Making a car-based leisure battery system

So we say that we made our car our home, but what did we actually do?

We installed curtains, a backup-battery system for leisure use, a storagesystem and a bed, dismantled the third row jumpseats and sold them, installed new mudflaps, new shocks and adjusted the brakes.

We did all this in a matter of two weeks where we didn’t do much else than drive back and forth between Home Depot, Walmart, Target and Pepboys Auto. We wanted the solution to be done in a hurry, but we also wanted to create something that we’d want to live with throughout the next five months. So that’s what you in Danish would call “whistling while eating crackers” (trying to do two things simultaneously).


What’s a backup battery system?

It’s probably easier to tell what it is, by telling what it is that we want. We want to be able to cool our food, and charge our laptops or phones, even when we’re in the middle of nowhere. We don’t want to rely on solar panels (we want to get them, but we don’t want to solely rely on them). We also do not want to discharge the car’s battery. So what is it that we need?


What we need

There are different ways of  doing it, but what we seem to find the smartest way is building a system  a that does everything automatically. For that we need a Voltage Sensitive Relay (VSR), that connects the car battery with our backup batteries. Of Course I did my homework before starting on this quest. I drew a diagram of how the system ideally would look. This is how.

Once the car battery is fully charged, the VSR allows the backup battery to be charged. That’s pretty neat. If you’re interested, here’s a thorough description of different methods of split charging.
We started our search for a VSR relay in Claremont, Los Angeles. And it was hard to find. Really hard. After two days of searching we found it. The type we found was called “SI-ACR with start isolation” by Blue Sea Systems.


Dimensioning cables

Installing the backup battery system was not the easiest thing that I’ve done. I bought the cables that I thought we needed, but I didn’t think of the simple thing, that we want to be able to use both 12v and 115v, and we’re probably going to want to charge two cellphones, and a laptop while having our fridge on. That means we need at least 150W, which means we need cables that can take 12,5Amps. That’s at least 16mm2 or 6awg as they like to call it in USA. So back into Home Depot, buy some larger cables than first anticipated, and then back to doing the installation. I managed to find some cheap cable that was 21,5mm2. It was a bit more that we needed, but what the hell…


Finding the right batteries

We also didn’t want to use normal car batteries as they take damage from being discharged more than 20%. Instead we wanted deep cycle batteries that can be discharged by 80% without being damaged. Also we wanted them to be sealed in order to not risc our health to the vapours of the batteries.
We managed to find two sealed, deep-cycle, 12V, 35 amphour batteries. That means that we all in all have (35+35)*0,8=56 amp hours at our availability. That’s roughly 16 cell phone charges. That’s more than enough.

I arranged the batteries, the relay and the 12v fuse-slots in a plastic container we bought at Target. I knew that everything would be tossed around as we will be driving over a ton of insane speed bumps and hard terrain in general, so I had to secure it pretty thoroughly.

I made sure to use the right size crimp-on fittings when installing the entire thing. The thing was though, that when they’re to crimped on to a 21,5mm2 cable, you need to apply pressure until your face turns blue 😫


The power inverter

As I’m from Europe, all of my stuff runs on 230V. Taylor’s stuff runs on 115V, and would get a 3rd degree burn if connected to 230V. So I chose a 140W 12V/120V Dewalt power inverter with one 120V outlet and two USB outlets.

Time for the test!

Once it all was installed, I connected my voltmeter to the leisure batteries, to check what status was. I quickly saw, that they were fully charged when we bought them.

That meant we had to discharge them a bit before we would know it the system works, as the relay wouldn’t start charging on already full batteries. So we connected everything we had, and we discharged the battery to 12,6… (which is hard to see in the picture below, sorry) to 12,0… to 11,5… 11,2… and the relay never started charging. What was I doing wrong???

I knew that the relay had three additional slots that I thought all were optional. 1) for a LED that notifies you that it’s charging. 2) you connect to the ignition of the car so the relay knows when a big current is coming. 3) the last one for ground connection. I emailed Blue Sea with a diagram of our system drawn on the back of a pizza box.

Within an hour they mailed me back with what they thought was the problem.

The “optional” ground wire had to be connected. I got the system out of the car, connected the ground-wire, started the car, and then I waited…
I waited for about thirty seconds. And then I heard the most beautiful “click” of a relay breaking in. The system worked. It’s alive!

So does it look like the diagram i drew before we started building it. Is this how it looks? Almost. The battery isolator and the VSR is one part. The 12V fuse box is on the other side of the VSR relay. I installed a fuseholder where the old fusebox is. The 115V and the 12V socket is one part. We don’t have the option of charging the batteries with 115V (yet). And as I installed a 12V fusebox before the inverter, and ad the 12V- and 115V outlets are installed in the same device, there is no need to install a 115V fusebox after it.

That’s it for now. A special thanks to Kevin from Blue Sea Systems for correcting my mistake!

Feel free to add comments or questions if you like!


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