HOW-TO: DIY JK Wrangler Dual Isolated Batteries

[Dan Grec]

Isolated dual batteries are a must in any Overland vehicle. The setup looks complicated and difficult in photos, though it’s actually very simple. Having two batteries means one can be used only to start the vehicle (let’s call this battery the “Engine Battery”) and the other can be used for all your electrical needs like a fridge, camp lights, charging of devices, running an inverter etc. (Let’s call this battery the “House Battery”). The isolation part refers to how the batteries are connected and disconnected with the engine is running and when the engine is not.
When the engine is running, we want the batteries to be connected in parallel (positive to positive, negative to negative) so we still get 12V, and the alternator can charge both batteries.
When the engine is not running, we want to separate the batteries, so that if the House Battery is completely discharged, the Engine Battery remains unaffected and will still start the vehicle engine without issue.
This “isolator” is commonly a large amperage solenoid that is triggered with an ignition hot wire.



Things To Consider

• Because the two batteries will be connected in parallel when the engine is running, it’s highly recommended to use two identical batteries to ensure even charging from the alternator.
• A cheap solenoid might save $30 now, though given it’s the fundamental piece of the puzzle, I recommend buying a good one that will last a very long time
• The system described here does nothing to prevent excessive discharging of the House Battery – if you leave multiple big camp lights on overnight, your house battery will completely discharge, which is usually not good for the longevity of the battery so you want to avoid this at all costs. Make sure you turn your electronics off, and/or buy devices with a voltage cutoff (i.e. my Dometic CFX-35 Fridge has a setting that will turn the fridge off when it senses the House Battery is too low)

Things To Buy

• Two Batteries. I use and recommend Optima’s YellowTop D34. They are specifically designed to handle the kind of charging and discharging the House Battery is going to go through. As a bonus they can be mounted on the side which is a win in the cramped JK engine bay and opens up all kinds of mounting options
• A dual battery tray. I have the Nemesis Industries tray which is specifically designed for the Optima D34 batteries. The batteries mount on their side, saving previous space.
• A solenoid for isolation. I’m running a kit from Painless Performance specifically designed for this purpose, and it’s excellent. (Note the Painless kit comes with everything you need, and a switch so you can control when the batteries are connected. The following instructions will not use that switch so they’re generic for all solenoids)
• Heavy gauge wire to connect the batteries, welding wire with crimps already on the ends works great for this. You’ll need one black wire to connect the two negative terminals together, and two red wires to connect the two batteries to the solenoid.
• Battery terminals for the House Battery. I really like the kind that have a large post for connecting the wires of all the accessories I’m running
• A mini add-a-circuit fuse holder – we’ll stick this in the JK fuse block to trigger the solenoid when the engine is running
• Zip ties to keep everything tidy
• Depending on what came with you solenoid, you might need to make a bracket to mount it. I used a scrap piece of a right angle bracket I had left over from something else.

Tools Needed
Only basic tools, though the specifics will depend on which dual battery try you use. You really won’t need more than a ratchet and extension bar, a few sockets (at least 1/2 inch and 10mm), some kind of saw to cut plastic (hand or electric is fine).
A deep 10mm socket is handy.

Step 1:
Follow the instructions that came with your dual battery tray to remove your existing battery and the large factory plastic tray, and install the new dual battery tray. Usually this involves cutting the factory plastic tray.



Step 2:
Install the two batteries in the tray – when using the Nemesis Industries tray it’s recommended to install the Hose Battery on the bottom, and the Engine Battery on the top. If your tray mounts the batteries side-by-side or front-and-back, check how well the stock jeep wires for the positive and negative will reach each location. Whichever battery suits those wires the best can be your Engine Battery.

Step 3:
Connect the negative terminal of both batteries directly together. These will always be connected with large gauge wire, and never need to be disconnected.

Step 4:
Find a good spot to mount the solenoid. You’re going to run heavy positive wires to the large terminals from each battery, so keep it close to the batteries. Check hood clearance, and make sure neither terminal will touch any part of the body nor hood.



