OH got ya .. that relay won`t accomplish that .
A solenoid/relay would accomplish that, just don`t make the switch monetary .

Quick search:

PQ290,

Glad we could help. This is what happens when you ask boys for men's advice.

Thanks. Brad.
(937)545-8991

Or a true battery isolator/combiner would also serve that situation well.

Could also just run this relay on the charge wire like I was saying....take your alt. wire to the in on this and two new wires from the outs to each battery. That's pretty much what the diagram shows and it won't over charge the battery that is not in use but if that battery were to drop it would sense that and kick some power to it while still keeping it isolated from the in use battery....kinda what it's designed for.

Or what happens when you try to explain something to someone who won't read instructions

BBYSTWY,

I am looking at the wiring diagram, and it does NOT show a connection from the alternator to the isolator. It shows everything BUT.

That's OK. We've already established that this isolator will not do what I would want it to do. I'll stick with what I've got until I can find something that will. It's not like it ain't working.

Thanks. Brad.
(937)545-8991

Great read on ACR’s:
https://marinehowto.com/automatic-charging-relays/

If you only have a few minutes, wait until you have more free time.

SB,

So, I see a project ahead of me. What's the output of the stock alternator on a Merc 496. '02, if that makes a difference.

Thanks. Brad.
(937)545-8991

I don’t know for sure, but Merc seems to love 55 amp alt’s over the years.

Brad, Okay, found a few spec pages. They (8.1/496’s) appear to have 65 amp alternators.

Brad i think what yor looking for is a diode battery isolator:
https://www.sbmar.com/articles/how-t...an-alternator/

everything on the boat stays the same but the output of alternator goes to the isolator and it can charge both batts at once and not back feed each other.