Trying to install 2 100w solar panels on small rv.

brian1724 · March 2020

Hi all, pretty new to this and learning as I go. I've recently bought a 2006 17' Casita. So far I've changed out the battery to a Battle Born LiFeP04 100Ah with a Vicron bmv-712 controller. I've also installed a Victron Phoenix Inverter 12 VDC - 800. I've just bought 2 100w solar panels. I would like to fix one to the roof and would like to keep one deployable. I know I can install one or the other by itself without a problem but with both of them I'm doing my best to figure out how they should be connected. I want them to be in parallel and I want to be able to connect and unconnect the deployable one with out causing any issues. I think I'm doing it right but with no one I know that has any experience in solar I'd like to be able to ask the group here if what I'm doing is correct. I figure a picture is worth a thousand words so I drew up a diagram of sorts to show wiring and what I want to accomplish. The solar charger I plan to use is a Victron SmartSolar MPPT 100/20. I hope I've explained everything clearly so you can understand want I'm trying to do. I sure would appreciate any help or feedback. Thanks Brian

BB. · March 2020

Welcome to the forum Brian,

Generally, you have the basics correct. However, an MPPT charge controller with a typical Vmp~18 volts (standard temperature conditions under test) is not really "useful" for the typical solar installation in warm to hot climates. Here is a detailed explination from Victron:

https://www.victronenergy.com/upload/documents/White-paper-Which-solar-charge-controller-PWM-or-MPPT.pdf

Ideally, what you would do is put the two (or more) solar panels in series so that Vmp-array is ~36 volts. This works nicely with longer wire runs from array to solar MPPT charge controller--Allows you to use smaller diameter cable and longer wire runs without excessive voltage loss.

However, if you want to run the second solar panel on an extension cord, you need a fairly heavy cable (and or a lighter weight cable kept short). For example, say you want to run the second panel 50 feet away and you have the choice between a 16 AWG cable and 10 AWG cable. Using a simple voltage drop calculator with 5.58 amps and 18 volts Vmp:
https://www.calculator.net/voltage-drop-calculator.html?material=copper&wiresize=13.17&voltage=18&phase=dc&noofconductor=1&distance=50&distanceunit=feet&amperes=5.58&x=65&y=13
16 AWG cable:
Voltage drop: 2.24
Voltage drop percentage: 12.44%
Voltage at the end: 15.76

https://www.calculator.net/voltage-drop-calculator.html?material=copper&wiresize=3.277&voltage=18&phase=dc&noofconductor=1&distance=50&distanceunit=feet&amperes=5.58&x=17&y=14
10 AWG cable:
Voltage drop: 0.56
Voltage drop percentage: 3.10%
Voltage at the end: 17.44

When you add the fact that "hot solar panels" (under full sun, warm weather), the Vmp-hot falls quite a bit--As much as 3 volts or so (for a 12 volt panel)... So if you have 18 volts - 2.24 volt wiring drop (if 16 awg and 50 feet) and -3 volt hot panel derating = 14.76 volts at the controller input... Just about the battery charging voltage (if you have 14.8 volts "available" from the array, and want 14.8 volts at the battery, no current flows.

In reality, because everything is a "curve", you will get current flow, just greatly reduced from what you expect or want.

So, one fix is to use 10 AWG cable for the remote panel. A second is to use two panels in series (one pair on roof, a second pair remote), Possibly look at using a PWM type controller (cheaper than MPPT) if using Vmp=18 volt array (no series connected panels).

Some other things to think about... One is will the remote panel "walk away" if you are not there. Another is that the remote panel needs to be staked into the ground (to prevent it falling in wind and getting shattered). Third, suggest not using "twist lock" power connectors but use standard push type plugs (if you drive away and forget the remote panel, you want to pull the plug instead of dragging your panel down the road and or ripping out your wiring).

It is difficult to find (if possible at all) to find off the shelf Vmp>24 volt 100 Watt solar panels. So, for an MPPT based system, it would be better to use 2x Vmp=18 volt panels in series on the top of the RV, and a second set of Vmp=18 volt panels in series for remote use. If you can fit the panels and have the space for 2x remote panels.

MPPT controllers are ideal when the solar array Vmp-array voltage is >> than the battery charging voltage (i.e., if you have a 14.4 volt chaging voltage * 1.3x MPPT minimum input voltage = 18.7 volts Vmp-array-hot, + another 1.25x hot panel derating = ~23.4 volts Vmp-std (mfg spec) rating... If you choose to use Vmp~18 volt panels, then MPPT does not really do much for you (in sub freezing weather, yes, you will get better harvest with MPPT).

Turns out there are getting to be more "24 volt" lower wattage panels... Here are both a 140 Watt and 90 Watt Vmp~36 volt panel:

https://www.solar-electric.com/solarland-slp140-24-140-watt-24-volt-solar-panel.html
https://www.solar-electric.com/solarland-slp090-24c1d2-90-watt-24-volt-solar-panel.html

These would work very nicely with MPPT and your 1+1 panel project.

