How to make a cheap isolated dual-battery setup for $50

[dwh]

ok so say i hook the keyed power to a toggle switch, and have it off most the time. would that just mean the house battery wouldn't get a charge?

Wouldn't get a charge from the alternator if the engine was running but the switch was off.

and what would happen if say the switch is left on while the vehicle was not running, would it just drain the start battery?

The batteries would equalize. If the house battery was low and the engine battery was high, then the house would rise and the engine would drop until they were equal.

[lostagain]

that's what i was thinking, thanks for clarifying that for me.

[GregB_00XJ]

This is basically the setup I have for my trailer connected to the 7 pin for charging. The RV lot set it up like this, even though I requested an isolator with third connector on it for a 2nd on board battery (on Jeep battery)? Was a little bummed that they didn't listen, but with the single Jeep battery and this setup I have happily used my trailer and charged it too for 4 years now. They also did a decent job of the install, so I was not too upset.

I want to put in that 2nd battery soon as I have added a bunch of lights, a compressor etc and would like to run all that and the stereo off of an accessory battery. Could you run 2 of these circuits (2 sets of fuses and 2 relays) or am I better to get a 3 pole more intelligent isolator?

Great thread...now where did I put my beer?

[robson]

How can you determine where to run to the fuse box? Would this be on a wiring Diagram to show what is hot and what is not?

[Rando]

I made a system very similar to this for a friend. The only additions were a On-Off-On toggle switch and LED on the control terminal of the solenoid. This was wired with the middle terminal of the switch to the input of the solenoid, the top terminal on the switch to a switched with ignition 12v source under the dash, and the bottom terminal wired to the aux battery. The LED was wired between the middle terminal of the switch and ground. Now you have three 'modes' selected by a switch on your dashboard:

• With the switch up you are in 'Automatic' mode where the batteries are combined when your engine is running.
• With the switch in the middle position you are in 'Isolate' mode and the batteries are isolated no matter what.
• With the switch down you are in 'Jump Start' mode which combines the batteries regardless of the state of your main battery.

The LED illuminates when ever your batteries are combined. One caveat with this system is the the continuous 80A solenoid uses a significant amount of power to stay on, so you don't want to leave your system in 'Jump Start' mode for the long term. Another option is to use a relay such as this: http://search.digikey.com/us/en/prod...079-ND/1242027 which have a higher coil resistance so they use less power when energized. Finally, these all will work just fine for jump starting - the contacts are rated to ~250A for 1s and ~150A for 10s.

You can have a pretty deluxe system for under $50 - which makes me wonder why people will pay $400 for some of the kits out there.

[dwh]

Could you run 2 of these circuits (2 sets of fuses and 2 relays)

Absolutely. Just jumper from the input of the first solenoid to the input of the second solenoid and the same with the wire to energize them. They'll draw something like an amp each so you probably won't even need a larger fuse to power them.

Instead of a fully dual system, the schematic will look more like a "Y".

[Dirtytires]

Nice,, I got the brew, and wire. I guess I should order the rest.. Good write up.

[evldave]

Been out on a work trip...let's get some updates going!

Here's an updated basic diagram, with the fuses for the solar array (buy two panels, then start calling it your solar array, people will think you are either really cool or some diabolical madman bent on world domination)



Now some additional comments...

Using an 80A isolator when trying to automagically jumpstart your car

While you may be able to jumpstart your car by using the smaller current isolator, there is a real chance you will 'burn up' the isolator. This happens when the charge between your starting battery and your house battery is great...ie your starting battery is at 8V and house battery is at 12V. When that differential happens, it's like water flowing down a steep hill instead of a shallow hill - more faster! More current! More cowbell!

Why is this a problem? When isolators 'fail' or 'burn up' - they don't always show it! In fact, the most common failure mode for an isolator being exposed to excessive current is to 'fuse' the contacts inside, with no evidence showing on the outside! OUCH that sounds bad! What does all that fancy language mean? The isolator will always be on, your batteries will always be connected, and you'll be draining both at the same time. Fine in town, not so great when you've been hanging out in the Nevada desert after the apocalypse and the zombie hordes are coming your way...and your truck won't start because you've drained both batteries drinking mail-order beer out of your cheapo fridge.

Wiring in two separate batteries, with one or two isolators

If you don't need your two house batteries separate from each other - you want a big battery bank...just wire the two batteries in parallel after the solenoid.



If you want your starting battery isolated from your house batteries and you want your house batteries isolated, you need to isolators. Just hook them both up the same way...sorry I got lazy making pictures, so you'll need to visualize it

Hooking up fully isolated, fully automatic, with manual bypass starting, house, and trailer batteries (whew!)

This is only for the most experienc...meh, that's a load of crap...this is really easy!!!!! I have basically this same setup in my truck and I did the whole thing drunk!



This diagram is really not that complicated. A few things to remember:

• I've upgraded the 'jump start' solenoid to 200A. This gives us plenty of extra capacity to flow current if we want to use the solenoid to jump start our truck instead of using jump cables (lazy bastard). Better yet, order the way good isolator and get 500A!
• I also upgraded the fuses to 200A. You know, because if I didn't do that I might blow a fuse or two (depending on the voltage difference of the batteries, you were paying attention earlier, right?)
• I also upgraded my wiring to 2GA. Remember, always overcompensate for the size of your...wire! We could have probably survived with 4GA, but what if we want to put in a winch later? hmmm, 2GA would likely run the winch, 4GA would definitely NOT run a winch. hmmm...500A isolator...2GA wire...winch, 2 fully engaged batteries...this might need it's very own 'how to wire up your winch really cheap' addendum...
• I put 'in dash' switches for each isolator. This is a little complicated...each switch needs 'power' from somewhere in your cab. I'm sure you can find it. Just wire the switches with power from within your cab and then run the wire to the solenoids. By doing this you can 'turn on' your solenoids without having to start the truck!
• The trailer circuit is 80A. This is likely the most you will ever need. Remember overkill!

How can you determine where to run to the fuse box? Would this be on a wiring Diagram to show what is hot and what is not?

The fastest way to figure out where to run your wire to the fuse box is a 12V detector thingamajig. Turn your key to 'ON' (don't start truck) and then start probing empty holes in her fusebox. At some point your thingamajig will light up. Write down all her locations that lit up your thingamajig and turn her OFF. Then start probing those same empty holes in her fusebox that lit up your thingamajig again. Find any empty hole in her fusebox that lit up your thingamajig when she was turned ON but got no light from your thingamajig when she was turned OFF - that's the circuit you want to use (ALL THE TIME, if you get what I'm sayin )

[dwh]

The "in dash switches" look very sad all alone out there on the end of those wires. Those...single...wires. They are sad because they know it takes two wires to tango.

[GregB_00XJ]

I love this thread... thanks for the advice, totally makes sense. Looking at getting a Cowl Snorkel (Specter) for the XJ, which will free up the Air-box space for the 2nd house battery. Already have the trailer circuit wired in, so it will simply be adding in the second setup for the house power.

Good stuff!