Double Up with a Lithium Dual Battery Install

Steve Cassano — 30 January 2020
We talk you through the installation of a lithium dual battery and DC/DC charger in your four-wheel drive.

If you’re handy on the tools, a whole new world of D.I.Y. 4WD upgrades opens itself up before you. If you actively nurture your skill set, this world of endless possibilities is only really limited by your imagination. Being able to take something into your own hands, save on labour costs, challenge yourself and have fun while doing so is a big win in anyone’s books.

Of course, there are some mods best left to the experts. For example, I recently swallowed my pride and allowed the folks at Out of Town 4WD, better known as Long Ranger, to install an auxiliary fuel tank. The trip out to Barnsley, New South Wales, was certainly worth it. 

But in this month’s column, I’ll be focussing on a favourite D.I.Y. modification of mine — a dual battery set up. I’ve briefly covered dual battery systems in the past, but not yet the process of installing an entire system. This time, you’re in for the whole hog.


To run a dual battery system I needed some basics. A method to manage the system, a deep cycle second battery, a menagerie of cables, wires, terminals and fuses, plus a means to securing it all. 

My first challenge was easy. Previously I’ve had success with a REDARC 1225D BCDC charger. Again, I intended to utilise this excellent unit, as it can manage new vehicle technologies and various battery types. 

The rest needed some planning before I delved online, and the most challenging question was where to mount the auxiliary battery in an already compact 4WD. Some lucky 4WD owners not only have plenty of space under the hood but a space specifically for that second battery. Alas, as with many modern 4WDs, my own offered no under-hood option.  

After exploring every nook and cranny, I was left with no other option than to have the battery reside somewhere in the cargo area.


I had in mind that I would build a custom battery tray of 2mm mild steel plate plus bracing, which would sit above the left rear wheel arch to minimise cable distance to the main battery. Whilst this looked like a safe and out-of-the-way option, it did present a challenge when it came to fitting a suitable battery and securing it in the confined place.

My first task was to measure the distance between the main starting battery and my intended position for the auxiliary battery. It’s best to keep this as short as possible to minimise any voltage drop and performance issues. 

Luckily for me, I found a safe route that measured around four metres, which meant I needed just 8 gauge (8B&S) power cable according to REDARC’s recommendation for up to five metres. For $22 I picked up six metres of red cable, plus another $13 for five metres of 10mm split conduit. I already had sufficient black 8 gauge cable for all the earthing I envisaged.  

This was followed with the two MIDI style fuse holders and a pair of 40amp MIDI fuses for just under $25. Avoid the common blade style fuses; they tend not to handle high current situations well and don’t have a small solid footprint like MIDIs. It’s recommended that each fuse be placed close to each battery, preferably within 300mm.  

I was able to position the main battery’s fuse holder along the firewall using some self-tapping screws (one of just two times I’ve drilled into the Jeep’s body). 

Next, for $17, I bought ten Narva 25mm cable lugs (avoid the cheap clones as they’re made of soft alloy and aren’t splayed) and terminated lengths of 8 gauge cables as necessary. A good crimp tool plus soldering ensured the best connections, which I then wrapped with heat-shrink.


Before I began creating a power board à la Steve, I decided to procure some 12mm medium density fibreboard (MDF) and matching car carpet to make it all pretty (and to cover my indiscretions), which easily allowed attachment of all electrical components in position. 

Once I was happy with the design, I fastened it to the steel battery tray I had sweated over in the previous weeks using 35mm M8 stainless button head screws and matching locking nuts. A good drill and bits makes life easy at this point.

I must admit, it took me many frustrating attempts to settle on a layout that I was happy with. So don’t be afraid to redo your layout several times till you’re a happy camper. After all it’s not a fixture that you can’t easily change or expand on in the future if the need arises.

First task (after “hundreds” of designs) was to attach the REDARC BCDC controller, which should be within 800mm of the auxiliary battery. Accompanying but not directly connected to the REDARC unit is a 40 amp circuit breaker (worth about $12) that connects the fuse block to the positive post of the auxiliary battery. 

A 6-way fuse box, as seen clearly on the next page, was purchased for about $18. I preferred this style of block as it houses both positive and negative posts and encases all the connection points, ensuring a cleaner, safer layout. Being light components, I attached them using some screws I had laying around the shed, unlike with the heavier REDARC unit, which found its home using more button head screws and nuts.

