6x12 cargo trailer conversion for camping.

I can't help you with the solar questions, but I would question whether or not you might get into an overload situation with the single axle trailer.

Typical 6x12 trailer (payload includes everything you plan to install or store in the trailer):

My concern is the battery system needed to provide 2.6kWh each day without killing them.

If you go with FLA then you system should be designed to provide 4 x that amount each day or 10.4 kWh. That is a huge system.

Or you can go with Lithium type batteries which will allow you a much smaller battery system but the cost will choke you.

IMO I would not power an AC system because that is about half your daily kWh usage and inflates the battery to be very big.

This could just be my noobness showing, but my thinking is that needing a/c means hot, which means sunny, which means lots of solar power. No sun means not hot, which means no a/c. I'm sure that's oversimplified, but that's kind of my strategy. Also, if the battery gets low, I'll just turn off the a/c. Imagine the size of the battery if I decided to cook with the microwave off grid, LOL.

But getting to the weight for a bit, I've given that a lot of thought. Someone suggested I make a weight table to help keep the weight of materials and things from getting out of hand. Here is what I came up with:
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Like the last table, green is what I'm sure of. (Notice there isn't any yet.) Yellow comes from advertisements and data sheets, and white are wild guesses just so I'd have somewhere to start. I'm sure I'm way off on some of these, but the point is that if the trailer gets too heavy, I'll have to come up with some kind of solution. At the very top left you see the goal weight and the advertised trailer weight. I'll weigh it myself though when I get the trailer. At the bottom I've got the current estimated total weight and how much I've got left to play with as the "balance" like in a checkbook.

From there I can calculate how much the tongue should weigh: about 12% of the total. So the rest should be on the axle. In real life, I can add the axle weight and the tongue weight to get the total actual weight of the trailer as I go. That's how I hope it will all work out anyway.


So back to the battery; this is more familiar territory for me. I have some tables I've been using for a while to help design batteries based on the specs of the cell type and the power requirements of whatever I'm designing it for.
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I start with what I know, which will vary from project to project. In this case, I know the nominal voltage that the battery will be, and how many Wh I'll probably use in a day. And also I figure a battery that can last a whole day just on it's own would be pretty good. From there, I can calculate Ah, average Watts, and Amps. I usually go with maximum continuous current, but this system is so variable, that I'm not worried about it yet.

Next is finding out what each cell will be able to do, and what configuration I'll need to connect them in:
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Most of that comes directly off the spec sheet of whatever cell type I'm going to use. In this case, 60Ah LiFePO4 cells. I don't know why, but for some reason, the 60Ah cells are less than half the price of 120Ah cells. To get the desired voltage, there will need to be 8 in series. To get enough Watt-hours, I'll need 13.54 cells total. But you can't have 0.54 of a cell, so I round up. I'll need 2 cells in parallel to keep the Watt-hours satisfied, since I already know there will be 8 in series. Each cell can deliver a whopping 180A of continuous current. I would find that hard to believe, except my motorcycle has a LiFePO4 starter battery capable of 300A, and it's been working perfectly for 4 years. Because of this, 1 cell in parallel would be more than enough to cover the required current even with everything going full on and peaking, which is why I wasn't concerned about only having the average current in the first table.

So to finally settle on how many cells in parallel are needed, whichever number at the bottom of the table is largest is the one to use. Now, an 8s4p battery will have 16 cells, not 14, so everything should be well within the power requirements.

The last table, is what the battery can actually do, based on the specs of the cells:
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At this point, I can check this table with the first table of what the project requirements are, and make sure I didn't mess something up. Also, when I actually get these cells, I plan to do a capacity test on each one, and run them at 80 Amps for a while to make sure they don't heat up. I picked 80 Amps because that's more than I should ever need to draw or put into the battery. Since they'll be in parallel though, maybe 40 Amps would do.

I'll also be adding an 80 Amp BMS, and lots of fuses and breakers. The voltage isn't dangerously high, but that would be a lot of current if there were ever a short somewhere. This may be overkill, but I plan to add fuses in between each parallel set of cells. The BMS can't protect the cells from internal problems, and if one cell were to have some catastrophic failure, a fuse could potentially save its buddy from going down with it.

