Well, how much load is on your battery from extra things? During running they both have to be together in order for the system to charge them. If your load exceeds the stator or charging system output, like my PS did on my ATV, it will just be a matter of time before both are drained. The extra battery just gives you some more time...it also takes longer to charge two dead batteries back up. Hard to know which way to go but here's the way I look at it:
The accessory circuit is for extras. It is fused with a 10amp fuse (as I recall) which is a safe amount of extra load above and beyond all the rest of the factory installed items.. except the wench. If I use that circuit and keep my loads below that limit, I should have no problems with the electrical system or the battery. I do this and have had no problems so far. Now, there may be a little more I can get out of the stator but I have not needed it yet and so far my max load on the acc circuit is 6.8amps even with the 24" light bar on.
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An isolator allows both batteries to charge, while blocking drain from accessories attached to the opposite battery.
The problem I'm finding is, the charging wire from the voltage regulator shares a common hot lead that runs to the other accessories AND connects to the battery.
I COULD snip the line from the regulator that goes into the harness, and run it directly to the charging post on the isolator, but I'm not sure I want to hack up the wiring harness.