Originally Posted by 11jdsummerfield
Hooking the batteries up directly to ecah other can lead to "Dual Battery Syndrome" basicly they don't last as long.
I'd love to hear what "Dual Battery Syndrome" actually is...
If you parallel two batteries, they don't know or care what's going on with each other, they just give off electricity. The power is going to move from the battery with the higher charge. And whatever is using the power will continously draw from both batteries alternately as one becomes less charged then the other. With that said, you should still replace both batteries (no used/new) and replace them with the same type (eg. Two group 65 batteries)
As for charging, the power is going to flow from the alternator to the battery with the lesser charge, and will fill up the batteries in the same fashion that they were discharged - whether they are or aren't isolated. All putting an isolator in does is let you discharge a battery and not affect the other. When your alternator starts charging, it will fill the battery that is more discharged first until they become evenly charged, and then will charge 50/50.
Although I shouldn't say that is completely true. If you have one battery that is only absorbing 40 amps, and you have a 120a alternator, the rest of the output of that alternator will go to the other battery or however much that battery is able to take. Electricity follows the path of least resistance.
Whenever my buddies and I go tailgating, I have an 0g wire run to the back of my truck, and I clip it on to an 8D battery that I carry on my rear carrier. That way I can play tunes ALL day long. Never had a single problem with doing it that way. On the run home, I leave it hooked up and charging, and by the time we're done with the 2.5 hour ride, my battery reads fine with our load tester, as does the big one, and thats only with my stock 60a alternator... gotta do that 3g soon