I wonder about cable size though. When the fridge and a/c are running normally, the battery would only see about 30A, even if everything else were turned on at the same time. Even if both units peaked at the same time, that would still only briefly be something like 72A. I haven't gotten into it yet, but I'm thinking the battery itself could see a maximum charging current of 55A. What I don't know is if I should make the cables big enough to handle 80A, or if sizing them to handle 55A would be enough?

To power the starting surge of any of your appliances, your Li battery needs to be able to supply that surge. Sometimes, Li batteries ( or their BMS ) have a surprisingly low max amps output surge.

Your Battery - Inverter cables need to be large enough that any combination of loads and starting surges, do not cause so much IR loss, that the inverter shuts down from undervoltage.

So if you expect to see 80A going to the inverter - even for a half second, the cable needs to handle that much so the voltage to the inverter does not sag and shut the inverter off.

I would look at a combination propane/AC/DC fridge, costs more but is just miserly on propane or they make some chest type refridge/freezer combos that run on 12v and are pretty miserly on power consumption. AC needs a lot of power to run and usually only needed for a few hours a day and only a couple months of the year, unless you plan to summer in southern AZ. A generator might be a better option. A microwave is a convenience not really needed a propane stove will do everything a microwave will do just a little slower.

What you plans for the vehicle has a lot to do with what you really NEED. Some states have park passes that allow you to stay for really cheap, my state has a pass that allows me to stay two weeks with power for $4./ngt. after two weeks I need to drive to the next park. There are lots of very inexpensive parks around the country, county parks, city parks, fairgrounds that are $10/ngt.

Ok, that makes sense. I'm thinking 4 AWG cables. Most everything will be together in a cabinet, so the cables should be a foot or less I think. I'll test everything before I install it.

I found the power consumption of the a/c a bit daunting at first, but on the really hot days, I'll really need it. At least for an hour or two to get the trailer cool enough to sleep in. I've accepted it as a challenge. I'd love to have a DC or propane fridge, but they're out of my price range for now. I'm trying to plan the trailer to be upgradable though, so I can always save up for one in the future.

I agree the microwave is a minor convenience. Another reason I decided not to try to power it off the battery. All my real cooking will be done outside on propane. But if we happen to be plugged into the grid, it would be nice to be able to re-heat a beverage or leftovers.

We'll be doing most of our camping in Washington. As far as I know, passes are only good for day use, and overnight fees are something like $25/night. I've only ever tent camped before though, so I don't know what RV sites are like. One thing I'd like to be able to do while traveling is to pull off to a rest stop when I'm tired, and sleep for a bit. That's legal in Washington as long as you don't stay more than 8 hours. The ability to power cell phones and laptops though will open up the possibility for my wife to work remotely, which means we could go camping for longer times and distances.We'll definitely be staying away from the hotter areas during summer though!

Finally getting to the solar: I've decided to put side vents in the trailer rather than a roof vent. That will give me the whole roof area for solar panels. I've never heard anyone complain that they have too much solar, so I searched around and found some panels that will fit the roof area almost perfectly. Here's a scale drawing of what I mean:
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I'll have the panels up on brackets, not sitting directly on the roof. I've heard that ventilation is good for panels. Also, there should be enough room to leave small gaps between each panel. Also allowing for better ventilation. And I can fit up to 10 of them. They're 100W each, so that would let me have a 1,000W setup if I can use them all. I've also read that 100W panels won't actually provide 100W. And what they do produce will vary with sunlight intensity. So I'm not expecting 16kwh per day or anything. But a few Watt-hours would be nice.

I only know a little about how to set this stuff up. I know they can be in series or parallel, like battery cells. I know that in series, one panel in shade will degrade the performance of all the panels in that series. Not sure the best way to set these up though. I know I'll need a solar charge controller that will be compatible with a "24V" or 8s LiFePO4 battery. The market seems to be dominated with FLA controllers though. At least from what I've found so far.

