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Will my idea for powering travel trailer work?

OnTheFlyPDX

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Jun 27, 2021
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Hi all. I’m sorry if these questions have already been answered before. I searched the threads as best I could (where I found tons of great advice) but couldn’t find a thread that’s entirely on point as to what I’m thinking about doing for my build. Also, sorry in advance for the length of this post, I tend to over explain.

I am awaiting shipment on 8x 3.2v 200ah lifepo4 cells from AliExpress. I haven’t purchased anything else, so if someone who actually knows what they’re talking about thinks I should adjust my plans (e.g., 12v system instead of 24v) I have some flexibility. My plan is to build in series for a 24v 200ah bank. We have an 18’ travel trailer (ORV Creekside 18ck), so space is a little limited. There’s a bunk over the main bed that we converted to storage space where I’d like to put the battery bank and system. To make things simple and so I can easily remove my build to install in a tool shed after our upcoming year on the road, I don’t want to fully integrate the battery into the trailer’s electrical system. I’m also being a little lazy and want to avoid fishing wire. But I still want to keep the convenience of using the trailer’s 12v and 120v systems as if it was plugged into shore power. Our goal is to be able to boondock for a week at a time. We will need to keep a couple laptops and a 5G hotpot powered, occasional microwave use, and run a hair dryer every couple of days. No illusions about running the A/C with this small of a build. With all that in mind, here’s my plan so far with a few questions:

Build a 8s 24v Lifepo4 with an overkill bms connected to a MPP 2724LV-MR all-in-one. I’ve read fan noise on the MPP LV2424 can be pretty annoying, whereas the 12v version of the MPP does a better job of turning the fans off when there’s no/low load. Anyone have experience with fan noise on the 2724?

I plan to wire a 30a receptacle from the MPP’s output and install it on the back of the trailer. Then I can plug the trailer’s shore power cord into the receptacle and run the entire trailer off the MPP’s inverter. I’m going to keep the 200ah 12v AGM that’s currently on the trailer tongue and powering the 12v system separate from the lifepo4 build. This way I don’t have to run the MPP’s inverter when all I need to power is the rear camera and fridge (on propane) when we’re driving to our next destination.

If I run it this way, do I need to turn off the circuit breaker for the trailer’s battery charger when plugged into the MPP? I’ve read that you have to turn off the breaker to avoid a power drain loop if the battery running the inverter is also tied into the rv’s 12v system. However, I don’t think that would be an issue with my idea of separate battery banks because the trailer’s charger will only be charging the 12v AGM.

I think I might just turn the charger breaker off most of the time anyway (and probably install an inline switch to disconnect the AGM battery entirely). Still seems odd to use one battery to charge another. I think I should be able to get by with just using the trucks’s alternator to charge the AGM battery while on the move. My understanding is that even if the tongue battery is disconnected, the trailer’s 12v system will run off the trailer’s 12v converter when on shore power (i.e., the MPP’s inverter in my case)?

Assuming my idea isn’t crazy so far, where I’m struggling the most right now is solar panels. At first, I was leaning towards 400-600w in briefcase panels (200w Renogy), but those are pretty expensive and I don’t know if I want to deal with setting up and storing them each time we move spots. The really thin foldable (think Dokio) or flexible panels (the one’s Will says to avoid at all cost) would be ideal, but everything I’ve read says they’re all junk. There’s almost always someone in my area selling lightly used residential panels for cheap, so now I’m thinking it best to install 600w solar (the MPP’s capacity) on the roof. Any thoughts on the best setup for that? Should I run 3x 200w panels in series? 4x 150w panels in series and parallel? 2x 300w in series?

If you’ve made it this far, thanks for your patience. Really looking forward to reading y’all’s answers and any/all ideas you might have about what I’m trying to accomplish here. Thanks!
 
For my RV build, to tap into the RV 12 volt system, I removed the positive battery cable from the self resetting circuit breaker and removed the negative battery cable. I put my solar system 12 volt output from a busbar and connected it to the appropriate trailer positive and negative. I use this to run up to 50 amps on the trailer electronic jacks.

