RV Electrical Upgrade: Part 3 – DC Charger & Truck 12V Socket

Sun Dec 02 2018

With the Battle Born Lithium batteries and Victron MultiPlus inverter installed in our trailer, we turn our attention to how we're going to charge the batteries whilst we're off-grid. Although we do plan to install solar panels, they rely on clear sunny skies. But what about the rest of the time? Can we charge our batteries from the truck? You bet we can!

RV Electrical Upgrade: Part 3 – DC Charger & Truck 12V Socket

Continuing in our RV Electrical Upgrade series, in this post we'll be increasing the charging capabilities. If you haven't already done so, make sure to check out Part 1 where we introduce our goals for the upgrade, and Part 2 where we install the Battle Born Lithium batteries and Victron inverter.


Whenever you connect your trailer to your tow vehicle with a 7-pin connector (the standard, round connector that most trailers use), one of those pins carries 12V power to the trailer. The minimum recommended wire gauge for the power pin is 12 AWG, which is rated for 20 Amps. If you look in your vehicle's fuse box, you'll probably find this circuit protected by a 20 Amp fuse.

So does that mean your tow vehicle can charge your trailer batteries at 20 Amps? No! Rarely, if ever, will that be the case.

Our 2016 F-150 is a great example of where you get nowhere near 20 Amps. It has a so-called "smart alternator" - essentially it reduces its output to maintain the starter battery State of Charge (SoC) lower than normal to allow regenerative charging during braking. The result of this is you can expect to see no more than 4-5 Amps on the 12V pin of the trailer connector. For reference, at 5 Amps of charging current, that would take 60 hours to charge our batteries, even at 100% efficiency. Erm, nope!

But all is not lost, we just need to be a little cleverer. Fortunately, this is not an unsolved problem - far from it! In fact, in Australia they've long been working around limitations such as these to power fridges and charge batteries in the back of their vehicles and their trailers. Time to learn some lessons from down-under!

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Goals of Part 3

By the end of Part 3 in our series, we'd like to have a mechanism to efficiently charge our batteries from our truck whenever we're hooked up. Not only will this let us charge whilst towing, but also allow us to use the truck as an "emergency generator" if our batteries get too low whilst boondocking and we need a bit more juice.

We want to do all this without damaging anything in our truck, or placing undue stress on the truck's engine and related systems. That being said, this is a guide for what worked for our truck - you may need to research whether this would work for you too.

How To Charge While Towing

If we can't just rely on the 12V from the 7-pin connector to charge our trailer batteries, what can we do instead? The answer is to use a device known as a DC-DC Charger or a Battery-to-Battery (B2B) Charger. Unlike a typical battery charger that uses AC power (i.e. you plug it into a normal socket or shore power) to charge a battery, a DC-DC Charger takes power from one DC source (i.e. a battery) to charge a battery. In this case we can use a DC-DC Charger to draw power from our truck battery (which is kept charged by the alternator, powered by the engine) and charge our trailer batteries.

After lots of research, I came across a company called REDARC - they're well-known in Australian overlanding groups for producing high quality components, but they're a relative unknown here in the US. I had the opportunity to meet REDARC's VP of Sales for North America, Steven Haubenschild, at Overland Bound WEST this year, and had a great conversation with him about this project.

Note: We are not affiliated with REDARC in any way, and purchased their equipment because we believed it was the most suitable for our needs. We paid full retail price.

We chose to use the REDARC BCDC1240D - a DC-DC charger rated for 40 Amps. That's almost 10x what we'd have been getting from the standard 7-pin connector - at the equivalent of around 550W of charging power, it would take just 7.5 hours to completely charge our battery bank from 0-100%. Still not quick, but much better than 60 hours! The REDARC charger also has a built-in charging profile for Lithium batteries like ours.

REDARC BCDC1240D DC to DC Charger Review

Installing a DC to DC charger in our RV is probably the most unusual but best upgrade we've ever done!

