DIY 6x12 Cargo Camper with Solar

May 25, 2017
Hey folks! Here is one of my most ambitious projects for a long time. Converting a cargo trailer into a camping trailer. This will be semi off grid, so both solar and grid power. Oh, for my metric friends, this is a build in the US, so it's a 6 feet by 12 feet trailer and 120VAC at 60Hz grid connection. I've got reasons for wanting to do this instead of just buying a camper.

- Customization
- Lower cost
- Sounds like fun

So here is a floor plan and wiring diagram. Keep in mind these are works in progress, and will probably change over time. Gotta start somewhere though.

Anyway, I thought you all might be interested. :)
Nice drawing - it really does help to lay things out before doing the actual hookup.

One thing that jumps out at me is the green boxes at the top-left that say "Solar Panel 40a12v". I don't know of any panel that produces 40a, so I'm curious as to what you intend :)
Looks like a big project :) Nice detailed plan. I like your plan to have the core of the system 24vdc. I made the same choice for my off grid cabin. Some thoughts relating to the 24v pieces. It might be a typo but why would you use 12vdc panels to charge a 24v battery? It is easy to get higher voltage panels and many mppt charge controllers can accept 1-3 standard 44 short circuit voltage panels in series and can charge 24v. On the other side of the system most of the appliances you list have 24v versions which would help with wire voltage drop energy loss and simplify the system. For my cabin I have all the dc wires with 24vdc and for the few uses that need other voltages I have usb c power delivery that runs off the 24v and can make many lower voltages with cheap sink boards. For most connections I don't want to be wired in I have Anderson PP 45A connectors. You can also get dc(12v or 24v) fridges which would reduce energy loss from the inverter. In my cabin I have an inverter 6kw that also works as the charge controller when I need to top the batteries off in winter.

On a different note you may want a switch that only has the alternator charge when you need it. Ideally most charging will be done by the solar and might as well not waste the engine resistance using the alternator to charge when you don't need it. By my math 180A 8s battery would be about 5kwh not 1229wh. depending on how much sun you get and how long you want to be unplugged that seams like enough for everything except the air conditioner. In my cabin we are currently using about 5kwh per day without an air conditioner but using my high powered desktop computer and electric induction cooking.

Have fun and I am sure we would all love to hear how the project progresses.
Thanks guys!
The two solar panel banks would be 5 parallel 100W panels, so 41.7A. I just rounded down to 40A. I figure I'm not likely to get 100% power from the panels anyway. And I found some that will fit 10 on my roof. My solar charge controllers are boost converters and can only do 600W each, so putting them in two groups will keep each controller well below it's limit.

All the DC appliances I've seen are way to expensive for me. But I will keep things accessible for future upgrades and repairs.

The manual off switch from the truck is a good idea. In fact, I've already designed how the power will come from the truck:
Still a work in progress. I don't know the length of wire I'll need or what gauge to help limit voltage drop. But the power will only be available if the truck is running, and if I have the switch (which will be on the dashboard) turned on. I also don't know how many Amps I want. Right now I'm guessing 5A wouldn't be too hard on the alternator. Probably won't charge the battery at all, but will send a little power to help run the fridge.

The cells I'm looking at are 60Ah, LiFePO4, but the spec sheet recommends only discharging to 80%. Or maybe they mean 80% discharge would be the bottom voltage of 2.5v. I'm not sure, so I'm assuming only 48Ah to be safe. At 3.2V nominal they would be 153.6Wh each, and 8 of them in any configuration would be 1228.8Wh. These cells are cheap enough that I could buy another 8 in the future and make an 8s2p battery and double the Wh, if I find I need it.

One guy just made a trailer with a 600Wh battery, and runs his A/C and a fridge and a stereo. I have no idea how much power stereos use. He says his solar powers most everything, and the battery only discharges very slowly. He's in Florida though, and I seriously doubt I'll get that much sun here in Washington, LOL.

A lot of the time, we should have access to grid power, but I'd like the option to have a day or 2 without it so we're not totally dependent. In that case, I'd probably use the A/C very sparingly, and we'd have a small generator if things got desperate.

The USB ports could run on as high as 30V, so they could be run directly off the battery like my CPAP will be. Depends on how I do the switching. I designed this console thing to go above the bed. A couple of reading lights, USBs for phones and whatever, 12V socket for my wife because she has this tablet thing for work that doesn't work on normal USB ports. It has to have its own proprietary thing. And a 24V socket for my CPAP.

