A LONG time ago (early 90’s) I spent my summers down at the marina. Most of the boats were sail and trawlers. At the time (from what I remember) they all had giant house bank batteries and normal starting batteries. They were usually isolated by a perko switch. The switch would be in 1+2 when the engine was running.

So.... my question is;
How did the alternator charge the two drastically different battery banks? Seems like one would be under or over charged. I thought the stronger battery would get ruined trying to charge the weaker one?

Thanks.