After a few overlanding trips in Nomad, it became apparent to us that the tiny 35ah stock battery just didn’t cut it for our needs. I often enjoy playing music from the car around the campfire, using lights, charging phones & camera equipment etc overnight. We managed to deal with the stock battery until we bought ourselves a really nice Snomaster fridge/freezer.

We tried the fridge out on a 7-day camping trip to Kruger National Park. We could only run the fridge while the car was running, so the stock lighter socket worked perfectly. However this was simply just not enough time running to freeze the contents enough to last the night. The fridge was not able to cool the contents fast enough. Thank goodness the campsites had AC power and we were able to run the fridge all night long. This worried us, as what would we have done if we did a very remote trip to Botswana somewhere?

The solution:
It was time to dive into a proper Dual Battery System.

Step 1: The plan

As always, it’s a great idea to start every project off with a solid plan, yes this always changes a number of times along the route, however, you need some guidance and a direction to start heading towards.

Location / Position

When doing a DBS, one essentially has two choices of battery placement. Both have their pros & cons and you need to weigh these up for your self.

  • Inside the engine compartment
    • Pros
      • Does not affect cabin space/passenger carrying capacity
      • Safer in the case of an accident
      • Does not have to be a sealed battery
    • Cons
      • Engine heat affects the battery
      • Battery size is severely limited due to space
      • More expensive
      • More custom work needed
  • Inside the cabin
    • Pros
      • Can fit a larger 105aH battery
      • These size batteries are cheaper
      • Can be installed as a removable component
    • Cons
      • Less safe in an accident
      • Takes up valuable cabin space
      • Not ideal for a daily driver / permanent install

Deciding where to put the battery was a big decision for me, and one that needs to be made early on in the project, as this determines all of the constraints for the project. I opted for the engine bay, as ultimately I thought it would be the more elegant solution especially as I use the Jimny as a daily driver, and I wanted the dual battery configuration for daily use, not only for long overlanding trips with the fridge. I knew this would be a much more difficult task and would require custom work/brackets etc. Long term I also knew that should I need much more capacity, I would be able to add a 3rd battery to the system which would very likely be a LiPO battery somewhere in the cabin. ( kept this in mind)

Before going online and getting excited and buying the whole internet, I decided to draw all the cables that we are going to need, this will let me think what we need, how many and where about the car to locate it.

Jimny Electrical System & Wiring for dual battery system
Please zoom in on the image to see more details about the wiring diagram. This was the plan for the electrical system.

The Battery

When selecting the battery the biggest constraint is most certainly SIZE. One needs to measure the area where they installing the battery and get the MAX dimensions that will fit in there. Keep in mind on the Height attribute that you don’t want battery terminals touching the hood or any other metal component. Try and work in at least 1-2cm margin for the batteries terminals. Also, note that some batteries have the positive terminal on different sides. In my case, I needed one where the positive was closer to the firewall as the hood was taller there. (Rather have the negative terminal touch than the positive).

After measuring my available space I was slightly disappointed as this pretty much limited me to a max battery size of about 65aH almost half of what I was aiming for. However I was also installing a compressor along with my battery, and I had to make some compromises. I can’t have everything in a small car.

I am not going to go into detail about the many different types of batteries and technologies, however, I think what’s important to understand is what you want to use the battery for. My second battery was going to be used purely for running accessories. This means I did not need high cranking amp abilities, but it did need to be able to discharge low and often. This is essentially what a deep cycle battery is designed to do. In most cases for an AUX battery, you will want a deep cycle. Some people if only upgrading their primary battery go for a what they call a hybrid/combo / semi-deep cycle battery which can still crank the motor and be discharged deeper. ( I have this installed in battery 1 position)

After refining my dimensions, terminal positions & sizes and battery technologies I was immediately limited to only 3 or 4 battery choices. I opted to go for the more expensive battery, for 2 reasons the Varta they did not have stock of, and the Discover was an AGM battery which essentially gave me a bit more usable storage out of the small battery, as well as some longer life span.

The battery which I choose was the Discover EV512A-70 AGM Battery.

(I am extremely happy with this battery) I have yet to run it flat and I would almost say it is the perfect size for me. Just run the fridge while camping. Generally, we drive the car at least once in the day.

