Charging deep cycle with 60A stock alternator

A friend has a 2019 Merc 150 4 stroke on his fishing boat. He has two deep cycle FLA batteries for his trolling motor and fish finder, plus a start battery. The boat has a charger that takes 120V for the deep cycle. The Merc charges only the start battery. On my sailboat I use a high output alternator with external regulator and a charge relay to maintain my house and start batteries. Would the Merc alternator burn out trying to set up a similar charging arrangement? Or can an outboard take a high output alternator with external regulator?

i am trying to free him up from the 120V a/c

thanks,
harry

Charge the deep cycle batteries in the slip/driveway. On the water, use a voltage sensing relay from the alternator to the start bank so that the alternator will kick over to the house bank after the start battery is full.
You won't burn up the alternator by putting a big bank under charge. It'll just be slooooowwwwwww to charge. You can run a fish finder for a long time on two deep cycle batteries but a trolling motor will suck them down rather fast. As long as the start battery is isolated from the house bank so that the engine always can be started, then no worries.
A 60 amp alternator will never actually charge at 60 amps in the real world. The valuation is under optimal conditions at the alternator's highest rated RPM which is like 18-20 thousand. If you get half that output you're doing pretty good. YMMV.

Excellent information. How about a small solar arrangement too? They are getting better all the time.