A friend of mine that owns a Bronco (FULL SIZE - late eighties) wants to run a seperate battery, charge it from the main alternator so that it can run sound and lights while sitting with the engine off on fishing trips, and asked me how to do it.

CONDITIONS:
1) Second battery charges from existing electrical system.
2) Second battery CAN NOT BE USED by primary electrical system (is SEPERATE)
3) NO TOGGLE SWITCHES!

My first idea was that you should run a seperate charge wire from the charge terminal on the alternator - to seperate it from the main electric system. The diode pack in the alternator won't let current run back through the alternator - AND THAT'S FINE TO THAT POINT....

My second thought was that from the charge terminal on the alternator to the second battery - AND BACK TO THE FIRST ONE is the same electrical POINT. So what needs to happen is a very high current DIODE or DIODE PACK needs to be on the charge line to the second battery, ONLY ALLOWING CURRENT TO GO IN ONE DIRECTION.

(with me so far? Good...)

Where to get that diode set....

Is it possible to get a set of diodes FOR AN ALTERNATOR at a parts house, and mount them in an insulated enclosure with a terminal for the alternator in one direction, and the battery in the other?

THAT would prevent reverse current flow, and allow charging - so that the battery would be an almost completely seperate unit....

CHARGING ONLY, and never supplying current to the rest of the wiring harness.

COULD THAT WORK????

 

Here is one way:


Here is the other way, disregard the onan note at the bottom and replace it with AUX Load:


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You are trying to re-invent the wheel. That's exactly what a battery isolator contains that you buy in the store. It's a large diode with a big heat sink. You just buy it near the size of the rated output of the alternator.

 

Cool. I never had a need for a thing like this, but it seemed like there had to be something already out there.

Thanx everybody - I'll pass it along.

 

I bought one of those diode isolator things years ago, never hooked it up. Just saw it a day or so ago in one of my hiding places!

One of the simplist things is shown on one diagram and is a "continuous duty" solenoid. It has a metal case instead of bakelite and was used in many an older vehicle that used auxiliary batteries, particularly school buses. This subject comes up fairly often and I posted that I had seen that solenoid recently in a marine accessory store. Hook the accessories to the 2nd battery and a charge leed through the solenoid which is energized by the ignition switch "run" circuit. You dont want the 2nd battery hooked in parallel with the main battery.

 

These isolaters have been used for years on Motorhomes!