You usually notice it at the helm before you smell it: the voltmeter sagging, the ammeter stuck the wrong way, a little alarm that suddenly feels louder than it ought to. The engine’s turning fine, the sea’s doing what it does, and meanwhile the batteries are quietly becoming a countdown clock.
First order is to stop the bleeding. Get the load down, keep a start reserve you can trust, and don’t let a handful of hungry gadgets decide your priorities for you. Once the draw is tamed, you can go looking—belt, heat, loose lugs, a regulator that’s sulking—without making things worse in the process.
I’ve seen plenty of “charging failures” that were nothing more exotic than one tired connection you could wiggle with two fingers. I’ve also seen the kind you don’t fix at sea. Either way, staying calm and methodical keeps it from turning into a bigger story than it needs to be.