Originally Posted by hellosailor
With regard to AGM sucking the life out of an alternator...Bear in mind that depending on the overall capacity of your battery bank(s) and how deeply you discharge them, you can control that to some extent just by noting how much charge you need (i.e. 50% of 400AH? 75% of 800AH?) and not letting the demand go too far beyond what your alternator/regulator can accommodate.
You're quite right about the heat. (4) 6V batteries would be 400 to 450 AH. @ 50% SOC they would most likely "accept" up to 50A. An internally regulated alternator would attempt to provide that load, and an OEM alternator, regardless of regulator (unless you have one like the MC-612 which has Amp Manager and Small Engine Mode).
So you need to match your alternator size to the acceptance of the house bank @ 50%, which usually results in a 100 A alternator working at 50% of its total capacity.
Millions have gone through this same exercise, and those are the numbers that come out and get you, most all of the time. Why? Balances output, storage capacity and input from the alternator.
hellos is right, and AGMs will accept a higher rate of charge than wet cells, so re-check your alternator sizing.