My boat has an ideal but tight space to fit the house batteries into. Right now there are two group 27 batteries in there with no battery boxes. The space is a shelf that is 22" wide in the front, 14" wide in the back, 14" deep, and clearance is limited along the back corner to about 9" high.
It's hard to get things in and out of, with tight access into the space. The space is the dead space in the reverse transom of my boat. There is a photo of the access here:
And a photo of the two batteries here:
My electrical requirements are light. 100 AH capacity is likely enough, 150 AH would make me very comfortable, and 200+ AH is overkill. There is no fridge, all lighting is LED, and we have pretty basic electronics (chartplotter + autopilot + vhf, no radar).
I'm tempted to go with 2 group 24 AGM batteries because they save me the space of the battery box and should meet my power needs. They would provide a combined 160 amp hours. The other option that I'm considering is trying to shoehorn 2 group 24 batteries into a single battery box in the same spot.
My boat has a Yanmar 2GM20F with either a 35 amp or 55 amp Hitachi alternator. It does not have a smart regulator, just the one built into the alternator. From what I've found my alternator should be able to supply about 40-50% of the battery capacity, which would mean an 80 amp alternator for this small battery bank. Is that really correct?!?
My shore power charger is a Xantrex Truepower 20, which also seems like it would potentially be undersized. It does have an AGM mode.
I'm trying to be semi-frugal about the upgrade (I'd rather spend money on sails), so 2 batteries plus alternator plus smart regulator is more than I'd like to spend. Can I get away with just the 3-stage charger if I go with AGM, or do I need the large alternator?