This is the wiring before the installation:

1624824840843.png1624824873552.png
This is the after picture with the RV OEM 12 volt hookup and the Solar 12 volt power supply cables bolted together and enclosed in this box.
1624824932472.png1624825017015.png
I am currently have 12 volts, but in a few months I'm upgrading to 24 volts. When this happens, instead of the RV 12 volt system being connected directly to my 12 volt batteries through a bus bar, there will be a 24 volt battery going to a 24 volt to 12 volt 70 amp DC to DC converter. I'm using a 24 volt DC to 12 volt DC converter because I don't need to run the inverter all the time to for 12 volts in my trailer. That can be pretty wasteful. The specific converter I'm going to use is an Victron Orion as is much more efficient.
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You can make your own briefcase by riveting two 100 watt panels together witch hinges, closing hasps and some sort of leg system, but honestly, these will not be much less expensive after hardware purchases than buying them pre-made.

I have 400 watts of suitcase panels and 950 watts of roof panels. At times, my suitcase panels will produce more than the roof panels because I can point and tilt them into the sun, and especially in winter months and at sunset and sunrise, they produce more power then the flat panels on the roof.

What you run depends on the charge controller you choose. They are rated with max volts in, and max amps out, like my victron 100/30 MPPT takes 100 volts and can make up to 30 amps out of it. That is about 400 watts at 12 volts or 800 watts at 24 volts. Because panels produce more volts when it gets colder and SCC can fry with too many volts, you have to look at the specs on the panel to make sure this does not exceed the SCC rating. With my panels, I can run three in series, but not four.
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For my RV build, to tap into the RV 12 volt system, I removed the positive battery cable from the self resetting circuit breaker and removed the negative battery cable. I put my solar system 12 volt output from a busbar and connected it to the appropriate trailer positive and negative. I use this to run up to 50 amps on the trailer electronic jacks.

This is the wiring before the installation:

View attachment 54356View attachment 54357
This is the after picture with the RV OEM 12 volt hookup and the Solar 12 volt power supply cables bolted together and enclosed in this box.
View attachment 54359View attachment 54360
I am currently have 12 volts, but in a few months I'm upgrading to 24 volts. When this happens, instead of the RV 12 volt system being connected directly to my 12 volt batteries through a bus bar, there will be a 24 volt battery going to a 24 volt to 12 volt 70 amp DC to DC converter. I'm using a 24 volt DC to 12 volt DC converter because I don't need to run the inverter all the time to for 12 volts in my trailer. That can be pretty wasteful. The specific converter I'm going to use is an Victron Orion as is much more efficient.
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You can make your own briefcase by riveting two 100 watt panels together witch hinges, closing hasps and some sort of leg system, but honestly, these will not be much less expensive after hardware purchases than buying them pre-made.

I have 400 watts of suitcase panels and 950 watts of roof panels. At times, my suitcase panels will produce more than the roof panels because I can point and tilt them into the sun, and especially in winter months and at sunset and sunrise, they produce more power then the flat panels on the roof.

What you run depends on the charge controller you choose. They are rated with max volts in, and max amps out, like my victron 100/30 MPPT takes 100 volts and can make up to 30 amps out of it. That is about 400 watts at 12 volts or 800 watts at 24 volts. Because panels produce more volts when it gets colder and SCC can fry with too many volts, you have to look at the specs on the panel to make sure this does not exceed the SCC rating. With my panels, I can run three in series, but not four.
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Thanks. I really like you’re build and future upgrade plans. I’m trying to avoid removing the 12v AGM battery that I already have on the tongue. That way all I have to do is remove the solar panels, lifepo4 battery bank, and the MPP all-in-one when it’s time to sell the trailer. This also helps me avoid fishing wires and buying a 24v-12v converter to tie the 24v lifepo4 bank into the trailer’s 12v wiring. And I think keeping things separate also avoids the issue of draining the lithium battery when I (inevitably) forget to turn the breaker off for the trailer’s battery charger. I suspect there are some inefficiencies with plugging the trailer’s 30a plug into the inverter instead of directly wiring the inverter into the trailer’s circuit panel, but surely it can’t be all that much. Interesting point about the inverter consumption though. I know these MPP units aren’t as efficient as the Victron products. If it’s too bad, I think we can avoid consumption loss by shutting off the inverter and relying on the tongue battery when we don’t need to use any 120v appliances. I read that the MPP unit’s solar charger still works when the inverter is powered off. Please let me know if I’m missing something?

