How to install dual batteries in your Jeep Wrangler TJ. Based on my '98 This tutorial is not for the weak minded, and it is not cheap. Your mileage may vary. Parts List Batteries You have to use a gel type battery because the batteries have to be turned in their sides. I used Optima 750CCA yellow top batteries. Isolator I used the Painless wireless kit. It was easy to install but the price is not painless https://www.amazon.com/dp/B00062Z5XY/?tag=wranglerorg-20 Battery Tray kit https://www.amazon.com/gp/product/B003CLNHKY/?tag=wranglerorg-20 Battery Terminals (2) https://www.amazon.com/gp/product/B01LLWKXV6/?tag=wranglerorg-20 Battery Cables (7 total) http://www.ebay.com/usr/genuinedealz?_trksid=p2047675.l2559 I buy all of my cables here. I have never had them show up wrong and never hada any issues. Best of all they are reasonable, and quality. Primary battery 6 gauge Red cable w/ 5/16lug to a 3/8 lug 12 inch (1) 2 gauge Black cable w/ 3/8 both ends lug 4ft (1) 2 gauge Red cable w/ 5/16 both ends 24 inch (1) 2 gauge Red cable w/ 5/16 to 3/8 lug 12 inch (1) 2 gauge Black cable w/ 5/16 to 3/8 lug 3ft (1) Secondary battery 1 gauge Black cable w/ 3/8 both lug 30 inch (1) 1 gauge Red cable w/ 5/16 to 3/8 lug 12 inch (1) Misc 1 yellow crimp terminal (key side of start) Heatshrink (to cover above crimp) 3/8 bolt with nut (1) 7/16 or 1/2 flat washers (2) 3/8 lockwasher (1) Battery terminal corrosion protectors (2) Shop Rags Rivets (could use countersink screws) Beer (Cheaper the better, you might go through a few on this project. Put on some coffee, and hunker down because this is a long read, but it will be fun. I am going to give you in my own words what I did. I did read all of the instructions of each of the kits, and I suggest you do too, if you take this on. I may miss something or not communicate it as well as the instructions. First step I did was disconnect the battery, and remove the hold down completely; then remove the battery from the tray. Then I removed all of the cables that were taped together in this harness: One ground bolted on the firewall One ground bolted to the side of the engine block above the starter The wire that goes from the fuse panel to the back of the alternator The two wires at the starter. The small wire that goes to starter has a pigtail between the battery and the fuse box. (You can skip this part if you are not replacing your cables, but my Jeep has 174 miles on it, and I didn't like the 4 gauge cables that come stock on the TJ, Plus my battery terminals were not able to to tighten down anymore. I upgraded all of my cables to 2 gauge on the primary battery and 1 gauge on the secondary battery. It starts amazingly better now and charges bettery too.) I removed that whole harness and carefully cut out the plastic conduit and tape that surrounded it. I say carefully because you are going to re-use the wire that goes to the alternator, and you are going to cut the small wire that goes to the starter and reuse that too. I cut the small wire right at the starter as close to the starter that I could and crimped a yellow terminal on where I just cut that would fit on the small key end on the battery. Then I covered the crimp with some heat shrink. I then opened up the Painless wiring kit, and installed the relay/solenoid on the flat spot on the fender just below and behind the air box. I had to put the relay there, then mark the wholes and drill them. The relay comes with installation bolts I used them from inside the fender well to inside the engine compartment so I could used the bolts as studs for grounds. I then ran the wires for the switch and wired it exactly as the instructions say. I wired the switch and lights to the center console. When the light is green it is charging both batteries. The switch has 3 positions. Up it is green and charging both batteries, in the middle it isolates the battery as if the car was not on meaning that the main battery is charging but the secondary isn't, but nothing on the second battery is draining your primary battery so even if you drain your secondary battery it will not affect your primary battery. The third position if you wired it correctly as the instructions say, will use your secondary battery to jump start your TJ in case you left your lights on or something. It will have a red light when in that mode. It is worth while to state that according to the instructions of the Painless wiring, they want you to not wire the red cable from the battery to the starter. They want you to wire from the battery to the relay then to the starter. It took me a while to figure out why, but I think it has to do with if your main battery dies and you use the secondary to jump it it will essentially bypass the primary to start the car. Anyway that is exactly how I wired mine and it works great! I am not going to tell you exactly how to wire it because you may run your wires through the firewall in a different place or you may add your switch to a different place, and the instructions do a great job of explaining how to wire it, and has an easy to understand schematic. Plus you may not even decide to use this kit; it is pretty expensive, but it has everything you need including plastic conduit. It is pretty quality. I have one in my Ram Truck also. Now I removed the factory battery tray, as the instructions said to being careful not to loose the bolts because you are going to re-use two of the