I installed the aux backup battery in the small cabinet behind the driver seat. Seems like a perfect place for it, and the battery I picked fits perfectly in there!

The battery is an Odyssey model PC925 which is more than capable of starting the 4.3L engine. It can actually be used as the main battery. It is a fully sealed, dry-cell, deep-cycle battery. The isolator/combiner I use is made by Hellroaring. It is an excellent unit. The way I have it connected, it keeps the aux battery charged all the time but never lets it drain, unless I flip the switch, in which case it kicks in. There is an LED near the switch that tells me when that battery is being charged.

I have used it several times, let the main battery drain all the way, and it has worked great. Starts the truck right up.

There are several different ways to do the install. I installed it just like the "4x4" recommended setup. In this method, a THICK wire comes from the starter terminal (or the main battery +) to the isolator. A thick wire goes from the isolator to the backup battery. You ground the backup battery to a good place and also wire up a few other wires for the isolator (for the remote switch, led, and a ground). There is also a resistor and a diode that go on the unit but their instructions are very clear and it comes will all the parts.

Source: www.hellroaring.com

Basically, the most difficult steps are to

1. pick a location for the aux battery and isolator
2. run a thick (0 or 2 AWG) wire from the main bat to that location
3. Pick where you are going to mount the switch

... the rest of the wiring and operation is straightforward.

A trickey thing that came up is that they recommend you ground the aux battery directly to the engine block. I was too lazy and grounded it under the seat belt bolt near that compartment. It seems to be a good ground point. If I disconnect my main battery, this backup one can't keep the engine running due to that bad ground (or has a very hard time). But to "jump start" when my main battery is dead, it works great.

I figured if worst comes to worst, I'll just use jumper wires from that battery to start the truck (or even swap batteries if things are really bad with the main one).

Source: www.hellroaring.com

You can see the switch setup above. I also added an analog voltmeter that I can use to check the voltage on either battery using the 3-position switch right next to it. The "AUX battery" switch has 3 positions. In the TOP position, it turns on the Hellroaring "combiner" and the aux battery kicks in. In the middle position (normal use), the backup battery is charged through the device, but does not drain. In the low position, the backup battery is completely isolated and neither charges nor discharges.