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- Dual Battery Installation -

I recently completed a dual battery and fuse block installation install in my FJ and thought I would share what I did in the hopes that it is useful. I also moved all of my aftermarket electronics over to the new fuse block, which was like reinstalling everything (read: PITA). Hopefully, this will be helpful to someone and if you find things that make it even better please add them to this thread so future installations benefit from the knowledge share. While a Dual Battery installation by itself is not that difficult, I found that rewiring all of my accessories was time consuming. This was a BIG project that took A LOT of effort and time. Be prepared to spend some good quality time with your FJ and be prepared to get to know it intimately. If you choose to undertake installing a dual battery system and have questions feel free to PM me. If I can help I will. Happy reading!

All of the images in this thread are "clickable", so feel free to click on and image to view a larger picture.


I decide to install dual batteries and a new fuse block in my FJ for a few reasons.

1.) I wanted to run all of my electrical accessories (offroad lights, rock lights, backup lights, CB, inverter, fridge, electric blanket, etc.) from a second battery.
2.) I wanted to isolate the starting battery to prevent a dead starting battery situation (don't like getting stuck in the middle of no-where).
3.) I don't like having my winch powered by my starting battery (goes back to that whole dead battery thing).
4.) I don't like having miscellaneous fuses and wiring all over under my hood and dash, hence the fuse block.


1 - Painless Wiring Dual Battery kit (available at
1 - Blue Torch Fab battery box (available at Blue Torch FabWorks)
1 - Optima Yellow Top battery (34/78 series battery purchased at local auto parts store)
1 - Blue Sea 12 Circuit Marine fuse block (purchased from West Marine)
15'- #2 Battery cable (purchased at local welding shop)
4 - #2 Battery cable lugs (purchased at local welding shop)
3' - 1.5"x1/5"x1/8" angled steel (purchased at Home Depot)
3' - 1.5"x1/8" flat steel (purchased at Home Depot)
5' - #6 red battery cable
1' - #6 black battery cable
4 - #6 battery cable lugs
6 - Stainless steel bolts, washers and lock washers (purchased at Home Depot)
1 - 15" #2 battery ground cable (local auto parts store)
1 - positive battery terminal (marine style with post & wing-nut)
10 - 12v LED's in RED and GREEN (
Miscellaneous items - Shrinking tubing, wire, zip-ties, electrical tape, 1/2" rubber grommets (and other stuff that I'm probably forgetting... :))

The Install:

Battery Tray & Battery Box - Installation

Once I figured out that I wanted the battery installed as close to the firewall as possible I started to fab a battery tray for the BTF battery box to set on. Here are a few pictures of it in progress and finished. This is the tray about half way finished. I added a few legs going toward the front of the engine compartment to stabilize the battery box. In the end, I chose to welded the battery box to the tray I made, then bolted everything to the fender. I mounted the battery tray and box to the fender with stainless hardware. I sprayed rubberized undercoat on the bolt heads once I was finished.

Installing the Solenoid

I installed the solenoid on the drivers fender. I moved a small electronic unit that sits directly on top of the wheel well in front of the master cylinder. I removed the bracket and bent it slightly until it lined up straight with one of the other OEM bolt holes in the fender. I then used the bolt hole where the electronic unit was located to hold one end of the solenoid. I drilled a new hole in the fender to support the other end of the solenoid. I installed the bolts and was done with the solenoid.

Wiring - Batteries, Solenoid and Winch Power

I created a new 2/0 gauge positive battery cable that is about 15" and had lugs on both ends. This cable goes from one of the terminals on the solenoid to the positive terminal on the battery. I then created another long (approx. 10') cable (the RED one in the pictures) that runs from the other terminal on the solenoid to the positive terminal on the AUX battery. I chose to run the cable behind the fuse box and battery, then across the front of the engine compartment. The cable is zip-tied to the OEM wire loom that runs under the front sheet metal just in front of the radiator. It's kind of a pain to get zip ties on the cable, but it's worth the clean look.

