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Off grid trailer power
#1
I plan to build my own trailer over the next couple years, and I'm looking to build a 4s 18650 battery for a few electrical needs.

The trailer build is here: DIY Trailer Build
The plan for the trip I'm designing it for is here: Epic Ural North

I plan to keep everything running on 12V so I don't need power conversion. I think my biggest power draw will be a laptop that I plan to plug in using a DC brick instead of the regular AC one. But until I actually pick out all the parts I want, I don't really know how much power I will need. So I will report back later as I make progress. Smile
-Mike G
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#2
It's been a while since I first started this thread, but I've been learning and designing, and I've finally gotten to the point where I can start figuring out the electrics and the battery. The link in the first post to the trailer build should still work if you want details, but here are the latest design pics of the actual trailer:


I'm toying with the idea of installing a couple of peltier coolers for hot days. Originally, I had considered a DIY evaporative cooler. But it would have been bulky, required lots of water, and I was concerned about condensation in such a small area. I think the peltiers will work much better. They're a bit power hungry, but I plan to use them at 25-75% capacity. Apparently, that will make them more energy efficient. Here's what I kind of have in mind:


I've been shopping for the various components I'd like to use for the trailer as far as electrical stuff goes, and I've taken note of the power requirements of each device. Another thing I had to consider is what battery chemistry to use. Lead/Acid was out; too heavy. LiFePO4 was out; I can't afford them. So I'm back to good ol' 18650 Li-Ions. So 3s or 4s? The question that forever vexes people who want to make a 12V battery out of Li-Ion. I've decided to go with 4s. For me, it makes sense to have the higher voltage, because it means less Amps per Watt. Because I use old laptop cells, I like to keep the Amps of each cell at 0.5A if I can. So that is almost always the limiting factor as far as how few cells in parallel I can get away with.

Anyway, here is what I came up with:


I'll have 4 USB charging ports, several 12V things, mostly lights and fans, and 1 24V outlet for my CPAP machine. After "translating" all those Amps back to 14.8V, I have an idea of how much current I'll be drawing from the battery if I ran everything at the same time. Comes out to something like 42A. Of course, I won't be running everything at the same time. That would be dumb. I played around with numbers for a while, and decided 30A is a good compromise between too much power, and not enough. Also, 30 is a nice round number, and I can get a breaker for that.

Note that in the above picture, I've got over 2000 Wh on the battery. Should be closer to 1400. I guess I fixed that after the screen capture.

Anyway, I know I won't be using everything at once, and I also know that I won't be using them all the time. I had to figure out both Amps and Watt-hours to know just how big to make the battery.


I may have made this image too small. I have that problem when sharing spreadsheet stuff.
What I did was to estimate at what capacity I would use each device. For example, the interior lights and reading lamp would probably need to be dimmed to be comfortable for me. The interior fan has two settings, Lo, and Hi, and I'll probably use it on low. The peltier A/C I'll use at less than full capacity. Say 50% on average. Things like that. Also, just because the 12V outlet can deliver 15A, it's unlikely I'll use it at 15A unless I have to inflate a tire. My electric tire pump has an initial surge of 15A then drops off to about 5-10A. Other things like the door light and the vent fans would always be run at full power.

Also, I had to estimate how many hours per day I would be using these devices. For example the floodlights are so I can see what's going on outside my trailer at night without getting out of it. Like if a bear is trying to steal my gas can for his dirt bike. Most days I wouldn't even turn them on. And since this is basically a sleeping box, I might run the interior lights for a couple hours and the reading lamp for maybe an hour.

After counting up all those things, I figured the battery will do an average of 10A for about 10 hours per day. But with the possibility of drawing as much as 30A at a single instant. And I'll need around 1400 Wh for the battery to last a whole day's use. Running this through all the regular math to build a battery, I came up with 4s64p, 256 cells total. Based on past experience I'll need around 45 laptop batteries' worth of cells.

What I haven't figured out yet is how to charge it. I'd like to be able to camp off grid, so I plan to have a solar panel and charge controller, which I think I've got figured out, but also, I might as well charge it off the motorcycle while I'm traveling, and I've got a battery isolator picked out. My motorcycle has a small car alternator on it rather than the stator and magneto things that regular bikes have, so it can generate quite a bit of power for a bike. But I need a 12VDC charge controller to safely charge the battery. I haven't found one yet. Also, when there is an electrical hookup, I'd like to charge the battery with 120VAC. I'll either need a controller for that, or a 120VAC to 12VDC converter capable of outputting, say, 10-30A. Haven't found anything like that either. I can balance easy enough with my iMax.

