I am looking for something that is like a check valve for electricity. My 95 E350 work van has a seperate alt for charging the boom system. The alt has just went out. I have to find an alt shop tomorrow. It is a add on 165 amp alt.

I think that I am looking for a resistor. I want to run a cable from the truck electrical system to charge the boom battery. Also I don't want it to drain the battery on the truck so I can't crank it.

I think a battery isolator may do what you are looking for. It allows you to hook two batteries up, letting them both charge off the alt, but only allowing them to drain from the circuit they are hooked to.

Any idea where to find it?

Jegs or Summit both carry some. There are other places as well. They come in different amp ratings depending on your alt output. Here's an example-
http://www.jegs.com/i/Powermaster/713/903/10002/-1

Bloodhound said:

Any idea where to find it?

Click to expand...

Your local RV/Camper stores will have them as well. You can also use a solenoid for an isolator.

I have heard of using a solenoid, but never knew how to wire it.

surewhynot said:

I have heard of using a solenoid, but never knew how to wire it.

Click to expand...

Tony,

As you know a solenoid is just a switch that is activated when the coil terminals are energized thus closing the switch.

A constant duty solenoid is used to isolate the batteries when the power to the solenoid is off. Basic wiring goes as such:

1) connect one of the large terminals on the solenoid to the positive side of the main battery or the positive side of the starter solenoid (this allows alternator current to be supplied to the solenoid & thus both batteries).

2) connect the other larger terminal on the solenoid to the positive terminal of the auxillary battery.

3) connect one of the small terminals on the solenoid to ground.

4) connect the other small terminal on the solenoid to a ignition switched 12V source.

When the ignition switched 12V source connected to the small terminal of the solenoid is energized the solenoid switch is closed thus tying both batteries together in parallel and the alternator is supplying current to both batteries(if the engine is running & the key is just not in the run position). When the ignition switched 12V source connected to the small terminal on the solenoid is OFF the solenoid switch is in the open position and the batteries are now disconnected from each other.

There are a couple of variations on supplying power to the 12V small terminal of the solenoid to allow for some variation in operation.

i) You can install as single throw switch between the ignition switched 12V source and the solenoid thereby allowing you to open the solenoid (and thereby isolating the auxillary battery from the vehicles charging system even when the vehicle is running or the key is in the run position). This is useful if the auxillary battery is shorted out internally and needs to be removed from the system to prevent problems.

ii) use a single pole dual throw[Center Off] switch between the 12V source and the coil side of the solenoid. By using a single pole dual throw switch the center terminal of the switch is hooked to the 12V+ small contact on the solenoid; one of the other terminals on the dual throw switch is hooked to an ignition switched 12V source and the other terminal is hooked to a constant on 12V source.
With this setup multiple possibilities are now available.
a) With the dual throw switch set to the ignition switched source the battery isolator solenoid is only connected when the key is in the run/on position tieing the batteries together.

b) With the dual throw switch set to the constant 12V source the batteries are connected together (in parallel) even when the ignition is off. This allows the batteries to be added toghether, effectively having the full (auxillary) battery jump start the low(Primary) battery. NOTE: if the switch is left in this position and a radio or other electrical draw is left on it will drain both the primary and auxillary batteries flat.

c) With the dual throw switch set to the center the auxillary battery is completely removed from the vehicles charging/electrical system even if the vehicle is running or the key is in the on position.

Here is a link to examples and pictures.
www.piranhaoffroad.com.au/products/dualbattery/dualbatteryinfo_isolator.htm

There are pros and cons to using constant duty solenoids as opposed to solid state isolators. In my experience solid state isolators have a higher tendancy to fail and the batteries can never be connected together in parallel to jump start yourself. However with a solid state isolator you cannot forget and leave the switch in the wrong position.

One trick when using the constant duty solenoid in conjunction with a single pole dual throw [Center Off] switch is to also incorporate indicator lights with the switch. Use a Green indicator lamp that is on when the dual throw switch is in the "ignition switched" position. Use a Red indicator lamp that is on when the dual throw switch is in the constant 12V+ supplied position. No lamp is lit when the dual throw switch is in the "Center Off" position.

Painless Performance sells a kit like this but it is mucho expensive and you can purchase the parts yourself locally for alot less money.

Great write up. NOw I understand. Thanks!

Thanks for all of the info. I found the problem with my aux alt. It ended up being a bad exciter wire. I wasn't looking forward to replacing a 165 amp alt.