REDARC BCDC1240D DC to DC Charger Review

The next question is how to get 40 Amps to the trailer - the 7-pin connector clearly isn't sufficient. Again, looking to Australia for inspiration, I settled on the Anderson connector. This is a rugged DC connector, suitable for use in wet and dirty conditions - it's self-cleaning contacts working to keep a good connection. Anderson make a model known as the SB50 (rated to 120 Amps, despite the name) but I chose to use the SB175 - a 340 Amp rated beast!

I chose this for a few reasons:

  1. The SB175 is physically larger than the SB50, and I'd read online a few people saying that its larger size makes it easier to connect and disconnect, especially when wet.
  2. The terminals for the SB50 only accept wire up to 6 AWG, whereas the SB175 accepts up to 2/0 AWG wires. Although 6 AWG has a rated ampacity of 55 Amps, the voltage drop along the 25ft length of the truck would be around 6.5%. As a rule, I use 3% as my limit (based on NEC and AYBC guidelines), and to achieve this I'd have to step up to 2 AWG wire.
  3. It's possible to operate the DC-DC chargers in parallel - up to 4 of them in fact - increasing the charging power. Although that's not something we need right now, I wanted to leave that option open for future - 80 Amps is definitely more than I'd want to put through 6 AWG cable!

One of the great things about Anderson connectors is that they're not gendered - i.e. there's no difference between a plug and a socket. There is, however, a difference between the colors - with very few exceptions, you can only join two connectors of the same color. For this project I chose red because, well, I like red. You may pick your own favorite color if you'd like.

Anderson SB175
These SB175 Anderson connectors are seriously heavyweight - perfect for this task!

The last part is making sure we don't deplete the truck battery itself. Our Ford F-150 came with a whopping 210 Amp alternator (that's the biggest one they offer, and it was pure chance we had it since I didn't ask for it when I bought the truck), so as long as the engine's running I'm not too worried about the current draw. To double-check, I actually connected to the OBD-II port on the truck and monitored the PID corresponding to available current from the alternator - even when sitting at idle, there's at least 80 Amps of capacity available.

But what about when the engine is off? We'll install an ignition-controlled continuous-duty solenoid so that the power is only available when the ignition is on - perfectly fine for our use case.

To make the design above a reality, here's what we need to do:

  1. Install an Anderson SB175 connector on the back of the truck, connected with 2AWG cable via an ignition-controlled continuous duty-solenoid (and a fuse) to the truck's battery terminals;
  2. Install a REDARC BCDC1240D DC-DC Charger in the trailer, with the input coming from a cable we can plug into the connector on the truck and the output going into our batteries.

It sounds complicated, but the install itself is fairly straightforward. We've also included links to the products we used to help you plan your own install.

Parts List

Here are the main items we used for the install. Other than the parts listed, the only other items you may need are some screws, bolts, nuts and washers for attaching parts to the vehicle and creating a ground lug if necessary. We already had some ring terminals but if you don't have those you'll want to pick some up. You'll also want a good pair of cable cutters and a hydraulic wire crimper.

REDARC BCDC1240D1Check Price
Anderson SB175 - 2 AWG, Red (SB175-02-RED)1$10.44 @ Powerwerx
Anderson SB175 - 6 AWG, Red (SB175-06-RED)1$11.50 @ Powerwerx
Dust Cover for SB1751$3.99 @ Powerwerx
Corrosion Proof Safety Boot for SB1751$4.49 @ Powerwerx
Trailer Vision Anderson SB175 Weather Proof Cover1eBay
Cole Hersee 24213 12V 200A Continuous Solenoid1Check Price
Add-A-Circuit Fuse Tap1Check Price
Blue Sea Systems MEGA/AMG Safety Fuse Block1Check Price
Blue Sea Systems 150A MEGA/AMG Fuse1Check Price
Blue Sea Systems AMI/MIDI Safety Fuse Block2Check Price
Blue Sea Systems 60A AMI/MIDI Fuse2Check Price
TEMCo Welding Cable - 2 AWG Black50ftCheck Price
TEMCo Welding Cable - 6 AWG Black25ftCheck Price


I bought this directly from REDARC as I found it hard to source elsewhere. It's worth noting that they also do a 25 Amp version, so if you don't need (or your vehicle can't support) 40 Amps of charging, then the 25 Amp version may be more suitable. The charger is a completely sealed unit internally (it's not IP-rated but the circuitry inside is encased in resin) so it can be installed outside if you want. The unit has pigtail wires attached, and comes with some butt-connectors for joining into existing wiring.