Maybe you all can help me? Here's the specs for each panel:

100 Watts each
600V maximum for the system they're hooked up to.
MPP volts is 19.12
MPP amps is 5.32
Open circuit voltage is 23.1
Average energy per day is 300 Wh (probably less in the PNW, LOL)

I was thinking maybe a 5s2p configuration. That would give an open voltage of 115.5V. I might just be confusing myself at this point, but the ones I've seen don't want more than 100V open voltage. Maybe I need two controllers? Like one for a 3s2p, and another for a 2s2p. I think they have to be at least 2 in series to get enough voltage for the battery. My understanding is that charge controllers are also buck converters, and not boost converters. So the solar voltage has to be higher than the battery voltage.

That's about as far as I've gotten with the solar planning. I haven't seen many trailers like mine with a/c running on the battery and solar. I certainly see why it's a challenge. There is one guy I know from another forum who is doing nearly the same setup as me, but he's using a smaller LiFePO4 battery, and fewer solar panels. He's been traveling around the country for the last couple months, and I'm very curious to see when he comes back how his solar powered a/c performed.

Last for today, I told you I'm a battery nerd. Though I'm not an expert by any means, I'm definitely a nerd. To prove it, here's a drawing I made of how the battery might be configured:
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I think putting all the cells in a single row will make it fit better in the cabinet I have in mind. I've never seen anyone fuse prismatic cells, but I also don't think I've seen anyone put them in parallel either. But for some reason, two 60Ah cells are way cheaper than a single 120Ah cell. Anyway, I almost didn't draw this, but then I figured it would be fun, so why not?

You’re really doing everything right to make this work, and there are some huge obstacles you are working on. So, I think you can overcome them, but a bit more work. I’ve put some thought into a trailer build like you mentioned, and this may be my next big project after my 24 volt conversion on my RV is complete.

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For your roof panels, its a jigsaw puzzle. Consider “used” craigslist panels that are much bigger, but have more wattage and can fit the jigsaw puzzle better. You can lose a bit of space with panel spacing if not planned for correctly. I lost a fist worth of space between my six panels for a tilt mount system, which adds up to an additional panel I could put up. Turns out the tilt mount is such a pain to use, like everyone else who installs it, I don’t use it so I wasted the time, space and money in buying it.

You can also do things like have slide out panels underneath the roof panels. As good of an idea as this seems, the price I saw on these were $600 per 100 watt panel. Also don’t know how bad they would be for the 35 knot winds that pop up out of nowhere at a lot of the places I camp at. I have started to think mounting panels on the side of the trailer could be better with the intent of having that side being the southern facing side. This is a better idea the farther away you are from the equator. For me in AriZona, side mounted panels would be great for low angle sun in th winter, but not so much for high angle sun in the summer when I need it for AC.

I’ve found portable panels to be a good solution for extra power, but they take up a surprising amount of space.

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For reference, I am getting 16 cells, 280 AH each lithium’s, that I will put into two 8S battery banks, weighing 100 LBS each, before mounting. Lithium batteries have a fairly “uniform” density, meaning if you see one pack the same size and weight that claims to be much higher powered, its not.

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You really need to look at the weight you have to play with. I have seen no mention of tanks: Black tank, Grey tank, and White tank. At 8 LBS a gallon you could easily and up with 800 LBS of liquids you need to have within the weight ratings.

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You can be on the right track with a lithium battery build for large amounts of power for short times like your AC. Today, I am testing 600 wh of lithium batteries to run at 300 watts constant while the sun is shining. For lead acid batteries, you just DO NOT want to run those at 50% of capacity constantly; 13% of capacity while charging seems to be the limit. This on a larger scale could be good to run your AC when the sun is shining, but once the sun goes down there’s not a lot of storage in these lithium batteries to keep the AC running, so an hour at night would kill it.

Thanks for the feedback!

I hadn't thought of slide out panels. At $600 I'd definitely have to make my own. They would have to be put away on windy days, but that's still a neat idea. Maybe I'll leave some space under the fixed ones so I could try this out some time in the future. I thought about tilting the panels, but since I'll never know how I'll be parked from one place to the next, I didn't figure it would be worth it.

My plumbing will be dead simple. A jug of water, gravity fed cold only faucet to the sink, and a drain into a little bucket. I'll also stash a composting toilet under a cabinet for emergencies, but that's it. I plan to keep maybe 4 gallons in the trailer, and any extra we might need, I can put in the back of the truck. There's nearly always a water source where we camp though, so it's never been an issue.

