Rbob, the correct term to be aware of is a battery's "charge acceptance rate" CAR.
AGM and flooded lead acid batteries would have a relatively high CAR up to about 50-60% state of charge, then their ability to accept high charge current diminishes rapidly. This essentially "throttles" the charging current coming from the outboard. Given that a start battery is rarely less than 90% state of charge, the alternator almost never has to work full out. This is a good thing for any engine with a standard automotive type alternator with internal rectifier/regulator that is air cooled, because it means the alternator is only running "wide open" for a short amount of time and thus never overheats.
However, with lithium house batteries many of us have, the CAR is very high up to about 90=95% state of charge, meaning they WILL demand high current for sustained long periods of time from the alternator (via battery to battery charger or ACR or whatever), meaning the alternator now is working hard for a long period of time, and if it's a automotive air-cooled type, it WILL overheat unless you have somehow designed to limit the charge current to about 50% of the alternator's nameplate rating. You'll see this advice commonly for diesel engine cruisers, and it applies to the Merc outboards IF the alternators really have an air-cooled rectifier/regulator like auto styles do. That's what I'm trying to find out.
