Auxiliary Battery Isolater/Starter Solenoid

by David Schwarze and Jim Arnott

Jim Arnott writes:

"There has been much discussion regarding the fitting of a battery isolator for running dual batteries. It really can be quite simple.

1. Get yourself to the local auto parts store and purchase a continuous duty starter solenoid P/N ST-80 (list price $46.85.)"

David Schwarze responds:

Or go to Camping World and get a similar device for about $15. Continuous duty, looks like a starter solenoid - cylindrical with two mounting tabs, two large connectors, one small one, silver in color. Camping World has a catalog, and presumably an 800 number so you could mail order this part.

"2. Install same on firewall using SHORT sheet metal screws. (3/8-1/2") Drill carefully, the gas tank is back there. You REALLY don't want to punch a hole in it. (Note: wrap a mess of duct tape around the drill bit about 1/2" from the point to make a stop.)

3. Run a 10 gauge wire from the large terminal on the left side of the solenoid to the positive terminal of the aux. battery. (Use crimp on terminals on both ends of the wire. You can't get away with just wrapping the wire around the terminals...)"

I used 2 gauge wire with soldered copper lugs - overkill perhaps but it makes me feel like a man, dammit!

"4. Run another 10 ga. wire from the large terminal on the right side of the solenoid to the large terminal on the starter solenoid. (There are other places to pick up the charging circuit, but this is the simplest for the novice electrician.) Make sure that the wire is grommeted where it passes through the sheet metal. Running it with the vehicle battery positive cable will work okay." 5. Okay, you've gotten this far, now it's time to tell the solenoid when to close. There are a couple of schools of thought on this, such as picking up the idiot light circuit so the solenoid will only operate when the alternator is charging, but I believe in the KISS principle... So... run a 14 ga. wire from the small terminal on the left side of the solenoid to the 12 v. side of the coil. That's the side that doesn't have the wire going to the distributor."

That works great - I went a bit farther and wired it to the ignition "on" AND another switch so I could interrupt the continuity if needed, but I usually just leave that switch on. The one difference between the ignition "on" position and the coil is that the former is off while the starter is cranking, which is a good thing.

"6. Run another piece of 14 ga. wire from the small terminal on the right side of the solenoid to ground. I use one of the attaching screws.

Viola!!! Aux. battery will now be in the charging circuit whenever the ignition is on.

There has been some concern expressed about the aux. battery "sucking the charge" out of the vehicle battery. Believe me, it ain't gonna happen through a 10 ga. wire in less that about an hour. Not without turnin' that puppy into a fuse."

I would go so far as to say that it will never happen. Even when my two golf cart batteries are dead, they do not drain my little 450CCA starting battery when they are connected through the 2 gauge. They pull some decent amperage for the first few seconds, but the starting battery still has plenty left and the alternator makes everything all right in a few minutes. If you're worried about it, wire the solenoid to the ignition switch instead of the coil as described above. Or put in the second switch and leave it off until the engine is running.

"That's it. As I've noted, there are variations on this theme, but this has worked for me on VWs, Fords (motorhome), GMCs (winch) etc."

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