Battery Bracket

In order to get this whole ordeal to fit into the position that I wanted, as well as fitting in the air compressor, I had to design a custom battery bracket. This was a whole other job on its own. Here is an image of the completed bracket.

{Write up on battery bracket coming soon}

Custom made secondary battery bracket with compressor mount

Charging System

There are essentially two options to go when it comes to connecting a secondary battery. Battery Isolator / Combiner or a DC to DC battery charger. Each has its own pros and cons.

  • Battery Combiner
    • Pros
      • Simple & Hardy
      • Cheaper
      • Small
      • Allows high current through
      • Can be used for reverse Jump starting.
    • Cons
      • Charges both batteries from the alternator (even though they are different types of batteries)
      • High in-rush current to flat battery when combiner kicks in.
      • Need a separate solar charger
  • DC to DC Charger
    • Pros
      • Build-in solar function
      • Steps up the voltage to FULLY charge the secondary battery
      • Limits current to 20A to the second battery
    • Cons
      • Expensive
      • Slightly larger
      • Less simple (more to break or go wrong)
      • No reverse jump-start functionality.

For the choice was simple, I was doing this once and I was doing it properly. I had invested a lot of money into getting a really good battery, I had every intention of protecting it and charging it to its maximum capacity. So I went with the DC-DC. This was actually perfect for the size of my setup and the car, and I had all intentions of adding solar to the system as well. If you factor this in, the price is really not bad. I did a lot of research into which charger to buy, and there was one clear winner when it came to reliability & value for money. Its not a fancy name brand that is overpriced, it’s actually a South African company that you have probably never heard of “WRND”. Coming in at half the price of its competitors, I was very sceptical. I hit the forums only to hear AMAZING feedback.

WRND 20A DC to DC Battery Charger with MPPT Solar Input

I ordered the device online, it came in a bland cardboard box with a very simple installation manual. Again I felt rather sceptical. However, the unit works amazingly and I will recommend it to anyone. Ignoring the solar feature the extra R1000.00 bucks of a battery combiner is totally worth it. Once I had all the gear I immediately set out installing the charger.

I found the perfect place to keep it, I decided to mount it in the cab to keep it away from water and dust etc (although it can be mounted in the engine bay). Another factor I had read online was the heat, these units have a safety feature that would automatically dial down the current if they started getting hot, So I thought in the cabin was perfect. I found a perfect little spot for it under the passenger dashboard. There is a bracket there that holds the ECU in place. I removed this bracket and securely riveted the WRND unit to the bracket, and then re-installed it. This was ideal as the unit would be securely mounted, was quite high up from feet and it could use the metal bracket as a heat sink and a decent grounding point to the chassis.

Centre Console

A big part of the whole dual battery project was the custom centre console that I had dreamed up in my head. (You will see it in my wiring diagram). This was the portal/control box to the whole setup. Its where I would control, monitor and have access to all that extra power.

{Centre Console Write Up Comming Soon}


Now with any electrical job, the cabling is always the most difficult and time-consuming thing to do, and to do it well takes 5 times longer. I think running the cables was one of my longest projects on the Jimny ever, I would say over a good 100 man-hours went into this whole dual battery setup. Anyhow as I have done with every build on this car I went completely overboard, being fairly knowledgeable about electronics & electricity I know voltage drop all too well. So with the cable, we went BIG. Unnecessarily big? No, Its only 25mm2 (4awg) cable. But way overkill for the application, after all the charger unit will only supply 20A to the battery. However I am always a fan of future-proofing something, and If I do it once, I want to do it properly.

I then popped online and ordered all the goodies which I was going to need. Cable, fuse boxes, fuses, lugs etc.

Sockets & USB Ports for the centre console
Positive power cable
Some more goodies
12 Way fuse box

Where and how all this fitted in was not an easy task to tackle, but with time and trial and error it eventually all came together. I mounted the larger fuse box under the steering wheel, as this was very central and a lot of the power-consuming accessories are in the dashboard. From there the power cable ran to the centre console where we mounted the smaller fuse box. This was used to provide all the accessories in the console with their own fuse. (see more in the centre console write up)

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