Got these cheapies off eBay and going to put 2 in the dash for main & 2nd battery, and one in the rear for 2nd battery.

IMAG0455.jpg

Red is to power the unit +ve anywhere from 4 to 30v
Black is earth or -ve.
Yellow is the +vs lead in from the voltage you want to test.

I've tested them 9v & 1.5v batteries and the seem to work alright, although you can't combine the red & yellow wires, you need 2 separate +ve sources, I should have bought the 2-wire gauge for that, d'oh.

My plan is to keep it simple. For the two in the dash run a wire each from the positives of both batteries and split them both into 2.

I'll power the gauge for the main battery with the 2nd battery & vice versa and then read off the other split and just run a common earth.

For the rear ill power off the +ve for the rear interior light and read voltage off the accessory cable to the rear.

Is it really that simple or have I overlooked something?