I'm also trying to keep as much of the electrical stuff centralized as I can. A lot of it will fit in this cabinet:

All the AC stuff will be in the cabinet except for an outlet in the kitchen for the microwave. Everything is still estimated though, and anything could change. I need to get hold of an actual trailer before I can start to be sure of anything. I'm hoping to hear from a company tomorrow...
Company says it can get one built in 8-12 weeks. And I've got 2/3 of the money saved up already! Looks like this project may actually happen. :)
Here's some detail I made of the front and back.
Nothing is to scale yet, except the microwave.

I've also started thinking about how to install the air conditioner.
Since the unit will be in a cabinet instead of hanging out a window, it will need air flow, and drainage for condensation. Still guessing on the actual cabinet size, but what other people have done is separate the hot exhaust from the cool air intake on the sides and top. I figure I'll paint and caulk the inside of the cabinet with outdoor rated stuff, and install a drain in the a/c pan, and one on the bottom of the cabinet, and route those down to the floor. The vent I have in mind is one that can be opened, so I'm thinking of putting in an insert to block the vent when the a/c is not in use to help keep weather out.
Slightly related to the trailer project is a custom panel I'm building for my truck, to replace the old AM/FM Cassette player. When it's done, I'll have a switch to manually shut off accessory power to the trailer. Also, it will only work if the ignition switch is on. So it can't drain my battery if I walk away with the keys and forget to shut it off.

I found a small Blue Tooth amplifier so I can get audio from my cell phone to the truck's speakers, and divided up the various wires from the old player to use for the new stuff.

I made a cardboard mock-up of the panel so I could see how all the things would fit. Or if they would fit. Then I started making permanent connectors.

Today I made the actual wood panel and fit all the parts on it, and got it fit to the bezel.

The clock runs on a circuit directly to the battery for timekeeping, and the display lights up when the dashboard lights are on. The only hygrometer I could find runs on its own battery, but everything else will run on the circuit that used to send power to the old player. I wanted a hygrometer because I have humidity issues inside the cab in the winter.

I wanted everything in green to match the rest of the dashboard, but it seems that's a hard color to find. Also wanted switches that light up when they're on, so they have their own ground pins. The first switch turns on the amplifier, and the last switch will be for the trailer circuit. The middle one will be for a backup camera that I haven't installed yet. I added a volume control for the amplifier, as its lowest volume is still too loud if I'm not driving. And the startup sound is really annoying, LOL.

Rather than run the trailer directly from the switch and circuit, I'm going to put in a relay, and add a fused circuit specifically for the trailer. I don't know yet exactly how I want to put that in. No hurry though, since I don't have the trailer yet.
Well, I've actually ordered the trailer! Should be here in a couple months. Also, total weigh, and weight distribution in the trailer is important. I've started building a list of stuff that will go in the trailer, and added a tongue and axle weight calculator at the bottom.
Thins in yellow are only estimates. As you can see, it's a total WIP, and everything is either yellow or not there yet, LOL. Also, it's pretty hard to know exactly everything that goes into the project, so I've found a way to make real life weight measurements as I go, without needing a commercial scale.
Who knew physics has real world applications, LOL.
I just finished a 7 x 14 cargo trailer -> camper. The base was 2000lb and it wound up 3800lb - e.g. 1800lb of build-out. It has 4 x 300w panels, 14s88p 18650 battery, PIP 3048, 9000BTU heat-pump, 2 x 30# propane, 40gal water tank, 9 cabinets, and eu3000is generator among the heavier things.

You've got your work cut out for you! I'll be interested to follow your progress :)
Mine is pretty simple. No propane or water heaters. No water tanks. Just a couple jugs. Generator will be in the truck. No bathroom or shower, though I do plan to have an emergency toilet. The trailer is supposed to weigh 1,100 lbs stock, and I'm taking out about 240 lbs of OSB as soon as I get it. I may leave off a couple solar cells. 10 is the max I can fit, but I'll have to see what I can use through a charge controller. I'm a total solar noob, so I'm learning this as I go. :)
Cells finally arrived in the mail!


