While that may not seem like a big deal, 4runners are not designed to have dual batteries so it took a little bit of creative engineering to make things happen. I had a lot of help putting this together from various people.
Here are some photos of the engine compartment:
This however meant the coolant reservoir had to find a new home. Right now it is wedged between the air filter box and the washer fluid reservoir, with an additional piece of hose added.
What runs the whole setup is the Luna Intelligent Solenoid, it allows both batteries to charge at the same time and isolates the auxiliary battery from the engine battery when the vehicle is not running. The really nice feature of the Intelligent Solenoid is the ability to override the isolation and jump start the vehicle if the main battery should be dead.
At some point in the future I will run the red wire behind the panel and have it come out at the top and reposition the black wire so that the installation is neater. I put this in literally hours before deploying for the weekend.
With a tarp draped out from the back it makes for a good sun shelter or light rain. For more inclement weather I have a 12 x 12 shelter with side walls that can be completely closed off or with the use of the tarp and the side wall the back of the 4runner can be placed in the shelter.
Over all I am pleased with the setup, it was not cheap to do. At about $460 for both batteries, $120 for wire, $400 for the Intelligent Solenoid kit, and about another $200 for connectors, fuse box and other things the grand total for this upgrade was $1180. I could have done this cheaper with different batteries, a Chinese solenoid that may or may not work when you need it too and not put in any of the accessories in the cargo area. But I wanted it done right and I wanted something that would work every time you need it too, so when possible I used the best parts I could afford.