Luckily Jeep had a vacant earth point under the carpet very close from my handiwork. To ensure a good earth, I recommend sanding any paint away and using star washers to maximise conductivity.

If there was one criticism (only one) of the REDARC unit, it’s the lengths of the attached cables. I wish they came in at least 500mm so as to minimise joins. In particular when it came to reaching my preferred earth point and placing the solar Anderson connector further away (yes, the BCDC 1225D has solar input). 

Joining these and similar large cables is not an easy task to solder without the use of a butt connector. Simply prepare the ends of the cables you wish to join, crimp them to the connector and then solder to ensure a perfect connection. You may have to search online for them, as I could not find them in the local hardware store. Alternatively, do as I did and use those yellow coloured connectors, strip the plastic cover off then crimp/solder your cables. Don’t forget to cover it with heat-shrink, preferably the black double-walled kind. 

REDARC have been good enough to colourise their cables to help identify what’s what. The black cable is the earth and, after extending with similar gauge cable using a butt connector, I secured it to the above mentioned earth point. 

Next was the four metre long cable from the main battery via the MIDI fuse on the firewall and it was joined to the red cable from the REDARC unit, again using a butt connector and heat-shrink.  

The REDARC yellow cable is the solar positive input source, which I extended (unfortunately I only had red) in order to reach my desired position. Using a $10 50amp Anderson plug I crimped the yellow solar cable to one of the supplied Anderson connectors. 

(Note: Anderson plugs can accommodate different gauge cables, so buy the size you need, in this case size 8). 

For the Anderson earth, I crimped/soldered some black 8 gauge cable that reached the earth point using a Narva lug.  Make sure you identify your positives and negatives.

The REDARC brown cable needs to go to the positive of the auxiliary battery, via the other MIDI fuse, which I attached to the power board. A short length of red 8 gauge cable from this fuse holder to the battery is all that’s required once you crimp and solder the necessary Narva lugs.

This left just three smaller wires which require different connections, depending on vehicle and battery choice. Cut to 'Securing the Battery' to read about what I did with these.


My biggest hurdle was finding an auxiliary battery to fit in a confined place, which still maximised capacity. Width and height were fine, but not length; I had a smidge over 10 inches, leaving me with few choices if I was to meet common 100Ah expectations.

While I’ve been a fan of AGM batteries, I had viewed several videos on lithium batteries (including those from a few infamous YouTubers). In particular, I was attracted to a company called DCS (Deep Cycle Systems) based in sunny QLD (see After a few conversations with Paul at DCS, during which I was able to explain my situation, he recommended the 75Ah model. They do have models all the way up to 200Ah and their batteries are suitable for parallel or in-series connections. My reason for choosing lithium? It’s about a third of the weight, offers around three times the useable storage, and has about six times the service life of an equivalently sized AGM, and with built-in BMS, no less.

For around $950, it’s a little pricey, though I look at it as a long-term, sensible solution. After all, it fitted like a glove and delivers great output in a smaller footprint. 

It looked great with a strong case (though the top cover was a little askew but won’t hinder performance), plus it has solid low profile stainless bolt style posts with fittings. It even had all the specs on the case and comes with a magnificent four year warranty!

In the two plus months I had it before installation, and having used it to do some testing of compressors and other electrical devices, I can confidently say it has shown no signs of weakness, nor any discernible loss of capacity.


While I was confident that the tray was strong enough, I needed to know it would not budge nor become a projectile in the case of an accident. To this end, I utilised non-slip matting and heavy-duty J-hooks and a RHS cross bar.

The final task was the remaining three smaller REDARC wires. In my case, with a variable voltage alternator, the blue is the trigger cable and needs to be connected to an ignition switch, like a cigarette socket which comes on when the car is switched on, and off when the ignition is off, in order to save any power draw if the car isn’t started for a long time (but check your 4WD for what’s needed). Luckily I found a dedicated point under the dash, though I needed to run an extended 10amp blue wire all of the way there. As it draws minuscule power, splicing in on an existing accessory was fine.

The orange wire sets the REDARC’s profile and is based on the battery choice and voltage output. For lithium, I soldered it to the green wire for correct functionality. 

The final step is earthing the negative post of the battery using 2 gauge cable I had on hand.

It’s early days but the set-up and in particular the DCS battery have served their purpose well and I’m happy, but I think I might still make even a few more layout changes in the near future. Happy Wheeling. 


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