Anderson Connectors

Anderson components can be a little tricky to find. Not only are there lots of permutations (you have to get the right color and the right sized terminals), but there are also lower-quality clones on the market. To be safe, I ordered all of the Anderson equipment from a company called Powerwerx - you know you're getting the right stuff then.

The weatherproof socket was the hardest item to source by far. It's not strictly necessary (and honestly, didn't stop the connector filling with mud at Parks Ranch Campground), but I used it to make the install look tidier. After fruitless attempts to find a retailer who would ship to the US (it's only sold in Australia), I ended up buying it through eBay and waiting for it to arrive all the way from Australia.

Wiring & Terminals

As with the rest of the install, I'd recommend buying wires from TEMCo - their copper wires are high-strand-count so they're super flexible and should be durable enough for this install. To be safe, I used split loom to protect all the wire on the truck, as well as the exposed wire on the trailer tongue.

We used 2 AWG wiring for the truck to reduce loss, and switched to 6 AWG on the trailer where flexibility in the wires is more important.


The continuous duty solenoid I chose for this install is undeniably overkill. As long as you don't have plans to go beyond one 40 Amp charger, then an automotive relay would be plenty sufficient.

That being said, I wanted an ignition-switched circuit that would give us plenty of power for this and any other future applications, so I chose a solenoid that was rated to far higher than we needed right now.

To switch the solenoid on, I used an "Add-A-Circuit" fuse tap to connect into the adjacent fuse box in the engine bay. I found an existing fuse that was only active when the ignition is turned on and used that to turn on the solenoid.


The purpose of a fuse in a circuit is not just to protect your devices, but also the wiring. It's therefore important to place a fuse next to (as close as possible on the positive side) each source of power, and make sure that the fuse is rated appropriately.

In this install, we used 3 fuses. The first, a 150 Amp fuse, is placed right next to the truck battery. There are then two more fuses, 60 Amps each, adjacent to the DC-DC charger - one on the input to protect the charger itself, and one on the output to protect downstream wiring and components in case the charger should fail and surge.

I used physical fuses and fuse blocks, but if I did this install again I'd probably use breakers - they're resettable and take up less space. Given our space constraints, I might swap to these in future.


To help structure this install, we'll split it into two sections - the 12V socket on the truck, and the DC-DC charger in the trailer.

Once again, Diana and I did this entire install ourselves - no external help in designing or building any of it. We're really pleased with the outcome, and it's working great for us so far!

Disclaimer: Mistakes in the electrical system can cause fire, injury or even death. If you have any doubts about working with electricity in your RV, consult with a professional electrician.

Truck 12V Socket

Much like we connected the negative (ground) the RV's electrical system to the RV chassis in Part 2 of this series, in most vehicles the negative side of the vehicle's electrical system is tied to its chassis too. On our truck, it's not recommended to connect directly to the negative battery terminal and instead use a frame grounding point nearby. Alternatively, you can use any other point on the frame as long as it's electrically well-connected to the chassis - i.e. don't use a door panel. You also want to make sure that the factory cable connecting the frame to the battery terminal is capable of handling the load. In our case the existing cable was fine and we can use the frame rails (the thick steel beams that run the length of the vehicle) to ground our connection to. This is a technique known as ground chassis return.

