If connected to the existing harness, the volt meter turns off with ignition and will show voltage for the main battery.
Your switch idea would be ideal for second battery, that way you can charge without having ignition on.@perkj I have a similar question as I am hoping that the one I ordered comes with two monitors which I will wire to the key and individual batteries. I am guessing on these since the design is to replace the ashtray and DC outlets, they should just plug into the existing outlets which are currently only powered with the key on.
I am considering a switch to both USB/Volt meters that will allow me to check battery voltage with a toggle ON/OFF regardless of key position. I figure if I am driving a flip of the switch to power the USB is easy enough to have the capability of switching ON/OFF to the outlets while I am parked and just want to check volts. Poor man's dual battery monitor if you will! lol