Installing a Second Truck Battery

Q. I've got a new 2005 Chevy Astro van, and am adding a second battery. The instructions I have for the isolation switch says to connect one positive lead to #1 connector, and the other positive lead to the #2 side. There is a "common" connector as well. My question is, do both negative, from both batteries, go to this post? Or is there a connection on the alternator I need to make? If it's on the alternator, is it from the original truck battery? Is the second battery getting grounded to the frame? This makes more sense to me, but I'm trying to clarify just what I'm doing so I don't explode somewhere on the road.

The type of switch I'm using is the old style barrel type. It has 4 positions: "1", "both", "2", and "off". The reason I chose this type is to allow me to charge either battery, or both, while driving. This van is not just an average van; it's a work truck. I'll be powering a huge inverter. The truck has a small, double fluorescent light fixture, and an outlet with 4 plug-ins. Off the outlet, I may be running two electric motors at one time, but not for long periods of time.


Both negative leads should go to the block. The original battery already goes to the alternator. You need to run a second lead from the original battery to the switch and a positive lead from the new battery to the switch. Then you should be good to go. Did the switch come with wiring instructions? It basically involves cutting your original positive cable at the battery and installing it either to a junction block or the comm. on the switch. You would also want to install a cable to your inverter to the same block or comm. on the switch. You would want the cable going to the inverter fused. Run both batteries to ground or a grounding block, and run new positive cables from each battery to no. 1 and 2 on the switch.

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