed to "condition charge" his new bank of Lifeline's to hopefully help it last longer..
AGM's certainly have benefits over wets but cycle life is just not one of the benefits I have observed in the marine environment