So if anyone knows of a good source for a charge controller, at least 10A output, for 4s batteries (one that just starts when you plug it in), I'd love to hear about it. Smile
rodagaster and completelycharged like this post
-Mike G
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#3
Interesting idea with the peltier. Perhaps experiment with water cooling it to make it more efficient.

I wonder if you could use a solar charge controller? I cannot find any info, but I wonder if it is possible to connect the PV side of a controller to a battery.
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#4
Nice setup. I agree with Geek on the Peltier idea. Those things are not very efficient. If you get a dehumidifier and redesign the housing and such, you could turn it into a small cooler. You would take the condenser coils and put them outside instead of having them in line with the evap coils.
Dehumidifiers are sold that run on 12VDC and are far cheaper than an A/C unit.

I would recommend going with 24VDC instead of 12VDC. You will incur a lot less losses and the wiring will be a lot more manageable. You can get some really good buck converters for decent prices and there's even some that can do 15A (about $30USD if I remember correctly)

For your camper, if you can take the siding off and clad the whole thing in heat reflective styrofoam, that will drastically reduce your temps inside the camper. This is what I mean: https://www.lowes.com/search?searchTerm=...insulation
Proceed with caution. Knowledge is Power! Literally! Cool 
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#5
I've changed tactics and instead of looking for a charge controller, I've started looking for circuit board type things that will do the job. I think I found one that will work. It's a CC CV Boost unit able to take as low as 10V, and boost it well above 16.8V. It also can output up to 30A. It's a rather big fellow, and comes with a heatsink and fan.


I've also decided to get one of those 12V power supplies that come encased in the shiny metal box. 120VAC in, 12VDC out. Again, 30A.


I'm thinking of maybe getting two of the CC CV boosts; one for the motorcycle limited to say, 10A, and one for the power supply at say, 25A. It's good practice not to run stuff like this at full capacity for long. 

I wonder if I should get one of those 30A "bms" circuit boards? I put a 20A one on a battery I made once for my lawn mower, and it cut the power quite effectively. My mower's starter was drawing 22A. So I guess they work.
-Mike G
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#6
If you are going to use that many DC-DC bucks converters, I think you would be better off with at least a 7s system.

For a trip like that, reliability is king, and you should be looking at brands like Victron and Samlex DC-DC converters. The Victron models with adjustable output voltage can also be used for battery charging.

Personally I like the Victron more because the price is better AND they have remove on/off so you can wire them to ignition or regular switches for control, eliminating the need for relays. They also come in isolated and non-isolated versions so if you needed to connect in series or needed isolated grounds, you have options.

https://www.victronenergy.com/dc-dc-converters

Victron Orion 12/24-10 fo 12 to 24 v for battery charging at 10 amps
Victron Orion 24/12-40A (40 amp DC-DC supply)

They have different sizes and amperage ranges for both, but those are the two I went with for my off-road 4x4 adventure setup. Most of my kit was all 12v lead acid, but ever since getting into lithium cells, I have been running 7s banks with similar challenges to yours to power my ARB fridge / freezer, ham radio communications equipment, lighting loads, and other accessories.
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#7
I'm intrigued by the dehumidifier idea. I will look into that.

Also interesting is going with a 7s battery. Same number of cells, but different configuration, and half the Amps. I did the same calculations as above, but with the new Voltage. Rearranged some circuitry, and came up with this:



I'd have a 7s36p battery at minimum, though 7s40p would be better. The CPAP would need a buck/boost converter to keep the Voltage at 24, but at only 4A, I bet I can find one. For the 12V buck converters, if I put them before the fuses, I may not need so many. Maybe just 2. This may work out very well. I'm going to look for components that will work for this. Smile
completelycharged and Korishan like this post
-Mike G
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#8
I think this should work for your CPAP machine...

Victron Orion Isolated 24/24-5
5 Amps RMS
16-35V Input

https://shop.pkys.com/Victron-Energy-Ori...gI3D_D_BwE

https://www.victronenergy.com/upload/doc...00W-EN.pdf
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#9
(08-15-2018, 04:01 AM)rebelrider.mike Wrote: It's been a while since I first started this thread, but I've been learning and designing, and I've finally gotten to the point where I can start figuring out the electrics and the battery. The link in the first post to the trailer build should still work if you want details, but here are the latest design pics of the actual trailer:


I'm toying with the idea of installing a couple of peltier coolers for hot days. Originally, I had considered a DIY evaporative cooler. But it would have been bulky, required lots of water, and I was concerned about condensation in such a small area. I think the peltiers will work much better. They're a bit power hungry, but I plan to use them at 25-75% capacity. Apparently, that will make them more energy efficient. Here's what I kind of have in mind:


I've been shopping for the various components I'd like to use for the trailer as far as electrical stuff goes, and I've taken note of the power requirements of each device. Another thing I had to consider is what battery chemistry to use. Lead/Acid was out; too heavy. LiFePO4 was out; I can't afford them. So I'm back to good ol' 18650 Li-Ions. So 3s or 4s? The question that forever vexes people who want to make a 12V battery out of Li-Ion. I've decided to go with 4s. For me, it makes sense to have the higher voltage, because it means less Amps per Watt. Because I use old laptop cells, I like to keep the Amps of each cell at 0.5A if I can. So that is almost always the limiting factor as far as how few cells in parallel I can get away with.

Anyway, here is what I came up with:


I'll have 4 USB charging ports, several 12V things, mostly lights and fans, and 1 24V outlet for my CPAP machine. After "translating" all those Amps back to 14.8V, I have an idea of how much current I'll be drawing from the battery if I ran everything at the same time. Comes out to something like 42A. Of course, I won't be running everything at the same time. That would be dumb. I played around with numbers for a while, and decided 30A is a good compromise between too much power, and not enough. Also, 30 is a nice round number, and I can get a breaker for that.

Note that in the above picture, I've got over 2000 Wh on the battery. Should be closer to 1400. I guess I fixed that after the screen capture.

Anyway, I know I won't be using everything at once, and I also know that I won't be using them all the time. I had to figure out both Amps and Watt-hours to know just how big to make the battery.


I may have made this image too small. I have that problem when sharing spreadsheet stuff.
What I did was to estimate at what capacity I would use each device. For example, the interior lights and reading lamp would probably need to be dimmed to be comfortable for me. The interior fan has two settings, Lo, and Hi, and I'll probably use it on low. The peltier A/C I'll use at less than full capacity. Say 50% on average. Things like that. Also, just because the 12V outlet can deliver 15A, it's unlikely I'll use it at 15A unless I have to inflate a tire. My electric tire pump has an initial surge of 15A then drops off to about 5-10A. Other things like the door light and the vent fans would always be run at full power.

Also, I had to estimate how many hours per day I would be using these devices. For example the floodlights are so I can see what's going on outside my trailer at night without getting out of it. Like if a bear is trying to steal my gas can for his dirt bike. Most days I wouldn't even turn them on. And since this is basically a sleeping box, I might run the interior lights for a couple hours and the reading lamp for maybe an hour.

After counting up all those things, I figured the battery will do an average of 10A for about 10 hours per day. But with the possibility of drawing as much as 30A at a single instant. And I'll need around 1400 Wh for the battery to last a whole day's use. Running this through all the regular math to build a battery, I came up with 4s64p, 256 cells total. Based on past experience I'll need around 45 laptop batteries' worth of cells.

What I haven't figured out yet is how to charge it. I'd like to be able to camp off grid, so I plan to have a solar panel and charge controller, which I think I've got figured out, but also, I might as well charge it off the motorcycle while I'm traveling, and I've got a battery isolator picked out. My motorcycle has a small car alternator on it rather than the stator and magneto things that regular bikes have, so it can generate quite a bit of power for a bike. But I need a 12VDC charge controller to safely charge the battery. I haven't found one yet. Also, when there is an electrical hookup, I'd like to charge the battery with 120VAC. I'll either need a controller for that, or a 120VAC to 12VDC converter capable of outputting, say, 10-30A. Haven't found anything like that either. I can balance easy enough with my iMax.

So if anyone knows of a good source for a charge controller, at least 10A output, for 4s batteries (one that just starts when you plug it in), I'd love to hear about it. Smile
Hi mate can i ask were you got your plans for the teardrop cheers .
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#10
Like the plan. For cooling go with a fridge/freezer and make ice with excess power. I still have 10 Peltier devices in a box to play with but they are a low priority after a few hours messing with one of them.... Ice is way better than batteries for energy storage.... and way more efficient...

Humidity control - compressor dehumidifier if 20C and above, otherwise a desiccant dehumidifier for cooler conditions as they provide space heating as a bonus and perform better in colder conditions. Humidity is usually an issue in colder conditions as the windows and general air circulation should provide enough in warmer situations.

"Like if a bear is trying to steal my gas can for his dirt bike." - LOL..

I have found that leaving a remote light on outside (few watts) makes for a good way of attracting all the flies and insects away from where you dont want them to be...

"I might as well charge it off the motorcycle while I'm traveling" - extra solar panel will work out more efficent than vie the engine...
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