Sprinter van: Electrical system

Installing the bench seat for Sefton was the most intimidating part of our Sprinter van conversion. Planning, designing, and building the electrical system was by far the most complicated part.

It took about a month-and-a-half and I'm not sure how many hundreds of hours. Late nights, mostly. This will mainly be about the installation; less about the design and planning. From a really high-level, our system--for now--includes the following devices:

• Renogy 40A DC-DC battery charger: I went with the 40A over the 20A after A LOT of research and thought, figuring the slightly larger (but not a 60A!) charger wouldn't cause significantly more harm to our 115A Mercedes alternator and the risk was worth the reward of always charging our house battery bank that much faster. Only time will tell (as is the case for this entire system, in fact) if I made the right choice…
• Windy Nation 12V 100Ah AGM deep-cycle battery (x2): Wired in parallel for 200Ah total system power. Based on my load calculations for all of our electrical consumption, that should give us about 3 days off-the-grid before having to drive enough to recharge via the alternator.
• Renogy 1000W pure-sine inverter: Part of our power calculations included what we'd need AC power to run, which we discovered wasn't much (charging our laptops, phones, and camera batteries is more efficient with 12V chargers). In fact, only two appliances required AC: our Nespresso Aeroccino milk frother and our collapsible tea kettle for boiling water. Yes, both of which are for coffee. Priorities.

Seems simple enough. If I had experience working with 12V power systems, it certainly would be. Except I don't. So it wasn't simple. Worth noting: Up for consideration down the road are both solar and shore power connections. We'll see how it goes based on our upcoming 5-week road trip.

Much like the risk involved installing a bench seat where our son would be sitting, designing an electrical system carried significant risks. Top of mind: burning our van to the ground. Unlike AC power, with which I'm familiar having done a fair amount of wiring on our old house and even some in our new house, DC is completely different. The biggest difference: since the voltage is so low, the amps are significantly higher. That means wire gauge has to be calculated very accurately for each and every load, rather than just stringing 12-gauge (or 14-gauge for 15A circuits) throughout an AC system.

I'll spare this post all of those details. For now, I'll just run through some of our installation process. Let's start under the hood and work backward from there.

Our Sprinter was a cargo van. That's important for our electrical system because the PDC (Power Distribution Center) mounted to the starter battery's positive terminal had an empty slot. Good ol' #4, which in the passenger version would have been the circuit for the rear air conditioning unit. Since it was empty, I could tap into it.

I had to figure out, however, what the heck kind of fuse fit into it. I did mention I'm not a DC system expert, right? Thanks to Sprinter-Source, I discovered it's called a MIDI fuse. After ordering one and it arriving, I was able to test it.

Boom. We were in business! Of course, I had to get 4-gauge cable from the engine compartment to the driver's side rear wheel well. Which involved (yay!) lowering the fuel tank. Again. Albeit just a bit, and most importantly, not entirely off the hanging brackets. Then fishing cable between it and the underside of the van's frame.

It also involved drilling more holes into our van in order to thread the cables up through the flooring and inside. As I mentioned when we installed the new stereo, the DC-DC charger also required a D+ terminal connection in order to power itself. So I ran an 18-gauge cable I had leftover from an earlier car stereo installation from under the driver's seat back to the rear wheel where all of our electrical components would be installed.

A quick test without the engine running (no power):

And with the engine running (power!):

With the cables for the charger in place, it was time to run all the cable for our various loads. This included circuits for our lights, chargers, fridge, water pump, fan, and eventual propane heater.

That's right, I threw together a little spool thing-a-ma-jig. To keep the cable runs inside the to-be walls, I also had to drill more holes (and file and paint them).

The longest runs were up to the passenger seat for some chargers and where we'll install the heater.

We won't be short on USB chargers! Throughout the process, the van looked pretty messy. Good times.

We did a lot of work in the evenings and at night once Squish was asleep. Sometimes I'd stop and catch a pretty cool sunset… 

Then get right back to work. Since we were doing electrical, we've also been chasing down a gremlin that has left the driver's side blinker inoperable. It's likely a grounding issue, but to rule out the obvious things first we replaced the multi-switch. After all, maybe there was a short somewhere in the switch.

Nope. Bummer, a new one didn't fix the problem, so I guess we now have a spare.

With all of the load cables strung, it was time to connect everything in the back before returning to the engine compartment for the final connections. Oh, I also had to build a battery box and the wheel covers in order to mount all of the components.

I also made a grounding point on the rear wheel well by grinding down the paint around an existing 1/4" hole.

Under the hood again, it would soon be the moment of truth. I took a deep breath before cutting the 4-gauge cable to the exact length I needed it to be. I knew after that cut there was no going back. At least not without buying another 20' of expensive cable and a couple of hours reinstalling it.

Oh, backing up a smidge… I first had to grind down the 4-gauge ring terminal because it wouldn't fit in the PDC slot.

For the negative return, against Renogy's recommendations and after much research, I opted to connect the cable to the vehicle chassis. Namely, I learned that wiring the negative return to the negative starter battery post is not recommended. In fact, that linked article starts off by stating in a big yellow box: The only connection to a battery post negative should be to another battery negative, the vehicle chassis, and/or the engine block. There should never be a direct negative post path to accessory equipment that has any sort of ground path to external devices. Seemingly in accordance with that, Mercedes doesn't make it possible with the negative battery post terminal connection. I'd have to order something that would allow for an accessory attachment. 

Without going into DC theory (much of which I wouldn't likely understand anyway), I'm confident that grounding the charger's negative return path to the chassis is correct. Again, only time will tell… For what it's worth, I did run my diagram and that idea past my friend Matt and his DC-electrically-inclined brother, Mark. They confirmed it was fine.

With the round trip from starting at the battery to ending at the battery, and the month-and-a-half and hundreds of hours in between, it was time for The Great Moment Of Truth.

Thankfully (oh so thankfully!), it ended well. Everything worked as it should. We tested the Maxxfan. It worked. Yes, thankfully everything worked. Now that's finished, we're hoping the rest of our build goes much quicker as we enter September and the final few weeks we have to get everything ready for our big trip.

Next up: Walls and ceiling…