Installing Renogy DC-DC charger in 2018 F-150


Wiffleball Batter
Posting this for anyone who might be interested in doing something similar.

Background: The idea behind this came from my previous vehicle, a 2004 Suburban. When we camp we use an Indel-B TB 41 refrigerator and in the past we've had "issues" with it discharging the battery enough to cause the vehicle to need a jump start (not to mention the possibility of spoiled food.) On the Suburban, the solution was easy: A 2nd battery installed with an isolator to keep the starter battery from discharging. On the 'Burb it was a fairly straightforward process both because of the "old tech" alternator and electrical system, and more importantly because GM trucks of this era already have a space under the hood for a 2nd battery (IMO the most vexing part of a dual battery install is where to physically locate the 2nd battery.)

That setup worked great (using a 68ah AGM battery as the "house battery" for 3 years, but in August of 2019 I bought a new truck, a 2018 F-150.

At first I considered a dual battery setup there as well. Unfortunately there were two factors that worked against that: The first was that there was not a physical space under the hood for the battery. The second and more difficult was that the modern engine has a much more complicated electrical system and my concern is that the battery monitoring system (which is needed for the stop/start technology on the engine) would go wonky if I introduced another variable into it like a 2nd battery running off the alternator and cause other problems with the system.

So instead, I decided to just make a separate (portable) battery box to run the fridge. But before I made one myself, I looked at the market to see if maybe something like what I wanted already existed, and it turns out, it does:

There are several varieties of these and they are all basically glorified battery boxes, but they have nice features like connecting studs on the outside of the box and 12v aux outlets, as well as a primitive volt meter (red/green/amber) to monitor battery life.

I modified mine right off the bat by adding a USB socket with a digital volt meter (which I had been planning on putting in the Suburban but never did) and a Battery Tender SAE harness:

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The SAE harness is so I can keep it plugged into a battery tender when it is at home. The battery tender keeps it "topped off" without overcharging it. I can also plug my solar panels into the SAE socket if I want to charge the box with my solar panel (100w.)

The battery box is made for trolling motors and can accommodate a wide range of battery types and sizes. Currently I am just running an inexpensive FLA 90 ah deep cycle marine/RV battery.

The battery works to keep the fridge running (though it's crazy heavy!) but I also need a way to keep it charged. Initially, I thought that maybe a 120v AC charger plugged into the 400W inverter (which is switched so it's only on with the ignition) would be a simple and easy way to keep the battery charged up. However, a bit of research revealed that this was not feasible because of the cheap nature of the factory inverter. With no factory inverter option, I was back to figuring out how to keep the 12v battery charged another way.

I decided that I'd try one of the new generation of DC-DC chargers that seems to be popping up all over the market. The DC-DC charger seems to promise a way to keep a good charge going to the aux battery without adding another variable to the truck's electrical system. Although I considered the Redarc 50a unit, I ultimately decided on the Renogy 20a charger both because it was less expensive, but also because at "only" 20a I would not have to run such massive wires that a 50a would likely need to operate safely. This was especially important because I plan to keep the charger in the cab of the truck.

So, now that I had a general idea of what I wanted to do, I now had to come up with a plan:


Wiffleball Batter
Part 2: The Plan:

I already knew where the charger would sit. The F-150 has a nice little storage tray under the back seat and the Renogy charger fits nicely in that space. I'll likely put some velcro on the bottom to keep it from sliding around but I don't see the need to screw it into the body work.

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Since the aux battery and the fridge both sit in the cab, this was the easiest solution for me.

The part of the plan I struggled with the most was simply this: How to connect the DC-DC charger to the main battery? If the charger was going to be in the bed of the truck, it'd be simple, but I really wanted it in the cab because that's where our fridge sits. I spent a lot of time looking at the firewall of the truck to try and see if there was a way I could run a wire through the firewall and into the cab, then along a door sill panel to the back. But I was not able to find anything that did not present other problems (like stringing wires across the engine compartment or behind the brake pedal.)

After a lot of looking and measuring, I came to the conclusion that the simplest and safest way to do it was to drill a hole in the floor of the cab behind the back seat. I hated to do it, the thought of drilling a hole into my nice new truck, but every other choice just seemed to bring too many other issues with it.

After much thought, I finally selected a location: On the passenger side, just behind the trailing edge of the back door, up under the floor. This is the view from underneath the truck looking up (the passenger side frame rail is at the bottom of the photo.)

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Here is what it looks like from the cab, looking downwards (you can see the running board on the left):

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This isn't the actual wire, BTW, it's just there to show where the wires will be. (As you can see by the picture that hole is just big enough for ONE 8g wire - I need to get 2 8g wires and a 16g wire through so the final hole might be 3/4" diameter.

(I'll also seal it well with silicon sealer.)

Putting the hole on the passenger side means that I can run the wiring from the battery (which is also on the passenger side, near the firewall) straight down and then along the top of the frame rail to the rear of the cab.

