Auxiliary Battery Setup

This is a project I have been working on for awhile, and it is 85% complete. The aux battery project itself was done in combination with the rear storage project since they would need to function together as well as independently.

Aux Battery Setup Ctek Dual DC-DC charger

The rear sliding drawer storage system was built to be removable, and would incorporate some aspects of the rear aux. electrical system, but I needed the battery and electrical system to be totally independent of the drawers when needed, as well as be safe. Electrical system fires are apparently one of the main sources of problems for Overland Expedition vehicles. A statistic that likely does not improve with time, vibration, wear and tear.

I started with this as inspiration:

and ended up with something very different.

I have seen people mount their batteries on WKs in a few different places. Under the hood, but that has included moving the stock electrical fuse and relay boxes. Anything that runs the risk of lowering reliability and increasing the chance of hard to troubleshoot electrical issues is not an option. My family owned a Land Rover growing up, and I owned a 1971 MGB GT, so I am very familiar with reading electrical diagrams and tracing electrical faults, even manually simulating the alternating signal on a DC fuel pump in the back seat of said MGB to keep it going while my gf drove it home.

I have seen people mount them under the left hand side rear tire next to the subframe, but in the event of a rear quarter crash, or off-roading and dropping that tie off a rock this was not an option. A Jeep forum member showed me where his was, and this made sense to me.

I bought a fairly solid aluminum minimalistic battery holder meant for Optima “6 cylinder” batteries. I bought it off Amazon, and here is the link:

When the link is dead I will kill it, and the photos will remain.

I still have to add some venting, but this should not be too much of a problem with these batteries and the Ctek charging system. As I understand it, hydrogen gassing tends to happen more often if a battery is being overcharged.

Step 1. Mounting Battery

After struggling with how to mount it, I opted (no pun intended) for what was available at my local hardware store.

Cut where red line is and fold over 90 degrees where black lines are. Vice, Sawzall and hammer helped

I bought a four foot length of lightweight drilled angle steel, cut a 1 inch line on the angle, and folded the ends over, overlapping holes, drilling out as necessary and ended up with “perfectly” fitting mounting struts. I used stainless steel bolts drilled through the fender well with the nut on the inside, and silicone seal to keep moisture and dirt on the bottom side of  the wheel well.

Below: By request are some additional detail photos showing how the Optima tray is mounted.

I purchased a length of slotted angle steel from my local hardware store. Made a cut along the angle, and folded the tabs over.

Here I used double nits and lock washers to make sure nothing would vibrate loose. As you can see I only used three out of the four mounting positions for the tray.

Above: Here you can see the process used to align and level the battery tray. In all honesty this seems to me to be the best tray to give battery a solid footing and take minimal space.

I used a fair amount of washers as necessary to level the battery tray, and nylon lock nuts as well as lock washers as needed.

Above: Attempt at making nice straight uniform lines to make an even cutout for battery poke through.

Below: Semi final product. Eventually I will add some border or moulding, flashing etc to it to give it a less chainsawed look. For now the fender liner functions to hold the electrical panel in place, still holds the cargo cover, and also serves as a mount for the Travall Pet Guard cargo separator.

Eventually I took a half assed attempt at putting the plastic fender liner back in. It all started pretty civilized, but in the end I was hacking i tup with a sawzall in my driveway at dusk.

Now that I had a battery mounted the question became how to get power to it. Currently I am not using a ground wire, but just grounding the battery to the steel unibody. Paint having been wire wheeled off, and bolted solid. Still I know this is not as good as copper, but for the charge coming through from the alternator, it should be fine. (for now). I will do some more advanced testing once the project is finalized.

As mentioned above, I started with a basic idea for charging a second battery, but decided to get something more or less foolproof, and reliable that did not require any manual intervention, and actually charge battery more effectively and faster than the alternator and a direct line of heavy gauge wire would.

Ctek D250S Dual:


CTEK DC-DC charger

It is called a “Dual” because it can also take a solar panel current direct and act as an MPPT charger. If you know anything about Solar and MPPT chargers, this Ctek box actually comes in at a pretty good price point just to be used as an MPPT, or at the very least a proper solar charge controller because most for sale on the market undercharge the battery. If you are unfamiliar with Ctek, check out a coupe episodes on Jay Leno’s garage were he explains the benefit, and how much better they are for batteries in the long or short run.

Using the United States Coast Guard USCG wiring chart for reference on all my wiring

The most important tools and parts for an electrical project like this are proper terminal crimpers, proper correctly sized terminals, heat shrink, (heat gun) and research.

