I got a 160 amp isolator to hook up dual batteries in the van, (a 74, BTW)and I see it has a hookup for the positive side of each battery and a hookup from the alternator field. I have 2 F hookups on the alternator, one going to the voltage regulator and then to the starter solenoid before going to the battery. I thought maybe I should go from the solenoid to the field of the isolator, but wouldn't that allow either battery to start the engine, thus preventing the isolator from actually isolating the batteries?


I'm planning on using 2 AGM batteries in the rear, and I know it'll take longer for the accessory batteries to charge.

This is the first time I've done a dual battery system, and above everything else, I'd like to prevent a fire. I'm sure you can relate.

Does anyone here have a dual battery setup that might help? I can post a pic of my wiring diagram if needed. Thanks in advance.

Post a pic or info on your isolator.

Not quite sure what you're describing.

The way I've done mine in the past:

As you can see, there's only 3 connections, one positive for each battery, and one for the "field" line from the alternator. Here's a pic of my engine compartment wiring diagram:



Here you can see that I have 2 "F" outputs on the alternator, but the one that ends up at the battery goes through the voltage regulator and starter solenoid first.

You might have the wrong one. Call them toll free.... http://www.surepower.com/isolator.html

You are making this too complicated, it does NOT hook to the "field." Look at the diagram that "DirectSubjection" posted, that is how they hook up. On your factory manual, you break the "main" charging lead to the alternator, marked "A20-10BK" which means "# 10 wire, black"

I DO NOT LIKE these units. I know people are going to disagree, here's why:

I used to sell auto parts, these things too. There's been at least TWO boat explosions I'm aware of that may be caused by these devices. Here's what happens:

These things are nothing more than two big diodes. IF THE DIODE TO THE MAIN BATTERY opens, the regulator 'thinks' that the system is going dead, and keeps trying to crank up the charge rate. Meanwhile, the auxiliary battery, still connected through it's diode GETS OVERCHARGED.

What I like to use instead is actually simpler, and has a few advantages. Buy what is known as a "continuous duty solenoid" which looks vaguely like a Ford solenoid. These can be left "on" for long periods, hence their name.

Just hook one large post to the main

Hook the other big post to the aux.

Ground one of the small terminals

Hook the remaining small terminal to an "ign on" source, AND YOU ARE DONE. Whenever the key is on, the two batteries are interconnected.

If you run the main down by accident--lights, etc, you can even jumper across the new solenoid terminals to jumper the two batteries, and use one to "jump" the other.

Here's some:

Thanks for the info, guys. The reason I went with this type of isolator instead of the relay type, is that in the limited research that I did, I found that the relay type when open can draw the power from the main battery to the auxillary battery if they are low enough that the alternator has to work harder to charge them. Is this true?

When the relay/starter solenoid is open, no power can flow from one battery to the other. We use the same type of relay to supply power to hydraulic pumps on tilt trailers at work.

If both batteries are new, just run them in parallel.

Double the cranking amps.

Both charge at the same time.

The alternator won't need to be all that large, you can parallel charge batteries with a standard battery charger. It's just going to take more time. The low battery draws more power until the two batteries reach approx the same charge, but all the charger/alternator sees is one low battery.
The switch in the keyed line is a good idea for times when you need the main battery to be the only one charging. (Night driving and low house battery will dim headlights)
For many RV apps, they put a charger unit for the second battery in the RV and run it off on site hydro when possible.

Sorry, the attachment from my other post won't work. I'll try and look for another pic of a + to + wiring setup.

Found one. Add a switch to the 12 volt supply for the Aux. relay, and you can charge or not charge the second battery as needed.
The nice added touch on this one is the momentary contact pushbutton that allows the second battery to be thrown in to the system as a booster for starting.