Well that's confusing, being that charging current to batteries is minimal. Sure, the wire has resistance, but since you're having a significant voltage drop on a (assumed) 15' run with a mere voltage test..... Something isn't right.
12.8-11.5=1.3V, and that's at idle. In other words, per every amp, you have 1.3W dissipating in that wire, so assume 100A, 130W of dissipation, AKA 130J per second in that wire. That's gonna get toasty. I don't believe you for some reason, and this is just an extremely rough estimate of how much is dissipated.
I would understand if there was a current draw, but idle doesn't make any sense.
I'm also assuming idle because the voltages are not anywhere near 14V.
xD
I do not believe the wire is the problem here Blazer, especially since it was over time. You first put the other battery in, the alternator did not have to work as much since the battery was topped off already. After time, that charge equalizes with the front, then the amp has to work a lot more to keep your whole system up. This really isn't something you can fix at this point, it is going to drain down to it's proper voltage eventually, now if it was any lower than these voltages above, I would start to say you have a draining conflict. In short, you HC(s) is(are) charging your front battery when they are put in parallel when the HC's voltage is above the front's, while the car is off. A 1/2 solution, or something to elongate it is to put an isolator in the system, so that the batteries are not connected while it is not needed. When you turn the car on, you charge them/top them off. When it's off, they sit there and eventually go down to their standing/floating voltage naturally, not charging anything to meet an equilibrium of voltage.
The wire is primarily being used to charge the battery bank, correct? Even then, 1/0 can supply 100A at 15V on a 17.35' run with .45V drop. It is not the wire.
