Nice work. This looks like HOW TO material....

 

I used the Surepower isolator relay on mine which closes when the alternator is putting out about 13 volts.

Only the aux battery is connected to the fuse box under the hood. Keep in mind if the aux battery is completely dead (or close to it) the ignition doesn't work. If that happens (and it has) I have a switch to operate the isolation relay that will put the main (starter) battery in the circuit and allows it to start.

If you simply connect 2 batteries together in parallel, you can run both batteries down and that kinda defeats the purpose of having backup.

 

Great setup!, so if I understand this right, the OE setup is basically connecting the the two in parallel without any protection , and the engine gets the starting power from both, they both dies together!,,,please correct me if wrong.

Many thanks!

 

Thanks. I don't know how the OE setup works, if it's parallel or isolated. Be cautious which isolator you use, if you go that way. Most cheap isolators use diodes, and theres a 0.7V drop across each diode. Either use a relay type isolator, or the solid state Hellroaring isolator.

 

Hardin what a great write up, I'm thinking of doing the same thing. I was looking at my setup today and I noticed two wire coming from the positive terminal on the main battery. I can see one wire going to the 175 amp fuse, but where does the second one go to? It disappears towards the passenger side firewall.

 

Thanks. The isolator continues to work very well. Two heavy gauge wires come off the isolator. The lower wire goes to the big fuse block as you noted. The only reason it ties there is it's a good point to connect the isolator, main battery, and of course the alternator all together. The wire coming off the upper terminal of the isolator runs over to the auxilliary battery. What I didn't show is at the positive side of that battery, I pick the 12 volts off to run my radio gear. I also run a ground line too directly from the battery. This is necessary for optimum performance of the gear. For lighting, and other uses of 12 volts, you could just tie to the chassis anywhere you wanted. Hope that describes it better for you,

 

In the second picture there is that orange cover that covers the positive terminal of the main battery. In my car I have two heavy gauge wires coming out of that. I can see one wire going to the 175 amp fuse, but the other wire I do not know what it is for, I was wondering if you do?

 

should go to the starter

 

Great write-up and pics! Even if it is a couple years since your original post, I'm just now reading it since I decided to finally install an auxiliary battery setup in my 2007 Tahoe, too. I have a more basic, dual-battery connected in parallel without any isolator relay. I have an Optima YellowTop installed in the secondary tray location (drivers side front corner of engine bay) wired positive-to-positive and negative-to-negative between the two batteries. Basically, my setup adds extra capacity to the factory configuration, but both batteries will get drained when I'm running the radio with the engine off.

I tend to sit in my Tahoe with the engine off and listen to the radio or watch a DVD on an overhead aftermarket DVD player, while waiting to pick up my kids from their various school activities. I don't have any other fancy audio system or accessories installed; just straight factory stuff and the DVD player. It seems like all of the write-ups about dual battery installations using isolators run a separate cable for power output directly to some aftermarket accessories. In your case, Hardie, you run it to your radio gear. I'm wondering how different the setup would be for me if I want to power the factory radio and other accessories with the auxiliary battery -- I want to avoid draining the starting battery while I'm waiting in the Tahoe and listening to my radio, for example.