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 Moonglow '95 FZJ80 Has Some - Still Needs More!
White 2008 GX470 - Nearly Stock
"I drive an ECO-FRIENDLY FZJ80...It's got a salvage title which makes it a REPURPOSED/REUSED/RECYCLED vehicle. Therefore, don't mess with me when I park in the "Reserved for Green Vehicle" spots in Boulder!!!