Originally Posted by Don L
In today's world I would never get an AGM battery! They have no advantage other than not spilling/watering and on a boat with a 40A charger aren't even going to get the "faster charging" (they don't really).
Instead get Lithium Iron Phosphate (LFP).
I see there are use cases for LFPs, but we've not yet had a boat that would make it easily or affordably possible.
Our boats have all been built with dual-purpose battery architecture, each bank acting as a mains starter that also services about half of the DC house/electronics loads. Quite common in US-built production powerboats. (Not sure, but Bruce's boat might be similar.) Making an architectural change to isolate a house bank is daunting, potentially there can be real estate issues. and then there's all that cost and complexity thing associated with LFP.
(Recent thread examining LFP for a thruster bank... ditto.)
For us, that whole service spilling/watering thing (and potential off-gassing) is a big deal, and AGMs scratch that itch... at a decent (enough) price point. And having a dedicated 70A charger is different from a split-use (multi-bank) 40A charger.
IOW, in today's world, and for our use, I would always (at least) consider an AGM battery... and I think there's still a viable place for wet batteries, too, where service is easy and off-gassing isn't an issue.
I think I did see info about a high-cranking amp LFP recently. Big buck$ and not much capacity for dual-purpose work, IIRC. Can't remember where I saw that...