Thanks for the ideas on building a solar briefcase. I saw a few videos where people hinged a couple 100w panels together like you described and came to the same conclusion that time/materials is more than just buying a premade one. I will probably make/buy at least one briefcase panel down the road to supplement this system and/or charge the ecoflow unit I keep in the truck.

The MPP unit I’m referring to (https://usamppsolar.com/product/top-pants-upper/) has a PV voltage range of 30-60vdc and can make up to 25amps. Using the Rich Solar 200w mono panel’s max voltage (Vmp) of 20.4v as an example, I think that means I can run 3 of those in series and be fine? Or should I be looking at the open circuit voltage (Voc) of 24.3v?
 
The MPP unit I’m referring to (https://usamppsolar.com/product/top-pants-upper/) has a PV voltage range of 30-60vdc and can make up to 25amps. Using the Rich Solar 200w mono panel’s max voltage (Vmp) of 20.4v as an example, I think that means I can run 3 of those in series and be fine? Or should I be looking at the open circuit voltage (Voc) of 24.3v?
If they're 20.4 VmP, you only want 2 of them in series. Input Voltage is a hard limit on an SCC and can cause it break. Also, when these panels are in cold weather, they output slightly more per degree cooler, and it can add up. When matching your SCC to panels need to take into account the record cold for the month you'll be there.

Also, I have a set of panels where the VoC is about 21 and the VmP is about 18. So with three in series at high noon with max production I see about 51 volts and 6 amps. When the sunrises and sets, I see closer to open circuit voltage of 60 with hardly any amps.
I read that the MPP unit’s solar charger still works when the inverter is powered off. Please let me know if I’m missing something?
It does work when the inverter is off. I misread and I had thought you were going to leave the inverter on at all times to run 12 VDC voltage. The tongue battery may work. I found my biggest DC draw is the propane heater fan. On a cold night that thing can be running all night long. That would drain a typical trailer battery real quick.

I have about 258 ah of usable energy in my FLA bank. I never worry about shutting the inverter off unless that propane heater is running all night on one of the shortest days of the year. With a 15 hour day, that inverter pulls 1 amp an hour, leaving all the lights on outside can pull 2 amps an hour for the 8 hours they would be on, and leaving the RV stereo radio powered up without shutting it off is a 1/2 amp which is about 30 amps.

I saw a few videos where people hinged a couple 100w panels together like you described and came to the same conclusion that time/materials is more than just buying a premade one.
The hinges on my suitcase panels blew away and I got to learn how to rivet them back together.
 
Thanks! This is all super helpful information. I should’ve been clearer about turning the inverter off when it’s not needed. Assuming I disconnect the tongue battery and the rv’s built-in dc battery charger is turned off, is it just the inverter’s idle consumption that makes keeping the inverter on full time problematic, or is there also a worrisome amount of power loss when the trailer converts 120AC to power the 12VDC system?

Really good to know that the SCC’s voltage is a hard limit. Back to the drawing board on that. Sounds like I shouldn’t be getting close to the upper voltage limit with temperature compensation you describe. Would, for example, a 2s2p of 150watt panels (18.8Vmp/22.7Voc/7.98A(Imp)) work? I think that puts it at 45.4 max voltage and ~16 amps, which is well within the SCC’s limits of 60V and 25A. Do I need to temperature compensate with this much wiggle room?
 
is there also a worrisome amount of power loss when the trailer converts 120AC to power the 12VDC system?
If the power is coming from your inverter than I think its not a good idea. I think my converter is only 30% efficient.

Depends on the battery bank. I have 258 ah I can spend, I like to use no more than half that 127 ah so I have an extra days of battery. I leave my solar panels on for driving. Also depends on the inverter. Mine draws 1 amp per hour.

