That's what I have done. I have one meter at the front for my start battery and one in the back for my aux battery. That way I can check my aux voltage every time I get a beer. So every 5mins :D
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My gauge in the front is in addition to the battery monitor I have in the rear, it's my 'reminder' to ensure I don't forget to disconnect the aux batteries & start battery if I've used the winch. I don't have a gauge just for my start battery, your post has been useful, putting in a switch so I can check the crank battery voltage sounds like a good idea.
Finally did this using double sided tape. See how it goes for the time being, will think about other options for permanent fitting later. I'm keen to see the volts whilst charging, and can check easily enough when stationary.
http://i8.photobucket.com/albums/a44...ps938b62ce.jpg
What about above the rearview mirror....I need to get a pic of mine to show you.
Mic
Mate, what is the 3rd wire connected to?
Red is +, black is - and green is the + of the battery you want to monitor. At the moment all the reds and blacks are on a cigarette lighter plug that goes into the dash, also 1 of the greens is connected to the reds so it reads the "car" battery volts, of the other 2 greens one is run to the aux battery under the bonnet and one to the 3rd battery in the back.