I agree that LiFePO4 is the way to go. It has it's drawbacks, but I think the pros outweigh the cons by far.

I've been working on a wiring diagram for a while, and here is what I've got so far. Remember that the solar part is not done yet:
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Red wires are DC+ (left out the - for simplicity)
Black wires are AC
Green things are sources of power
Purple things use power
Blue things are just various components.

Not a professional wiring diagram, but it's something I can follow. I figure the solar will be connected all the time, and there will only be 1 power inlet for the generator or grid power. I'll have a little power coming from the truck while I drive. Should be enough to power the fridge and inverter if I'm driving at night. I'll have that set up so power is only available if the truck ignition key is on.

One big manual switch between the battery and everything else. Maybe it will double as the junction box I have shown separately. There's a transfer switch so the fridge and a/c can get power from either the inverter or grid power, but never both. I think the peak current draw from the microwave may be a bit much for a 15A circuit breaker, so I'll probably upgrade that to 20A. The inverter will be loaded with safety features, but I'll put it on an external DC breaker anyway.

Here's the circuit that will send power from the truck:
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The ignition switch and the manual switch will have to be on for the relay to send power from the alternator to the trailer. I'm not sure how many amps I'll be sending. This will be a long cable going all the way back to the trailer, so there's going to be voltage drop. I hope to lessen that a bit by using 12 AWG cable. I've already make a custom panel in my truck with a switch for the trailer power:
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The clutch plate is the limiting factor in my truck as far as how much weight I can tow. But I installed a temperature gauge anyway that I want to route down to the transmission to see how hot it's getting. I'll also have to find out how hot a transmission is supposed to be.

I plan to have a little shelf above the bed for glasses, phones, very small monkeys, books, keys, that sort of thing. Underneath it I'll be making a sort of diagonal panel so we can have USB ports, power sockets, and reading lights. I haven't worked out the exact positioning or scale yet, but here's the wiring diagram:
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I have all the parts I need, just need to build it. It will have the USB ports and the 24V power socket for my CPAP, running directly off the battery, and the lights and 12V power socket running off the 12V regulated power. The switches normally wouldn't need a ground, but these ones light up to show when they are working, so the little LED inside needs a negative wire. The reading lights I already had left over from another project, and I found some nice little dimmers that work perfect for them.
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They're plenty bright, so the dimmers will be very handy. I'll also get some filters for them both for protection, and to warm the color up a bit.

I found a solar charge controller that I think will work with putting my panels in a 5s2p conviguration:

It can handle 160V of open circuit voltage from the panels and can handle more current than I'll get from the panels even in full sunlight. And it's able to work with LiFePO4, and can even be programmed if I decide to change the preset parameters for some reason.

I've been working on a floor plan. Not everything is to scale yet, and I don't know the exact position of the wheel axle, but this is the layout I'm imagining.
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I know the width of the side door, but not it's exact position either. Though I think I'm pretty close. I'll be able to measure everything once the trailer actually gets here.

Putting "filters" on LED's to make them "warmer" is a waste. to get them warm enough you will loose half the light output. Just get warm white LED's in 2500-3000 K lighting range.
The phosphor mix is what changes a blue LED to some version of white. Cool white (daylight) 5000K phosphor mix is cheap, warm white is a couple cents more expensive.

Trailer is finally here! I picked it up yesterday. Also, the cells came in the mail. I'll do a capacity test on each one first, and then a load test. They're rated for 180A continuous discharge, but I'll only need them to hold a decent voltage for a few seconds at 40A. I'll rig something up later to test that.

Thanks for the update. Keep us posted on your progress.

FWIW, I skipped the capacity test on the 2 X 8S 24 volt lithium batteries I’m making. First, I go them from a reputable vendor where I saw their tests of my actual cells and others had tested and posted about this vendor. Second, would not really change the way I put this battery together, Third, capacity testing those 16 cells would have taken two weeks.

I’m also looking forward to your progress and wish to build something like you’re doing, just a little bigger to put a shower in. I will likely not act on that wish.

I took the trailer on a cargo run this weekend.
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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.