I will stop here before I make things more confusing... Any questions/corrections?

-Bill

brian1724 · March 2020

Thanks so much for your knowledge. I'm reading the links you posted.

As it stands I have only space on the roof for one panel and the only 100 watt panel I could find which would fit in between my fan and bathroom vent are the ones I show in the picture. Size is 35.6x25.9x1.2 in. I will fix one to the roof and attach an adjustable mount to the other one. I thought it was good to have two panels of the same or similar specs. As far as cable size I was just going to buy a roll of 10 awg stranded wire off amazon and use that for everything. I believe it is overkill on most of the runs but I thought it would just simplify things, which helps me. At the most the one deployable panel would be 25'.

Thanks again for your info I'm going to do more research and then probably have more questions. My ignorance of solar is extensive!

I live in Las Vegas and tried to find a company that was willing to work with me on a small install like this. Rv places were of no help. They knew less than I did about solar as far as I could tell. And then the other solar companies in town are the big solar panel companies for homes. So I've been on my own. I was a carpenter for 30 years so I consider myself fairly handy and of average intelligence so I figure if I just keep reading and asking questions eventually in may sink in.

littleharbor2 · March 2020

You might find someone to work with you on an install through Craigslist.

You want to avoid any shadowing on the panel. If you are squeezing it between two things on the roof it sounds like you may have issues with at least with the A/C unit. If the panel is snugged right up to the shroud you are risking shadowing on the panel until the sun is directly overhead. That would cost you a number of critical hours of sun.

brian1724 · March 2020

This is the only area I can fit it on the roof. I haven't configured the mounting yet but I'm going to do my best to make it adjustable. Depending how I'm parked and how high I mount it I would probably get some shading from the max fan that's why I thought it was better to connect them in parallel so if I did get shading it wouldn't effect the other panel if I had it deployed. At least that's what I read. Thank you for the Craig's list idea. I'll look into that!

Image: https://us.v-cdn.net/6024911/uploads/editor/4i/27d74pkdmo31.jpg

brian1724 · March 2020

If I connect the panel in series but leave the deployable one disconnected for lengths of time does it have any detrimental effect on the panel that is fixed on the roof or cause that one not to function as intended? I know it's a newbie question but in my mind I'm thinking links in a chain and if ones not connected then the chain fails. Maybe a poor analogy.

BB. · March 2020

What you do not want to do (in general), is put two (or more) panels in series, and shade (partially/fully shade one panel/cell)... The cells "in the dark" go high resistance. There are bypass diodes for every ~12-24 cells, that allow the current go "go around" the dark cell(s)... But (I believe) there is an issue with bypass diodes as used/installed in solar panels. Diodes do generate heat when in conduction (forward biased), but solar panels have very poor or zero heat sinking for the bypass diodes... So I am concerned that these bypass diodes can run hot and fail over time.

So, if you have a situation where you are parked under shade (keep trailer cool), and place a remote panel 50 feet away in the sun. Paralleling the panels is really the way to go. Each string will output the current in proportion of the sunlight hitting the panel. The one in shade, near zero. The one in full noontime sun, near full Imp.

Placing the two panels in series, and keeping one in shade--You do not get any advantages (and you have the drawback of ~1 volt bypass diode drop on the "dark" panel).

-Bill

brian1724 · March 2020

Thank you Bill. That is why I initially chose parallel. If I can park in the shade that is what I would prefer to keep the Casita cool and thus why I wanted a deployable panel that I might be able to find a sunny spot for. I really don't have high power needs and I do have a ge inverter generator but would much rather just try to keep some amps going to the battery. Even if only one was getting sun and maybe I could get 10 to 20amps off it. All my lights are led. My fan doesn't even use 1 amp per hour. Stove and fridge are propane. I do have the inverter to charge my laptop and drone batteries. I have a dc plugin to charge my phone. I'm just trying to make it so I'm not tied to rv parks. I would like to be able to boondock for 5 days or so and not feel stressed that I'm about to run out of power. Lot of BLM land here about, free for the staying.

At least for 14 days at a time.

As for the mppt solar charger, I picked that because I have the victron battery controller and it bluetooths to my phone. I thought the victron solar controller would play well with everything else including my battery and also send data to the smart app. Also Battle Born recommends that controller for use with their battery. So those are some of my reasons for the way I initially set things up.

Thank you for the input. When you are ignorant on a topic you don't even know the right questions to ask or the terminology to ask it correctly. So I appreciate the help.

Brian

BB. · March 2020

You are very welcome Brian.

The Victron line does offer a lot of integration options. Just read through the manuals (a few times) and understand the "black boxes" you will be using.

I scanned quickly through the manuals (because of your questions), and they offer a lot.

-Bill

littleharbor2 · March 2020

Now that I see the proposed panel location it doesn't look as dire as I first envisioned. You can always try and spot your trailer with the front end pointing south and you shouldn't have any shading issues.

Trying to install 2 100w solar panels on small rv.

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