This is the type of solenoid I used with my dual bat system. As Dave mentioned, I use the signal from the fuse box under the hood that is labeled ALT...so that the solenoid only connects the two batteries when the alternator has a signal. This way, the two batteries are completely separate when the vehicle is off. If I want them connected, I use a marine switch to tie the two together...in case I need to jumpstart myself.
Horribly Painted '95 FZJ80 Has Some - Still Needs More!
"Just because California does it, doesn't make it a good idea, or the right way to do it! If you like California so much, please move there. Don't make the rest of us live in the same HORROR!!!"