In order to get power to the new fuse block, I created another cable out of #6 wire. I ran the cable from the AUX battery through the firewall and into the cab where the fuse block is located. I installed the fuse block in the passenger's kick panel. I also purchased a 15" #2 ground cable and ran it from the AUX battery to a bolt located on the passenger's fender. There is a bolt located in the same location at the ground on the driver side fender. I used this same location as the ground for the AUX battery. I then created a short 15" #2 cable and ran it from the AUX battery to my winch control box. The power line from the AUX battery to the fuse block has an 80amp in-line fuse to protect the electronics that will hang off the fuse block.

Wiring - Fuse Block

I ran the #6 wire from the positive terminal on the AUX battery through the firewall into the cab. The cable is connected to the fuse block that is mounted behind the passenger's kick panel. During this installation I moved all of my accessories over to the new fuse block (what a PITA!). I mounted the Blue Sea Marine 12 circuit fuse block with zip ties. It is mounted to the existing plastic wiring mount and the surrounding electrical wiring. The fuse block is very stable and the passenger's kick panel fits back on with plenty of clearance. The power line come in through the firewall from the AUX battery. There is a factory grounding point directly behind the fuse block that works well for grounding the fuse block. All new electrical accessories will be powered by the new fuse block. The first one will be my CB radio that I will install in a few weeks.


3,329 Posts
Discussion Starter #2
Re: Dual Battery Installation Write-Up - Pics


Wiring - Switches

I decided to use the driver's glove box as a location for all of my switching. I have not used my glove box for anything and I never plan on purchasing the over-priced Toyota NAV unit. I used some 16 gauge sheet metal to make a switch panel that mounts inside the glove box. I made some labels with a vinyl label maker, but I haven't installed them yet. I have also considered having Tap Plastics build me a switch panel with the name of each accessory engraved into the plastic, but for now this will work. I had pre-purchased all the switches required for my planned mods from I drilled a bunch of 19/32" holes in the panel and mounted it in the glove box with all the switches pre-mounted. The wires from each component are fed up through three holes in the back of the glove box. I made sure to leave enough wire so that I can remove the glove box if required. I created a wiring harness that feeds all the switched electrical accessories up into the back of the glove box and connects to each individual switch. There is also another harness that routes the positive feed from each switch to one of the LED's mounted in the front dash. Even thought the switches are out of sight, the LED's let me know when is actually powered up. While I was adding switches I decided to add one for the A-TRAC Hack and the DRL Hack, so now "A-TRAC+Locker" or "DRL Off" are only a flip of a switch away. :)

Wiring - LED's

Since the switches are out of sight I elected to install LED's on the dash so I would know which accessories are on/off. These are normal automotive LED's that install into a 5/16" hole. I created one large loomed wiring harness that run from the back of the glove box to the install location on the dash. Each switch that powers an accessory also supplies power to an LED on the dash.

Wiring - Connecting the switch to the Solenoid

There are two wires that run from the switch to the solenoid. The switch is mounted in the driver's glove box. The Painless kit comes with a 3/4" rubber grommet that gets installed into the firewall. You need a 3/4" drill in order to drill the hole and insert the grommet. Then pull the wires through and connect them to the solenoid. I had to move the small silver finned electronic device that is to the left of the solenoid. I also had to bend the bracket so it would sit where you see it in the picture. The solenoid is mounted using one (1) factory nut and a hole that I drilled.

System Operation / Testing

The toggle switch that comes with the Painless kit has three (3) positions. In position one the GREEN LED is illuminated and the system is operating in normal mode with both batteries available for charging, but are isolated from one another. In position two the RED LED is illuminated and the system is operating in emergency mode with both batteries combined for starting and charging. In position 3 the system is off and only the factory battery is available for starting and charging. All testing went fine and the system is now working as described by the manufacturer. :)

Installation Comments

The Dual Battery Installation by itself is not that difficult and was not that time consuming. The Painless Wiring kits was well put together and had great instructions. This should have been my first electrical modification as it would have saved a lot of re-work on the already installed electrical accessories. I had to move all of the electrical accessories (IPF lights, Backup Lights, IPF Fog Lights, Rock Lights, switching, CB and other stuff) from the factory battery over to the fuse block that I installed as part of this project. I still need to move the 400 watt inverter over to the fuse block and AUX battery, but that's going to have to wait as I tired of doing electrical work... :)

There is a lot of good information in this thread starting around post #70 there are a few others that have started/completed their dual battery installations.