To make sure we have a high quality ground connection, I created a new ground lug at the back of the truck, rather than rely on any existing bolts, etc. I drilled a hole through a thick piece of steel welded onto the frame rail and used a drill with a wire brush to clear the paint away around it.

Drilling Hole
Once you've drilled the hole, it's important to remove the paint around it so that you can get a good electrical contact.

Using a bolt, some washers, lock washers and a couple nuts I created a really secure ground lug. I sprayed the whole area with some CRC Battery Terminal Protector afterwards to prevent any corrosion.

Ground Lug
This ground lug will give us a really secure ground connection - and we can reuse it in future to ground other things if we need.

Moving our attention to the positive side of the circuit, let's start in the engine bay. The first thing I did was mount the fuse block to hold the 150 Amp fuse. The engine bay of the F-150 is actually really tight on space, so I mounted it on the side of the battery box - as before, installing the fuse will be the very last thing I do.

Solenoid Install
The blue tape shows the location I chose for installing the solenoid. I think I should probably clean the engine bay though!

I found a spot near the battery to install the 200 Amp Continuous Duty Solenoid that would provide ignition-control of the charging system. Using some blue painters tape to mark the location, I drilled and tapped some holes. Since I couldn't reach the back side to use a nut and bolt, I tapped a screw thread directly into the holes.

The solenoid installed securely in position.

Adventurous Tip: The hardened steel in the truck chassis is brutal on taps - I broke two. In the end, I found the DeWalt drill taps to be the best solution, and make sure you use loads of lubrication to keep the area cool.

As close as I had managed to mount the solenoid to the fuse box, it was still too far for the Add-A-Circuit, so I spliced in a short length of some spare wire to extend it, and wired the solenoid up to the ignition-controlled circuit.

Tapping into an existing fuse to trigger the solenoid to switch on only when the ignition is on.

The last thing to do is build some big cables and connect everything up. I cut the first 2 cables (battery-fuse and fuse-solenoid), crimped the connectors, covered them in adhesive-lined heat shrink and encased them in split-loom, then connected them in place.

Battery Cables
These cables are super strong and are more than capable of carrying the current we need for this project.

For the long cable that runs from the solenoid to the back bumper, I left the cable long, put it in the split loom and pulled it through. I was able to follow the existing wiring loom of the truck, along the chassis frame rail on the driver side. I took my time, and used a lot of cable ties, but ended with a nice clean cable run. Once it was in position I was able to cut and crimp the cable at the engine-end.

Anderson Connector
Here's the Anderson connector that will be attached to the back of the truck, with the ground cable to connect to the frame already installed.

At the other end, I put the Anderson terminal on the end, then placed it into the Anderson connector along with a short cable I built to join the negative side to the ground lug I fitted earlier. The last thing to do was install the weatherproof socket and screw in the Anderson connector. The result is a really secure, clean install that provides 12V to the back of the truck.

Weatherproof Socket
Here's the final result - a high-current 12V socket on the back of the truck, ready to connect to our trailer.

Once it's all done, you can install the 150 Amp fuse in the fuse block in the engine bay and check you see 12V (actually, it'll be more like 13-14V) on the Anderson connector when the engine is running.

DC-DC Charger

With the truck wiring done, it's time to turn out attention to the trailer. Fortunately, this is much easier.

The instructions that come with the REDARC BCDC1240D are great and include really clear wiring diagrams. We mounted the charger in our electronics bay under the bed inside the trailer - next to our batteries, charger and other electrical components.

The charger has a pigtail with 7 colored wires in - let's deal with the simple ones first:

  • Green / Orange - joining these two wires together is what sets the charger to use the Lithium charging profile, so I connected these;
  • Yellow - this is for connecting to a solar panel, which we're not doing (yet);
  • Black - this is ground, so I put a ring terminal on this and connected it into the ground bus bar with 6 AWG cable;
  • Blue - we pull this high (i.e. connect to 12V) to set the Low Voltage triggers - this essentially disables the standard triggering which we don't need because we installed our own solenoid.

This just leaves two wires - Red and Brown.

