Originally Posted by blt2ski
Something not mentioned, I could be slightly off on the numbers. IIRC AGM and maybe gels lose 3% per month of power when not in use, where as lead will be 6%. Gels may be 6% too. For those on a mooring with out solar or other ways of keeping the batteries topped off when not in use, this too could be a reason to go with the AGM vs lead cell.
My 20amp truecharge IIRC has a place for AGM, GEL and flooded batteries. As Gel's take the least charge input vs lead with agm able to take the most.
With an older motor as the OP has, it would not surprise me if the alternator will need an upgrade. As mine from the mid 80's with a 20hp yanmar, a size 31 agm could take as much as 80-100% of its out put to charge it. So I stuck with a Lead cell. I was looking at a $500 alternator to upgrade etc too to go with at east one or two size 31's in my boat any how. I just went from a 24 to a 31. Gained enough for how I use my boat.
At the end, not sure there is a true right or wrong, only that you have the correct charging system for the type/style of battery and how big the pack is. This does take some number crunching.
Yes that could be a reason but AGM batteries are so much more expensive that I don't know if it is worth it just for that.
I have equipped my boat with AGM batteries and made a complete overall on the charging system and in my case the results were very good, but I have cruised nonstop for 3 months and during that time have been at only 6 marinas. I am not a big spender of energy and to have them charged at 80% It was enough the small time I run the engine in the morning before set sail and the small time after taking the sails out before set anchor (about or less than 1 hour a day).
I fully charged them on the times I have been in the marinas (more than a 24h charge) and in some passages where the engine was run for 7 or 8 hours, sometimes more. Never had them below 12.34V and most of the time the lower they get before being charged was 12.50. That small daily charge brought them to 12.80/12.90. if I remember correctly fully charged by the engine or the marina charger they get to 13.20 or 13.60? I cannot remember. Anyway, comparing with what I had in my previous boat, this set up is just wonderful.