Guys, let me clarify something off the bat: dual batteries do nothing for you if the engine is running. PERIOD!!!!! With the motor running, 100% of the loads should be coming off of the alternator. If they are not, then you are overloading your alternator and you have other issues that a second (or third, or fourth) battery is not going to fix. It is just delaying the inevitable. A dual battery setup is only useful if you are planning on using your truck's electronics when the motor is off. End of story. With this being said, if you are using electronics when the truck is off, all those electronics should be powered off of the second battery which is a deep cycle style of battery. Using a standard battery is only going to kill the battery after a few deep discharges.
Freeway, if I was to do a dual battery setup, I would be looking at a module like:
Powermaster Part pow904 - Battery Isolator
Please keep in mind that like anything else, the size of module you need is relative to your alternator. You can not get away with a 130 amp battery isolator if you are running a 250 amp alternator. The isolator needs to be rated for atleast (if not more) than what your alternator is.
Joey, as my first paragraph says, the battery will not help you if you have the truck running. That is all your alternator. If you are just out playing and have only say headlights, the truck and all of your lights running, you will have no issues. The lights are only adding 30 amps to the system. This is well within what the truck can handle. Now, if you are trying to run the truck at idle, all the off-road lights on, wipers running, heat going full blast, etc, then yes, you may find that your lights will start to dim some and the battery light will come on. You will notice that as you raise the RPMs of the motor, the lights will get slightly brighter. IF this is what you see, then you are maxing out the alternator that you have and you need to step up to something a bit bigger (I would recommend a 200 amp unit). If you are going to be running the lights when the truck is off, then as I said in the first paragraph, you need a second battery to power the lights and then you need to isolate the two batteries so you don't kill both batteries at the same time. If you need more specific information, let me know. I will tell you what you need, what you can get away with, and attempt to keep it within a price range.