Step 5:
Install the min add-a-circuit fuse holder in the JK fuse block in the location indicated on the picture below. This will trigger the solenoid to turn on when the engine is running. Drill a hole just big enough for the wire in the bottom/side of the JK fuse block cover, feed the wire through and put silicon around it to seal the hole from dust and water.
Connect the other end to the small trigger terminal on the solenoid.





Step 6:
Connect the other small terminal on the solenoid to an earth – this could be the negative of either battery, or the body of the Jeep. Remember, they’re all connected together now, all the tire, so it doesn’t matter what you use, it’s all the same thing.

Step 7:
Connect one heavy gauge red wire onto the positive of each battery, and to each of the large terminals on the solenoid.

Step 8:
Connect all your accessories (fridge, lights, etc) to the House Battery positive and negative.

Step 9:
Connect all the stock Jeep wires onto the Engine Battery positive, and then lastly connect the stock Jeep wires to the negative of the Engine Battery. Leaving the negative until last ensures you won’t get a really big spark if you accidentally touch something to the body during the whole setup.

Step 10:
Zip tie wires together and out of the way, and attempt to make everything as neat as possible, which is not easy.

Step 11:
Test out your new isolated dual battery setup – When you start the engine you will hear a “clunk” as the solenoid connects the two batteries. You will hear the same “clunk” when you turn the engine off again.

Enjoy hours of lights and icy cold beverages without worry of draining your Engine Battery!

-Dan

I also wrote up this HOW-TO on my website, here DIY Jeep Wrangler JK Isolated Dual Batteries

 

[vitara]

Anyone caught out by running with only one battery is a chancer and probably a fool. Good tutorial.

 

[Jeffrey_A]

Good write up. Saw you used the painless product?

This guy has a write up to avoid the painless deal. As a bonus he instructs when to drink.

http://www.expeditionportal.com/for...ke-a-cheap-isolated-dual-battery-setup-for-50

 

[Dan Grec]

Good write up. Saw you used the painless product?

This guy has a write up to avoid the painless deal. As a bonus he instructs when to drink.

http://www.expeditionportal.com/for...ke-a-cheap-isolated-dual-battery-setup-for-50

I went with the Painless because I need it to be dependable for 80k miles and two years in Africa. Not something to skimp on.
As a bonus, the Painless kit comes with a switch and the details so you can be in control of when the batteries are connected, and when they are isolated (it's a three position switch)

And yeah, I try to keep my website family-friendly

-Dan

 

[John Shaffer]

Grecy

Did you mount a dpdt switch in you cab? I don't care for the painless dpdt and prefer a Contura or Carling style for my Toyota 4 but I'm not sure of the wiring. Any suggestions if you do this ?

Thanks

 

[Dan Grec]

Grecy

Did you mount a dpdt switch in you cab? I don't care for the painless dpdt and prefer a Contura or Carling style for my Toyota 4 but I'm not sure of the wiring. Any suggestions if you do this ?

Thanks

Hi John,

I decided not to use the switch at all, I wanted everything to be fully automatic.

Just don't wire in the switch at all. Follow the directions as above - put the two big lugs on the solenoid to each positive of the two batteries. Join the two negatives directly together. One of the small lugs goes to ground, the other one goes to a wire that's hot when the engine is running.

So now the solenoid will automatically connect the batteries together when the engine runs, and it will disconnect them when the engine is turned off, without the need for the switch at all.

I hope that helps,
-Dan

 

[John Shaffer]

Hi Dan

That in itself is very simple. Though I thought that switching was a preferred way to go.
I'm running 2ga. Wire from the pass side to the stock battery side on the front driver side. I could run a new 4' #2 straight to the solenoid. I'm using the aux. optima d34 blue to assist the trailer battery and maybe to run solar input to either battery.

My 4x4 guru shop was gonna assist on the switch but maybe I don't need it. Though I don't plan to run down the main battery so much, I like the idea of a assisted backup and to be able to run accessories and power assist my trailer (w jumpers).