2,925 Posts

To read the original full thread and post questions/comments on this topic, please go to

Here is how I built my own dual battery kit, all measurement provided are approximate, make your own measurement for everything. This will be 2 parts, the tray and the wiring.

Part 1 – The Battery Tray


36" 1/8" x 1" aluminum strip
36" 1/8" x 1" aluminum L strip
Battery Tray ~ 10.5" x 7.25" - Amazon

Battery Strap - 38" - Amazon
1 - 1/4" x 4" Bolt
1 - 1/4" x 3" Bolt
2 - 1/4" x 1.5" Bolt
9 - 1/4" Stainless Steel washers
5 - 3/8" Stainless Steel washers
13 - 3/8" Zinc washers
4 - 1/4" x 1-1/4" Neoprene washers
4 - 1/4" lock nut
4 - 1/4” OD X 1/4” ID X 1” long Nylon spacer

Home Depot - $~20
Amazon - $~13
Total for Tray ~$33

I cut the side off the tray that the plastic battery retainer bar would use, no room for these with the Cold Air Intake, I doubt the plastic would holdup off road.

Drilled a ¼” hole in each side of tray.

I made an aluminum frame to support the tray and disperse the weight. I also made the right side higher to help shift the battery weight back toward the fender and less on the “stilt”. Top strip ~10.5”, the 1 strip on the left and 2 on the right are ~7.25”. I drilled holes through them to match the tray.

I cut a ~ 7.25” of the L strip to fit inside the tray. This will keep the battery snug, prevent left to right shifting. I drill holes to match to existing on ones.

I used 4 of the 3/8” zinc washers under each end of the L strip, it made it flush.

Some of the referenced parts.

All relative to how you drill the holes in the battery tray and the fender.

Example of the order used for the stilts and supports (a few extra parts used later). The general order was going top to bottom. Bolt, ¼” washer, tray and or aluminum, nylon spacers (some cut in half), ¼” washer, 3/8” washer, ~1.5” strip of aluminum to help disperse weight on fender, neoprene washer to cushion. Under side of fender, ¼” washer and ¼” lock nut.
On the front right, used 3, 3/8” Stainless washers under the tray, then the aluminum. (raised the tray height)
On the front left, used an extra zinc 3/8” washer under the top ¼” washer (reduced the amount of bolt on the underside of the fender)
On the rear left, used 4 extra zinc 3/8” washer under the top ¼” washer (reduced the amount of bolt on the underside of the fender)

Bolts in underside of fender (extra washers from above remove the excess sticking down)

Tray mounted

Battery mounted and tied with the battery strap, no movement, totally solid.


2,925 Posts
Re: How to install your own Dual Battery kit

Part 2 – The Wiring


20Amp mini fuses - Littelfuse MIN20BP MINI 297 – Amazon $3.14
Fuse Holder - Littelfuse FHM2BP MINI In-Line Fuseholder – Amazon $5.39
ANL Fuse holder - Raptor RGFHW-ANL Rose Gold Fuse Holder ANL 1/0 In and Out – Amazon $11.35
ANL 250amp Fuse - DB Link ANL250 250 Amp ANL Fuse – Amazon $4.99 Amazon Total $38.13

4 pack Screw on 1g ring terminals - Lightning Battery Cable Compression Fittings – jcwhitney $25.99
S&H $6.99
JCWhitney Total $32.98

Battery Cable Cutter - CHANNELLOCK 911 – Summit Racing $19.95
Battery Cables & terminals - Summit Racing SUM-G1208 – Summit Racing $79.95
Solenoid - Painless Wiring 40112 – Summit Racing $91.95
Heat Shrink - Taylor Cable 41056 – Summit Racing $6.99
S&H $11.95
Summit Total $210.79

Grand Total No Switch $~302 (including battery tray) I’m sure it can be done for even less.

Switch DPDT - VJDJD66B-00000-000 – OTRATTW $9.00
Switch Lense - VVASC2J-100 – OTRATTW $3.


