Oddly enough Ripple has a 12v lighter built in so we just run a cheap plastic voltmeter that plugs into that. The 'lighter' is switched along with the depth gauge and instrument lighting.
I wouldn't 'hard wire' it to the batteries. Definately put it on a switch.
A current meter isn't a bad idea either but, same deal, it should be on a switch. The only things we have straight off battery are the automatic bilge pump and the radios.
s/v Ripple 1977 Hunter 27
She's a beater but we're in the game.