For the bigger gauge wire, 8 gauge and below, this is the real deal in my opinion:

Amazon (again): TEMCo Lug Crimper Tool TH0012 – 8 AWG – 0000 AWG(4/0) DIELESS Indent Electrical Battery Terminal Cable Wire 5 YEAR WARRANTY 

After looking at the issues with hydraulic crimpers, and matching dies to crimpers, I opted for old school mechanical power. I must say this is one of the manliest tools I own, and I still have machetes and snake sticks left from my time living in the rain forest in Costa Rica dealing with Terciopelo aka Fer-de-Lance.(venomous snakes) Anyway, this makes crimping AWG 6 child’s play and $60 -$80 it is a bargain. Try to buy pre made battery and alternator cables  and you will see this thing pays for itself very quickly. 

Given that I am also wiring up the solar powered system in my trailer it will get more use. Big wire is not used out of necessity for voltage or amperage, but to lessen voltage drop which is prevalent in DC applications. (See USCG chart above)

Keep in mind when buying heat shrink tube you need it big enough to cover the terminal housings, not just the wire. (Not that I made that error) Let’s just say I have a lot of extra terminal ends, and heat shrink laying about.

Before anybody feels the need to comment on the sloppy cutting of the inner plastic fender, and the sacrilege of cutting into sacred irreplaceable Jeep “Tupperware” I was unsure I would even put these plastic inner wheel-well  liners back into place. In the ned it was a little difficult to find the lines and cut, but if anybody is considering this, please contact me and I am sure I can come up with more accurate templates o measurements to help. Finally I put it back because it made it all simpler to mount the wiring board, still use the cargo cover when desired, and protect the stock wiring hidden by the plastic.

The Ctek ended up getting mounted on a piece of plywood down and out of the way. Probably important to remember, some of those steel terminals are direct to the battery, so proper insulation, or coverage is needed. Mine are normally covered by the rear storage drawer.

Step 2. Running wire from Alternator/battery

I removed the passenger and rear seats, (super easy) as well as the center console, which I have removed a dozen times battling the Shift Control Module problem. (Final solution, replaced it)

I used a backup set of 8 gauge booster cables. I don’t know if they are UV resistant, but they should not see any sun to speak of.

Above: Here is wire going through firewall. I put it through large grommet on right of steering. I used plastic cable protector here, and for much of the 15 foot run, but also used red rubber coolant hose under the floor carpet and rear seat.

Above: There is a nice natural channel above the heating duct to run the wire to right of LHD WK driver’s legs. You can see the imprint on the carpet from the line of the plastic center console.

Above: here you can see I taped a small low voltage wire to the side of the main charging wire. Although I did not really plan this out in advance, from experience, I know running wires while you have the chance is a good idea. The light gauge orange wire may be used for a switch from the front panel, to a relay i the rear for pretty much anything. No idea at the moment.


Above: Here I followed the natural curvature of the floor, and used the stock wire protection as much as possible. Here I used rubber heater hose I had laying around (Elephant hose mod) as protector. I feel the corrugated black plastic would be easier crushed and compromise the wire and insulation inside.

Above: Coming up the right hand side, I used Zip ties to hold it in place. Another reason I put the stick inner wheel-well grey “tupperware” back in place.

Rear seat back in place.

Above: Rear seat back in place. Being careful not to pinch wire with rear seat bracket.

Above: Cut the carpet slightly to allow a cleaner exit for cable and insulation.

Above: Zip tied the front section, and put console back on.

Above: With main wire now in place, time to put inner fender with “laser cut precision battery hole” back into place. IN all honesty may two main concerns for the batter were that it was “level” stem to stern with the jeep, and that it cleared the slide in rear drawers easily. In order to mount a fuse and relay panel I was considering building a frame, over the battery and all sorts of things, and then decided, “well if the rear storage space is going to be full most of the time, I will not be able to see anything anyway, so who cares about light through the windows.”

Left: At some point I began wiring up the fuse block. It is a Bluesea Marine panel, and has both a positive and Negative post. That means all pos and neg wires run to this panel.

Here you can see an AWG 4 gauge  wire with ring terminal and heat shrink waiting to be crimped and terminated.

Originally I had the fuse panel on the outside for easy access, but after a bit more use, decided I needed the real estate for the Dc-AC Inverter. Originally I had holes going through from relays n the back to fuses in the front.

