We've used AGM's both for starting our Volvo
and for house bank for 11 years, so both our shore-powered charger and alternator regulator
are set to AGM
parameters for best results.
Figuring out your starting bank is pretty easy - how many CCA do the big Volvo's need? A pair of group 34's or group 31's will probably do. We use a pair of g34 AGM's, same as for the Cummins diesel in our truck.
Before you spend a ton of money
on new batteries, wiring
, etc. you really ought to calculate your amp-hour needs for the house bank. That depends on the electrical
devices you use between charges, their current
requirements, and how long each one will be running. A spreadsheet like Excel is great for this - it's an iterative estimating process. I could email
the one we used.
Since you shouldn't go much below 50% and can't necessarily expect to recharge to much above 85% every day, you should have a house bank about 3X as big as the AH you require between charges.
AGM's are tolerant of somewhat deeper discharges than flooded, but they're not OK with taking them all the way down. I'd suggest no lower than down to 40%, or better no lower than 50%. The less you take them down the longer they'll last. Our just-replaced house bank (a pair of g31 Deka AGM's) lasted 11 years of cruising the PNW every summer, mostly anchoring
overnight, with no generator
AGM's also can sit for a very long time unused and not on a charger with little self-discharge. Many leave them untouched over the winter, even up in Alaska
An amp-hour battery monitor
is very helpful if you want to take good care of your house bank.