Would it be possible to install a switch between the battery and the input to the carnetix 1900 where I could either apply power to the 1900 from the battery or another 12v source? Basically, I would want to have a 12v AC adaptor always connected to the switch and I would like to be able to plug the adaptor into my house power and throw the switch to change the 1900's input from the battery to the ac adaptor. I sometimes go out on jobs where I'd like the be able to leave the carputer on, without draining the battery.

If I were to do this, how would I do the grounding?

Also, is this a situation where I could use the second input of the 1900 for the AC adaptor instead of installing a switch?