With 20a of power, using a chart I figured that 8AWG would give me a nice safety margin (I could probably have gotten away with 10AWG since the distance is probably going to be less than 9' but I think 8AWG will be safer.) I'm also going to run a 16g wire from the DC-DC charger back to the fuse block for switching. I'll run it to a fuse that is only "hot" when the ignition is on, and that way it will act as an isolator to isolate the aux battery from the starter battery when the ignition is off.

Of course, all wires will be enclosed in wire loom.

Anyway, that's where I've got to so far. Wire, connectors, rubber grommets, a nice crimper, and some other odds-and-ends are on their way to me from Amazon now, so hopefully by the time the COVID-19 "lockdown" is over, I'll have a functioning DC-DC charger. I will, of course, post pictures as this progresses.


Wiffleball Batter
Oh, I forgot to add that I will be charging the battery box with a short (~2') section of 8g dual cable going through a pair of Anderson connectors that I bought for another project and never used (this seems to happen a lot with me but at least then I have the parts I need for other stuff!)


Wiffleball Batter
You are going to put a grommet on that pass-through right? Otherwise the sharp sheetmetal will ground out the wire.
Called it! :D

(See the bolded and underlined text in my original post.)

That's just the pilot hole. The final hole will probably be 3/4". I have to get 2 x 8g and 1 x 16g wire through it.


Engineer In Residence
No worries, I actually did look for that line, but must have missed it on my phone. I even searched in page for "grommet" before posting. Meh.
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You are going to love this Renogy DC to DC charger. I have the 40a version that I have connected to Anderson plugs so I can swap it between my SUV and my truck. Works great to keep aux battery for the refrigerator charged. I don't bother carrying a generator any longer. 120w of portable solar and the Renogy is all I need.


Engineer In Residence
Is there a gap between the set bottom and that trim piece? If its decent sized, you may not need vent space. Otherwise its gonna get toasty under there I predict.

You can probably expect 50-100W of heat generation in the unit.


Well-known member
It has over-temp protection, but avoiding that much heat would be better regardless. With a small gap at the top, a similar space at the bottom might offer enough convection combined with the Renogy fans to get the job done.


Thanks for adding another project to my list.

Using what you have done as inspiration I’m thinking of adding a battery box in my bed.

But I want it to be portable.

When I go on a trip I want to be able to install it and when not I want to be able to pull it out.

So I need some type of recessed outlet in the bed to plug the Renolgy into.

So I am thinking I run a wire from the battery underneath to the bed to a switch. And then plug the Renolgy into the switch when it is being used.

And then use the battery in the bed to run a fridge.

You have given me some ideas.

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Wiffleball Batter
Is there a gap between the set bottom and that trim piece? If its decent sized, you may not need vent space. Otherwise its gonna get toasty under there I predict.

You can probably expect 50-100W of heat generation in the unit.
It has over-temp protection, but avoiding that much heat would be better regardless. With a small gap at the top, a similar space at the bottom might offer enough convection combined with the Renogy fans to get the job done.
I actually don't think that's going to be an issue and here's why: When the charger is being used, it means the fridge is in the back seat.

And if the fridge is in the back seat, then the seat has to be up (otherwise it won't fit.)

I can't envision any circumstance where the charger would be in use and the seat folded down. My plan is that when I'm not on a road trip where the charger is needed, I'll just pull the 30a fuse from the battery that supplies power to it and keep the fuse in a zip-loc bag in the glove compartment.


You should think about a grommet where that wire passes through the metal. Lol

Which fridge do you have? About how long does the 90ah battery run it?

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Wiffleball Batter
You should think about a grommet where that wire passes through the metal. Lol
Grommets are for sissies. I like to live on the edge! :LOL:

Which fridge do you have? About how long does the 90ah battery run it?
It's an Indel-B TB-41 A/K/A TruckFridge TF-41. Purchased in June of 2014 and still going strong 6 years and 100+ camping trips later. Has been everywhere from Colorado to Ontario to New York to Washington State, California and the Mexican border at Big Bend.

As for how long it lasts - not sure. I've only been using the 90ah battery box since September of last year right after I got the F-150. My previous setup had a 68ah AGM battery for the house battery (limited to 68ah because of size restrictions on my old Suburban) but it was being recharged by the 145a alternator so I never ran out of power on it, even when parked for 2 - 3 days without ever starting the truck.

My hope is that I never have to find out how long it will last because I'll be able to keep the battery charged up - that's the whole idea here.

Even though this setup has a 90ah battery, I'll have to watch the voltage level because with the DC-DC charger I'm only going to be able to put 20a of charge into it vs. 145. I can also charge it with a 100W solar panel if needed (the SAE harness will plug into my solar panel) but that's at most ~5 - 6a of charge on a nice, sunny day. The solar panel is normally connected to the trailer (2 x 6v golf cart batteries, 230ah capacity total) to keep that charged up but if needed I can switch it to the battery box.

Eventually I will likely switch to either an AGM battery or a Li-Ion if the prices on those come down from the stratospheric levels they are now.

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