My Setup:
I have an Optima Redtop in the stock position with an 80A battery isolator mounted below the tray (in the spot where my vacuum actuator was factory installed). This redtop is connected to one side of the isolator, let’s call this side A. When I initially installed the isolator I simply moved the existing alternator wiring to side A of the isolator and ran a 4g wire from the alternator to the input of the isolator. I then vaguely remember running an excitement wire from the isolator back up to the alternator or something as such (I did as the instructions indicated). Then, from what we’ll call side B of the isolator, I ran a fused 4g wire to the rear of the truck to an optima Yellowtop which is connected to an amplifier, the overhead off-road lights, aux power connectors, cb radio, and the onboard air system.
I also have two solar panels on the roof which are connected to a set of diodes which split the connection and allow either the side A, or Side B systems to take the solar charge but not from each other (think mini isolator). These panels simply float charge the lowest voltage battery and counteract the parasitic drain of the electronics. These panels produce 30w max in full summer sunlight, and I do not believe they have anything to do with this situation, but figured it was worth mentioning.
As well, I also have a remote start unit that I installed myself, and it IS installed correctly, but again, worth mentioning
Alternator was replaced in 2008, isolator installed in 2011 I believe, and everything has worked fine until recently.

My Problem:
When I start the truck my Low Battery indicator illuminates and my voltage reads at between 9-10v (This is electrical system A). However, the truck has no problem starting and running, as well the battery indicates ~12+v at the battery terminals when running like this. To complicate things; the first time this happened it either caused, was caused by, or had nothing to do with the rear battery (system B) boiling and spitting fluid out of its vent ports. I noticed the low voltage before I heard the battery boiling. I immediately stopped and unhooked both the fuse to the rear wire and the rear battery. When I re started the truck the electrical seemed fine. Until it started showing low voltage on system A again. Now it does it anytime I start the truck, even though the truck clearly has enough juice to start strongly.

Thoughts?

 

What year, make, & model?


With a fully charged battery, all accessories turned off, engine at ~1500 RPM, charge voltage ideally should be 14.2 volts.


If the purpose of the isolator is strictly to charge the batteries, the 80 amp isolator, and the 4ga cable, are not nearly heavy enough to handle the amperage. As a result, alternator life will be shortened, and the isolator and cables will overheat. Depending on which alternator you have, maximum output is at least 105 amps. To be on the safe side, the isolator needs to be rated ~15% higher, than the amperage you are sending through it. In other words, ~120 amps. There is a formula to calculate what gauge cable is necessary, and it depends on amperage and the length of the cable. If you want to operate the starter from the rear battery, the starter draws ~180 amps, so the isolator would need to be rated at ~210 amps. The cable gauge would need to be calculated accordingly. Just a guess would be 0 or 00 gauge cable.


Typically when isolators are used, a positive cable from each battery is connected to it, and a third cable from the center of the isolator connects to the original positive cable that was removed from the OEM battery.

 

I realize my sig picture has disappeared; I will get that fixed soon. The truck is a 1996 Blazer LS 4dr 4wd.

Your not wrong. About any of that.

For the amount of amperage pulled, the 4g wire was fine, only ever burned one fuse when i hooked a second battery to the rear. Voltage drop to rear is neglegiable as well, the rear battery supplies power to devices in the rear, so the battery acts more like a filter capacitor when those loads are on and the truck is running. While I agree the isolator needs replaced with a larger one regardless, I need to get the truck back to working before I fiddle with components that may or may not be dead.

However, since we seem to share a depth of understanding on this subject, lets hypothesis:

1. If the brown or white(or yellow, hard to tell ) wires in the alternator plug were damage what would happen?

2. If the isolator croaked, what would happen?

3. if the alternator ( or regulator) croaked, what would happen?

And more importantly how would YOU go about identifying the culprit? yes i am quite capable of using a voltmeter, just not diagnosing alternator issues (yet). Thanks in advance.