They all arrived physically intact, and all were around 3.2V. A good voltage for storing these guys. So now I get to test each one to find out if they are really 60Ah. Chinese companies tend to exaggerate a bit on their cell capacities, so I'll be happy with anything around 56Ah or higher. I'm all set up to test lots of small Li-Ion cells, but not these large LiFePO4 cells, so this is going to take a while.

Another good bit of news, is that the trailer is now in stock and ready to roll. I may pick it up as early as tomorrow. Next big purchase will be the solar panels. I can't afford to buy them all at once, so I'll be getting a couple per paycheck.

Some sad news though, is that my grandfather has passed away. So the first use of the trailer will be to help clean out his house so we can sell it, or whatever mom decides to do with it.
Thanks. :)

Well, the first couple cells are about 50Ah. A little disappointing, but not really surprising. We often get what we pay for after all. But still, plenty sufficient for what I need. These LiFePO4 cells are behaving a bit different than Li-Ions that I'm used to working with.

I did some Ohm's Law math, and found out that to do a 40A load test on a 3.2V cell, I need a 0.08 Ohm resistor, that can handle 128W. I have no way to measure resistance that small. So unless I can come up with something creative, I don't think I can load test the individual cells. Maybe my math is wrong, but I'm using R=V/I, so 3.2V/40A=0.08 Ohms. The same math predicted my 500W 120V space heater should have a 28.8 Ohm heating element, and I got a very close reading of 28.6 Ohms.

I thought maybe a voltage booster, but I haven't found any that can boost 3V and still handle that kind of Watts. Once I have the inverter, I can test the whole battery. So no point in buying a bunch of extra stuff. I wonder though... If I'm using 2,000W on the inverter, that would be about 40A on each cell of the battery. Could I get an accurate measurement off each cell, or would being connected up mess up the reading? I suppose in the end, it will either work or it won't.

Random thought: Anyone ever use a smoke alarm to automatically shut off a circuit? Like using a relay or something? Might be a thing to add to the inside of the electrical cabinet.
I took the trailer on a cargo run this weekend.
The truck did pretty good. I don't know how much weight it had, but the truck had no problem pulling it, or stopping. Backing up a hill is pretty hard on the clutch though.

All 16 cells have now been tested. They all passed with an average of 50.5 Ah. Plenty for what I want. I've also got the inverter in the mail. So time to hook it up to a mini fridge and an A/C, and see how it goes.
I've been wanting to test out my battery and inverter to see if they can run a 5000 BTU A/C and a 1A mini fridge for the trailer. I got the battery temporarily put together today, and hooked up to the inverter. Since it's still cold out, I had to run a space heater for a while to get the A/C compressor to kick on.

The short answer is yes. The battery voltage barely moved under the peak load of both units switching on at the same time. And the inverter had no issues whatsoever.

To get the A/C blower and compressor to turn on at the same time, and also the fridge, I put them on a heavy duty power strip and just turned the strip on. The A/C is using less power than I thought it would. I was expecting it to need 4A, but it's only using 2.84A. Just 0.36A with the blower only. The fridge uses about 1A, as expected.

These are old units though that I happen to have on hand. The trailer will be getting new ones. I don't expect a new A/C of the same BTU would use more power than the old one, but for all I know, it's possible. I bought that old mini fridge USED when I was in college over 20 years ago. The A/C is only about 10 years old.

So a nice proof of concept anyway. On sunny days, the solar array will make more than enough power to run everything and charge the battery. But I can also run everything off the battery in the evening.
Yes Sir - I completed my 7 x 14 conversion a year ago. I has a 7s88p lithium-ion (260ah @ 48v). Using MPP Solar 3048 (accepts grid in + battery) and 9000 BTU mini-split. About 500 miles on 'regular roads' and no issues. Everything including hot water at the sink and cooking and TV and mini-split and small refrigerator run 24-48hrs depending on the outside temp. Absolutely love the mini-split.

The one issue - I put panels on the roof and they work except that I've yet to find a campsite that isn't in the shade. So PV is problematic and generator is good for extended trips.
I plan to get a small inverter generator for in case of shade or bad weather. Also, if we happen to be at a campsite with an electrical hookup, I won't have to worry about budgeting energy.

I took some pictures of my temporary setup trying out the inverter, fridge and A/C. Only one of them turned out any good:

I later figured out that I could "fold" the battery in half and save a lot of room on the table. It's connected up with fuses for now, since I don't have enough bus bars.