Ratchet set
Deep sockets for ratchet
Heat Gun – for shrink tube
15mm wrench - for twisting the ring terminals
Vice – Used for holding wire when threading the ring terminals
Allen wrench - for fuse holder
Wire Cutter
Wire Stripper
Wire Crimper

8mm x20mm Stainless bolt
2 - 6mm x 20mm bolt,
4- 3/8 washers and one
2 - 6mm lock nut
6mm x 45mm Stainless bolt
16 gauge wire

A pic of the parts

I started with the placement of the Solenoid. On top of the driver side fender was perfect, the existing holes were an added bonus.

I used one 6mm x 20mm bolt, 2- 3/8 washers and one 6mm lock nut on each side. Put the ground wire with the ring terminal on it through one of the bolts.

Bolts from underneath the fender

I cut the negative cable to ~28”. Slide one of the heat shrink tubes onto the wire, strip the end so it will fit into one of the ring terminals.

Thread on the ring terminal

Slide up the heat shrink tube and use a heat gun for a nice fit

Make sure to have the battery terminal and the ring terminal parallel.

Pull down the flap under the passenger side wheel, there is a threaded unused hold in the frame, clean it out with a pipe cleaner and some liquid wrench. This bolt shows the location, it is a 8mmx 20mm stainless.

The cable mounted to the frame

Looking down from above, the cable is far from the exhaust and engine.

The battery terminal covers don’t fit over the crimp, I sliced the bottom and zip tied it on the negative terminal of the aux battery.

Install the supplied wire clamps onto the metal posts, use 6mm locking nuts to hold them on, there are six posts.

The next cable is the positive, disconnect the negative cable on the aux battery. I cut the positive cable, from the tip of the terminal to ~11.5”

Put the 250A fuse into the fuse holder, strip the end of the positive cable and place it into the fuse holder. Make sure the fuse holder is perpendicular to the battery terminal.

I test fit the fuse holder into where it is going.

The cable is going to hold up the fuse holder with easy, but it could vibrate against the body, I cut strips of Velcro and placed them on the feet. I didn’t attach the adhesive until I was done.

Strip the end of the spool of positive battery cable and connect it to the other end of the fuse holder, place everything where it is going to make the next measurement, run the cable along the body to the solenoid and make the next cut, make sure to account for the length of the ring terminal. This piece was ~ 58”. Slide a shrink tube over the and end screw on another ring terminal.

Here is the entire cable run from the battery to the solenoid.

Now measure a piece of cable to go from the other terminal on the solenoid to the main battery positive terminal. I cut a piece ~ 19”

Slide a heat shrink tube on each end then screw on a ring terminal to each end, the terminal should be parallel. Attach both ends. Notice I bought a large bolt for the battery terminal, I couldn’t fit everything on the stock one, I use a 6mm x45mm stainless.

I cut the cover for the positive terminal of the aux battery as I did for the negative and zip tied it on.

2,925 Posts
Re: How to install your own Dual Battery kit

The small terminal on the solenoid activates the solenoid, attach a wire, 16 gauge with a ring terminal and run it through the firewall and into the cab. The white wire in the pic.

Attach a ring terminal to a wire, 16 gauge to the large terminal on the solenoid that the wire from the aux battery attaches to. This wire will provide power to the switch for “jump mode” and active the solenoid. The red wire in the pic.

Attach a fuse holder to the wire, place a 20A fuse and then run it into the cabin.

Pic of the firewall

Pic of the firewall from inside the driver side foot well, under the dash.

Reconnect the negative cable on the aux battery

The wiring for the switch will be a follows. I used a switch from OTRATTW.

-The white wire was attached to the “power out” or “out to relay”
-The red wire was attached to one of the “power in” for the switch, depends on which position (up or down) you want the “emergency mode” or “jump mode”.
-Another wire is attached to an add a circuit mini to a fused slot in the fuse panel that is active when the ignition is on, this was attached to the other power in for “Normal mode”, both batteries are connected for use and charging.
-The middle of the switch, neutral, no connection is the “off mode”, the aux battery is not connected. You should leave the system like this, so you can’t possibly drain both batteries. Use normal now and then to keep a good charge on the aux battery.


Fuse Panel for switch wiring
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