The 'Why'
Belt dust despite great efforts at alignment.
Our alternator ( purchased from Maine Sail ) is a Balmar 100A, we had it serviced a couple of years back and the shop ran it up to 140Amps, it's powerful.
We have a Balmar MC 618 external regulator and have it throttled down to 75% to keep the load down to prevent squealing despite keeping the belt tension set appropriately.
I'm about to install 300Ah of LiFePo4 batteries and they can suck up the power ( hence the MC 618 ) but I'll be happy to fill the new batteries when we run the engine.
I have upgraded our electrical system substantially over the past year, a full Victron System, with the intent of replacing the Lead Acid Batteries which don't last very long here in SoFla.
Here's a link to my blog posts that show what we have done.