Consider this if one battery has been drained to say 8 volts and you start the engine flip the switch, and you can have a surge of amperage that can explode the solenoid and/or battery. Some facts about solenoids, they are stupid for use as an isolator. Sorry for coming late to dead post but one of my customers sent me the link and had a question. Cable in these photo is 0 gage stranded copper. A relay is used to power the solenoid to take the heavy load off the ignition switch and auxiliary switch. The solenoid in the middle of the batteries that connects the two positive posts is a continuous duty 250 amp solenoid. Start the engine and it will charge the low battery. If your primary battery dies because you left your lights on in the fog, flip the switch and instant jumper cables. Start your engine and the secondary battery will charge. You can run your accessories and drain the secondary battery and not touch the primary. Whenever you turn on your ignition switch, the batteries will be hooked together. Below I have included a wiring diagram that allows you to isolate the batteries, hook them together and charge them, and use the second battery as a jumper if you run the primary battery down. I have already had one request for a wiring diagram for dual batteries that can be isolated.

Jeep dual battery isolator relay