The Red wire is the positive input to the charger, and this needs to connect to the 12V feed on the truck using the Anderson connector we just installed. We also need to install a fuse. I mounted a fuse block (for a 60 Amp fuse) next to the charger, then built a cable to connect the Red wire to one side of the fuse block. We'll deal with the other side of this in a minute.

The Brown wire is the positive output from the charger. Again, running this through another fuse block for protection, I connected this to the +12V side of the electrical system.

By this point we had everything connected together except for the cable that would plug into the truck. I again used 6 AWG cable for this - more than sufficient for the current we would be using and a little easier to work with than the thicker 2 AWG I used on the truck side. We also used lots of electrical tape to hold the two wires together, then wrapped both of them in split loom and taped that up too. Don't forget to put the dust cover and protective boot on too.

Trailer Cable
Wrapping up the positive and negative wires that will plug into the connector on the back of the truck.

The biggest thing to watch here is the length of the cable you create to run out to the tongue - too long and it might drag on the road when you're towing, and too short and it might snag as you turn. To be safe, I measured it to be exactly the same length as the existing 7-pin connector cable.

Finished Anderson Connector
The completed Anderson connector on the trailer-side.

Both the positive and negative wires from this Anderson connector run back into the electrical bay under our bed. The positive wire connects to the other side of the fuse block that we connected the Red pigtail wire to earlier. The negative wire connects to the ground bus bar.

Completed charger install
Here you can see the REDARC charger, just above the red switch in the center; the two fuse blocks are mounted to the left of it.

That's everything! When the Anderson connector on the trailer is connected to its counterpart on the truck and the engine is running, we have 12V running from the truck alternator into the input of the charger, which in turn charges out batteries.


Overall, we've been really pleased with the performance of the charger since we installed it. Whenever we're towing, we recharge the batteries at a rate of about 12% per hour - a huge improvement beyond the charging power of the 7-pin connector. From our phones, we can connect to the BMV-712 battery monitor by Bluetooth and check the battery charging status whilst towing too.

One thing to be aware of with the REDARC charger is that because it's a resin-filled unit, it struggled to dissipate heat effectively. REDARC technical support have been great - I've had an email and phone exchange with one of their engineers to investigate options. The unit works best when mounted on metal - not on our plywood sheet under the bed. At full charging power, the unit was heating up to around 135F and after an hour or so we were seeing the power reduce - a built-in throttling mechanism to prevent overheating.

Charger Heating Up
135F isn't going to cause a fire, but is warm enough for the unit to restrict the charging power.

Our solution was to stick a small 12V computer fan on top of the unit - this brought the operating temperature under full load down to about 95F - well below the limit at which it starts to reduce power.

Given the unit is completely waterproof, if I did this install again I'd probably mount the charger to the RV chassis near the tongue - the chassis would be an excellent heat sink for it. We'll see how we get on with the fan, but we might move the unit anyway.


Connecting our truck to the trailer with an Anderson connector to power a high-current DC-DC charger is undeniably one of the most interesting aspects of our RV electrical upgrade. It's not something I've seen anyone else do on their RV in the US, although we've taken a lot of inspiration from existing implementations in Australia.

Overall, we're more than happy with the setup. Not only is it great to charge the batteries when we're on the move (if our batteries are full, we turn on the inverter and run the fridge off AC rather than propane whilst towing), but it also lets us use the truck as an "emergency generator" if our batteries need a bit of a boost. I've been keeping a close eye on the truck and have seen no evidence of any problems.

Thinking back to the goals we laid out in Part 1 of this series, we wanted an electrical system that would let us get off-grid to boondock in some amazing locations as we travel between the National Parks. Without a doubt, this upgrade is enabling that goal.

Would you add a charger like this to your RV? Leave a comment below and let us know!

Make sure to check out Part 4 in this series as we install a Progressive Industries EMS-HW30C electrical and surge protector, and connect up the AC and DC parts of our system to the RV.

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