I don't know the efficiency of the trailer converter. I think mine uses 700 watts, or about 50 amps to push 15 amps to the battery, or about 30% efficient. That's really bad and not doable. I'm going to do a 24 volt upgrade and may replace my converter with one that is 80% efficient, so that may be doable.
 
Okay, okay. This is starting to make a lot more sense now. I had a feeling that inverting 24vdc to 110vac and then converting to 12vdc would have some inefficiencies, but not to the degree you’re describing. Our trailer is 10 years old so I doubt it has a very efficient converter. Time for more research!
 
When I first turned on my solar, I did not shut the breaker off. That is how I was able to measure the 700 watts. That was my batteries feeding the inverter feeding the converter to charge my batteries. Not a good loop. I shut that down quick.
 
Okay, okay. This is starting to make a lot more sense now. I had a feeling that inverting 24vdc to 110vac and then converting to 12vdc would have some inefficiencies, but not to the degree you’re describing. Our trailer is 10 years old so I doubt it has a very efficient converter. Time for more research!
It sounds like you’re going for 24V because
A) it is a bit more efficient
B) you want to use one battery

Are you opposed to 12V? I ask because to my thinking if you add another LIP battery, use a good mppt controller that will take higher DC volts and charge to 12V, and still run your panels in series, you could eliminate the 12V agm battery and still run everything. There’s nothing in your camper that can use 24V, it’s all 12V already; why not just make good 12V? (FWIW disclosure: I run a 1200W true sine inverter 24/7 currently and have a 4.5cf electric fridge on 12V. It will run a vacuum, coffeemaker, etc without issues. I’m only running 4 100W panels 2S2P right now. All my everyday lights are 12V LEDs- only a couple 120V lights left snd I can’t remember when I used any of them last. I have no RV converter; shorepower is plugged into a GFCI 20A 120V outlet that is fed from the 1200W inverter. If I had a standard RV ‘converter’ I’d remove it and duplicate what I’m running now. It’s very simple always works, and necessary 12V devices are no problem.)

If you’re completely stuck on 24V you could add either a decent PWM off your panels that will take increased voltage for the 12V AGM- or a DC2DC charger.

Personally for what you are trying to do I’d not use 24V batteries. Why complicate? Not that 24V is complicated, but for your stated goal of not rewiring stuff 12V is just so simple to integrate immediately.
 
It sounds like you’re going for 24V because
A) it is a bit more efficient
B) you want to use one battery

Are you opposed to 12V? I ask because to my thinking if you add another LIP battery, use a good mppt controller that will take higher DC volts and charge to 12V, and still run your panels in series, you could eliminate the 12V agm battery and still run everything. There’s nothing in your camper that can use 24V, it’s all 12V already; why not just make good 12V? (FWIW disclosure: I run a 1200W true sine inverter 24/7 currently and have a 4.5cf electric fridge on 12V. It will run a vacuum, coffeemaker, etc without issues. I’m only running 4 100W panels 2S2P right now. All my everyday lights are 12V LEDs- only a couple 120V lights left snd I can’t remember when I used any of them last. I have no RV converter; shorepower is plugged into a GFCI 20A 120V outlet that is fed from the 1200W inverter. If I had a standard RV ‘converter’ I’d remove it and duplicate what I’m running now. It’s very simple always works, and necessary 12V devices are no problem.)

If you’re completely stuck on 24V you could add either a decent PWM off your panels that will take increased voltage for the 12V AGM- or a DC2DC charger.

Personally for what you are trying to do I’d not use 24V batteries. Why complicate? Not that 24V is complicated, but for your stated goal of not rewiring stuff 12V is just so simple to integrate immediately.
I’m leaning 24v primarily because I like the MPP all-in-one’s price point and simplicity. I plan to use the MPP and lithium battery bank in the trailer for only a year and then move the system to my tool shed when we sell the trailer after our trip. So I’ll need a larger inverter for some of my handheld power tools. Seems the 12v all-in-one systems either have smaller inverter size or have a larger inverter but cost more than I want to spend. I think I’m going to give my idea a try first, and then just get a 24VDC-12VDC converter to tie into the trailer’s 12VDC if the dual battery system doesn’t pan out.
 
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