Kinda like what Painless says about the switching:

"Painless says; and similar maybe in Dirty Parts
"Switch actions:

Switch centered; the main battery is used only for vehicle operation. The solenoid is not activated and the auxiliary battery will not be charged.( so probably no power to the solenoid)

Switch activating green light position; the main and auxiliary batteries are connected for charging and starting and the green light is illuminated when the key is in the on position.

Switch activating the red light position; the main and auxiliary batteries are connected together and the red light is illuminated at all times regardless of ignition switch position."

Much thanks

John

 

[Dan Grec]

It's up to you.

Do you want the ability to manually control if and when the batteries are joined together?

If yes, install the switch as the directions say.
If no, don't install the switch like me.

I personally decided it was another point of failure, including operator error (i.e. I flip the switch for some reason, then forget about it and kill the starter battery).
No switch means there can be no operator error

-Dan

 

[John Shaffer]

Thanks Dan

I kinda like the option to switch as needed. But the painless switch can be improved on using a Carling or Contura. (I've got a 10 blade) and it's ready for my stock switch port on my panel but it's not clear the way wiring for it. Dirty parts has a lot with the same switch but alas it's not the kit I have.

I'm still researching how I want to do it. I'm all about minimizing failure points.

Thanks

 

[SheepnJeep]

I have a question regarding your install without using the painless wiring kits switch.

If I understand how you hooked up your deal without a switch, you simply charge both batteries when running and only access the house battery when shut down. If you turn the key does that flip the solenoid giving you double the cranking amps, or does it only switch when the engine is running? Do you have any way at all to use both batteries when not running or have you deleted that option?


I am thinking about running dual batteries in my willys jeep that has an old school and very power hungry Koenig electric winch. It drains the battery when the jeep is running and really drains the battery when not running. I would like double the amp hours in case the jeep is dead, (but needs to be winched out) but I guess I also like your setup that doesn't allow for forgetting to flip the switch to charge the batteries. Maybe there is a way to have a low voltage cutoff on the starter battery to keep one from draining it on accident.

Long story short I need both batteries when running or not but an idiot proof way of keeping them both charged.

_

 

[Dan Grec]

I have a question regarding your install without using the painless wiring kits switch.

If I understand how you hooked up your deal without a switch, you simply charge both batteries when running and only access the house battery when shut down. If you turn the key does that flip the solenoid giving you double the cranking amps, or does it only switch when the engine is running? Do you have any way at all to use both batteries when not running or have you deleted that option?

On my system they are only joined when the engine is running - when the starter is firing they are not joined, because if the house battery is drained, that would put a big drain on the starter battery, which then might not have enough to start the engine.
I have no way to use both batteries when not running, other than if I rewire something, or use jumper leads.
In the way I built mine, the starter battery is always for the engine to start, and the house battery is for everything else, and solar.

I am thinking about running dual batteries in my willys jeep that has an old school and very power hungry Koenig electric winch. It drains the battery when the jeep is running and really drains the battery when not running. I would like double the amp hours in case the jeep is dead, (but needs to be winched out) but I guess I also like your setup that doesn't allow for forgetting to flip the switch to charge the batteries. Maybe there is a way to have a low voltage cutoff on the starter battery to keep one from draining it on accident.

Long story short I need both batteries when running or not but an idiot proof way of keeping them both charged.

_

It's always possible to use jumper leads to jump the starter battery from the house battery, if you ever required that.

-Dan

 

[SheepnJeep]

I see. Maybe for my application it would be best to simply run dual batteries in parallel for double the cranking amps and add some type of low voltage cutoff to keep from running the system completely flat. The vehicle could be push or crank started as well in an emergency.

Thanks for the reply!

-Jesse

 

[Two Jeeps]

I have a dual battery setup on a CJ7 and I used a Cole Hersee 48530 Smart Battery Isolator. It can only handle 200 amps compared to the Painless 250 amps but it was very simple to wire up. It can also be setup with a switch and have a indicator light letting you know the batteries are connected. I haven't gotten around to hooking up the switch or the light yet.