Above: Photo is from testing, prototyping phase. Red LED I added, because I wanted to incorporate a few things that “added a cool factor” and while at the electronics store grabbed things that I felt might come in handy. In all seriousness, this is switched to the off position for the on board Air compressor relay. Using a 40 amp relay with a Viair 450 (or something like that I will check) allows me to leave it on all the time when a hose is attached. The light is just an extra to let me know it is off or on and is visible from the driver’s seat, which is how I positioned it in the board.

Above: Relay board now looks like this. I considered moving the inverter to the other side, but for the high amperage cables needed, I figured it was best here. There is enough room for some air flow top and bottom, and the cable organization works perfectly this way. This is a Stanley 750 watt non-sine wave inverter. I did not do any great amount of research on this, just grabbed it on 50% off end of season sale at local hardware store for $75.  I have used a rice cooker plugged in to cook rice I have plugged my RV TV in to it and it draws almost nothing, and I have plugged in my Ctek battery maintainer charger to keep the RV trailer battery topped up overnight, so essentially going Solar/alternator to DC battery via Ctek DC-DC to Dc-AC inverter to AC-DC charger to let me run DC lights etc in RV. Maybe to the least inefficient, but all part of experimenting with new system at the moment.

Fitting the window shaped plywood electrical board into the window opening took a few tries. When I ordered two sheets of 1/2″ Marine Grade plywood for the drawer system, it came protected by two sheets of chipboard, which gave me a lot of material for mocking up stuff. It fits into the natural contour of the plastic interior window liner, behind the Travall Pet guard.  Holding it in place I drilled a hole and with large flat washers mounted a 1.5″ 5/16th bolt coming out of the plastic wall. There is a hole in the plywood that slips over it, and I use a flat washer and wing nut to tighten and remove. Its not perfect, because it can be dropped easily, but works for now.

For the switch panel I found a “Marine switch panel” on Amazon. I am a little suspect, but it feels pretty good, and 6 switches, and panel plus USB, lighter socket, and dodgey Volt meter for less than $50, is hard to pass up. Considering it all comes wired up, and a pretty nice job at that, and it is a winner, at least for now.

I say dodgy Volt meter because it does not give you real accurate battery level. For that you need a shunt and proper meter.

I have a template fo the wooden board shape, and will be making one for the other side, as well as replacing this one eventually, maybe.


Above: One word of warning I will give, and you will pick up on this pretty quickly. This is what the relay board looks like to the civilian passerby in the right/wrong light. I plan to spray tack a mesh fibre to the window to block any view inside. I have gone through US-Canada border a couple times so far, but do not want to return to my Jeep to see it surrounded by a SWAT team and demolition team trying to diffuse my refrigerator battery during these heightened times of security.

40 Amp Relays also purchased off of Amazon. Five of them for $12 with mounting brackets. They click lock together which can be a good thing to save space and make assembly more rigid, but will be a real pain to take apart, or replace with one that is not exact same design

N.B. I use an 80 Amp relay for the inverter.

For the switch panel something else I need to adjust for security purposes is the lighting. Currently I have set it up in the safest way for electrical shorts and load protection (I think), but I have no easy way of turning off LED switch lights or voltage meter while still keeping power flowing. The switch panel has three hot leads running into it all separately fused. One for LED backlights, one for Voltage Meter, USB charger plug and lighter socket, and one for something else.

I will probably set up a big mechanical kill switch, but for now main power flows direct from battery to the  100 amp fuse panel. The run is so short, I am not sure it is worth the extra hassle to put in a breaker or fuse. With only 1.5-2 feet of AWG 4 cable, I am unsure what could happen that fused protection would help. Anyway, more to the point, all those pretty LED lights sure do attract passerby’s attention at night.

5 Replies to “Auxiliary Battery Setup”

  1. The writeup is a macro steps explanation. I do not go into explaining how to remove seats, consoles, etc. If anybody is looking for more detail information, I have it along with photos, but did not want to make this writeup to long and detailed.

    Please feel free to write or discuss here and let me know if any f this seems wrong, or a good solution. Cheers.

  2. I found this write up and your storage drawer write up from Thank you. I only have one critique. Your pics are small and at least in Chrome cannot be clicked upon to expand. It would be helpful if I could make the pics bigger. Regardless, thank you!

    1. Chris: Thanks for the feedback. Still struggling with the wordpress format. Slow loading times and awkward interaction with photos are an issue for sure.

      To see the larger photo, which is there, right click on the photo and choose to open in a new tab. They are considerably bigger that way. If you need a more detailed photo to understand or replicate, feel free to send me, and I will e-mail you. Cheers.

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