Today, I got out my solar charge controller since I have the battery temporarily assembled. It's a PowMr 60A. People seem to love it or hate it. I don't have solar panels yet, so I can't test it at full power, but I hooked up my benchtop power supply to stand in for panels, and I was able to get it programed for my lithium battery. The directions were difficult to follow at first, but after reading them about 10 times, I finally started to follow the logic of what they were talking about.

I remembered that I have a pocket oscilloscope, so I brought that out to see how pure my pure sine wave inverter really is. Once again, I took several pictures, but only one turned out good.

I had my little 12A space heater plugged in at the time, and whether it was running or off, the energy meter I had plugged in measured 60.18 Hz. The sine wave also didn't seem to change when the inverter was under load. The wave looks a bit pointy, but it's also a very cheap device. In fact, its a cheap knock-off of a cheap device. And I know it doesn't work exactly right. So take the readings with some skepticism.

For a comparison, I put the scope on an old UPS inverter that I converted to a portable power box a while back. You can see a huge difference.

A fun little device, but no good for precision oscilloscoping. That's fine though, I wouldn't know how to use a full sized one anyway. What's important though, is that these technical parts seem to be working together ok. That's a bit of load off my mind.
Looks like I may have enough money to finally buy the solar panels. This has renewed my interest in figuring out exactly how to wire them up. I'm still a noob at solar, but I've watched a lot of videos, and done a lot of reading. I think I have a viable plan:

Took a while, but it finally dawned on me that a "12V" solar panel has nothing to do with 12V. The numbers in the diagram are rounded somewhat for simplicity. These particular panels happen to be the perfect shape to cover the whole roof. Vents and other traditional roof things will be on the sides of the trailer. I've never heard anyone complain of having too much solar, so I decided to take advantage of the whole space.

Here are the specs for the panels I found:

Hopefully the diagram makes sense. The idea is to have 5 sets of 2 panels in series, and then connect those 5 sets in parallel. This setup will need a 5-into-1 pair of connectors, and I think a 10 or 12 AWG cable to connect it to the charge controller. The controller I bought recommends a breaker between the panels and the controller. I'm looking into the din rail style ones.

I've also read that inline fuses are sometimes needed to protect the panels from themselves if one gets a short circuit. Makes sense. We fuse our individual battery cells on the bus bars, so a similar concept for solar panels. Not sure if I need any fuses in this case, but I'm thinking 10 or 15A fuses on each series, so that if one series goes nuts, the others can't just dump power into it.

Anyway, have I missed anything?
One thought - charge controller MPPT voltages must be higher than the battery or it won't charge the battery :) Its like trying to charge a 48v battery with a 36v charger - the 36v charger will never get high enough to allow current to flow into the 48v battery.

I'm not sure your of your battery bank voltage - assuming it's 12v or 24v. For example, 2s would be 38v'ish (on specs above) which is close to 29v max charge of an 8s Lifepo4 battery since MPPT can vary up/down. I think 38v -> 29v is OK but you want to make monitor as you don't want 30v -> 29v kind of thing. If you are doing a 48v, then 2s is not enough. If needed, you can do 3s if your charge controller will allow that high of a voltage.
Good point! My victron MPPT says that solar voltage has to be 5V over battery voltage for it to start. I run a 2s solar array and my panels run OCV around 69vdc but they like to run closer to 60v. On a 48v (15s) battery I have to run them in 2s or drain the battery down a little bit for the solar to kick in on a 1s config. So I just run 2s and it is all good.
The battery is an 8s2p LiFePO4, so it will range from 20-29.2V. The MPPT controller seems to work down to at least 32V on the benchtop power supply, standing in for actual solar panels. I'm not sure how panels and MPPT controllers work together in low levels of light. Will the voltage drop severely, or will the controller reduce the current to try to keep the volts up? As long as the panels can produce at least 32V though, should be enough to charge the battery. As long as there is some current too.

Not sure what the voltage difference needs to be. My battery was at about 26V at the time I was fiddling with the controller. There is a low PV fault mode, that I'm guessing cuts power from the solar panels if the voltage difference is too small. I made the battery large enough to handle a full day (based on my best guess for power consumption) without charging, so even a partial charge will get us through the night no problem. Well, that's the hope anyway.