I ran my power into the switch. I put my ground on a spot that other stuff was grounded to under the helm with the understanding that I might have a noise issue and be required to run a wire back to the battery. No noise issues whatsoever. A monitor seems pointless to me. So long as you are never using the "BOTH" function with the motor turned off you are pretty much in complete control of which batteries are used or charged. I'd prefer to spend the money on an isolator before a battery monitor. Just my $0.02.
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Paul
2007 Four Winns Horizon 220Mercruiser 350 Mag MPI w/ Bravo III Drive
2004 Ford Explorer Limited (V8-AWD)
Pomme de Terre Lake (Southwest Missouri)