factory bolts. Now unwrap your new battery kit, and read the instructions for installation carefully. I then added the first tray using the countersink bolts as suggested and the two factory bolts back on the tray. Then I laid the battery on it's side with the positive closest to the engine. then installed the second tray on top of the primary battery. At this point I loosely put in my new cables. I loosely bolted the ground to the engine block and firewall, then battery, I loosely installed the positive wires to the starter and the relay and battery with the new terminals. I loosely installed the 6 gauge red wire from the battery to the fuse box, and lastly installed the wire back on to the alternator. Also don't forget to plug the key wire back into the pigtail. Now with all of the new cables in place loosely, make sure the cables are free from everything, then tape them up with electrical tape like they were stock. now tighten them all down and tighten down the terminals on the battery not forgetting the terminal covers and corrosion protectors. Now at this point your TJ should start again, and charge. You should stop and test it, because once you are finished it is tough to get to the primary battery. This is really where you don't want to half-ass. Take your time and make sure everything is perfect. If there are issues work them out before going further. Some things to check for: Make sure you re-tightened down the end on that goes on the alternator. The other end goes to the end of the fuse box. Make sure they are on and tight. Also the 6 gauge wire will go from the battery to the fuse box. Make sure it is tight on both ends Make sure the key switch wire (the one you cut and added an end on is tight on the little stud on the starter. Make sure the red wire is going from the correct side of the relay to the big lug on the start, and finally make sure all of the grounds are tight. If your jeep starts, and you switch inside is in the up position with the green light it is time to add the second battery. Shut the car back down, and be careful now because you are installing the second battery with your primary battery live. Now add your next battery tray from the dual battery tray kit. if you haven't yet, (Don't put the all thread in first). You may have to pull up your fuse box. There are two levers on the air box side you push towards the air box and pull up and the fuse box will pop out, and get out of the way. We will get back to this later. This is also in the instructions of the battery tray kit. Now with the second tray sitting on the primary battery, look over the top, and run the all thread through the tray into the bottom tray. Do this a little at a time on both sides. Don't tighten one then the other because you may have to wiggle the tray a little to get the all thread in. I started the all thread a little bit on the fender side of the TJ. I had to wiggle the top tray a little to get it to go in. After about few threads, I installed the all thread on the other side the same way. Once I got both sides started I double nutted each all thread and threaded the fender side till it stopped, and I tightened the motor side, till I could feel it coming out the bottom of the bottom tray. Then I broke the double threaded nuts free and ran one nut on each side all of the way down till it hit the top tray but didn't tighten it. Now add the second battery. Lay it down the same way so the positive is on the engine side. Then go ahead and add the top strap that holds it down to the all thread, then put the remaining nuts on, and tighten them; then go ahead and tighten the bottom nuts that you left loose earlier. Now install your secondary battery cables. You should only have two 1 gauge cables left. One black one and one read one. Add your battery protectors. terminals, and covers on your battery and run your read cable to the other post on the relay, and for the ground I ran it from the battery down to the frame where the upper control arm bracket is welded to the frame. On the top of the bracket there is a hole, and I bolted my secondary ground right to that hole. The only thing left is remounting the fuse box. On my 98 I had to relocate the bracket forward about an inch. The bracket is riveted on to the bracket bolted on the inner fender well. I just drilled out the rivets, they were super easy and moved it forward about an inch and moved the front slightly closer to the air box, and re-drilled new holes and re-riveted it back on. Be careful hear making sure tat everything lines up meaning that the wires are long enough for as far forward as you want to go and make sure it doesn't interfere with the hood when it closes. If you don't have a rivet gun you can probably use some countersink screws instead. This seems like a tough job based on how long this write up is, but really it only took a few hours, and if you read all of the instructions to the different kits it is pretty cake. This is really expensive, but I recommend this to anybody relying on their winch. I have been stuck in mud before for hours, and was glad I had beef jerky and dual batteries. Hit me up with any questions or if I may have missed something, or if I need to clarify anything. I really recommend this for winches. I also have a double power socket in the rear of the bed of the jeep so I can run fridges or whatever off of the spare battery without killing the primary battery.