Best set up for my RV solar project

[hazed]

Your wire size depends on how many amps it has to carry. The panels will list amperage but if you know the watts it is no big deal to figure. Watts= Volts X Amps. When wired in series, panels add voltage and the amperage stays the same. In parallel, panels voltage stays the same and adds amperage.

Using an example of a 100w panel at Imp at 18vDC. of 5.56a. All 7 or 8 in series means the wire must carry 5.56a. You could easily get by with 16awg wire rated for 10a. lets say you run a 4S2P arrangement of 8 panels. 4 times the voltage and twice the current. With 11.2a you would need 14awg wire rated for 15a.

Your SCC must be rated to carry the max possible voltage which is expressed by panel Voc rating. Say it is 22Voc per panel, a 4S2P setup means that your SCC must be rated to withstand 88Voc and a bit more for cold conditions likely 100Voc minimum.

Makes sense. I am trying to carefully track the wires inside the walls, as it is a fairly new rv and the better half is not sure about the solar, lifepo4 and mini split swap. Getting to 8 panels will help with wiring. Just trying to figure out the more preferred wiring before I mount the panels and route the wires.

 

[Rocketman]

At 8 panels - (we really need to know the specs on the panels)
Voc Vmp & Imp

Still guessing…

I see a few options with Victron Solar charge controllers (there are lots available- these are ones I am familiar with)

You could go 2s4p into a Victron Mppt 100/50 or
4s2p into a Victron Mppt 150/60. One more option is two charge controllers 2s2p into a Victron Mppt 100/30 (but that’s 2x more wires coming down - but maybe it makes sense).

Good Luck

 

[Rednecktek]

So how does your shading situation look? That'll decide whether to do a 4s2p or spend the extra on a combiner box for 2p4s.

 

[hazed]

At 8 panels - (we really need to know the specs on the panels)
Voc Vmp & Imp

Still guessing…

I see a few options with Victron Solar charge controllers (there are lots available- these are ones I am familiar with)

You could go 2s4p into a Victron Mppt 100/50 or
4s2p into a Victron Mppt 150/60. One more option is two charge controllers 2s2p into a Victron Mppt 100/30 (but that’s 2x more wires coming down - but maybe it makes sense).

Good Luck

Attached are the specs on the panels I bought.

 

[hazed]

So how does your shading situation look? That'll decide whether to do a 4s2p or spend the extra on a combiner box for 2p4s.

Probably won't see too much shading for what my future plans are. Let's say 80/20. Could be some driving figured in, so charging off the engine too. I am not one to sit in one location too long, if that makes sense.

 

[Rocketman]

With eight of those panels
Voc 20.5v
Vmp 17.6
Imp 6.03

With 4s2p that will be:
Voc 82v
Vmp 70.4
Imp 12.06
That will work with both a Victron 100/50 and a 150/60

With 2s4p
Voc 41v
Vmp 35.2v
Imp 24.12a
That will work with both a Victron 100/50 and a 150/60. You do need the verify that you have 10 awg wire from the roof to the SCC. You will also need to fuse each string.

100/50
At 800w your system will be slightly over paneled with the 100/50 (it can max produce 700w), but your panels are flat - so you will very seldom see 700w (My array1 has 800w into a 100/50 and I can only recall four or five times in 4 years that I have seen 700w). You will need #6awg or #4 awg from the SCC to the batteries/bus bar. This one is a little cheaper $184.45

150/60
This SCC will capture the occasion bump into 700+ watts, You will need #4 awg from SCC to batteries/bus bar. This SCC can connect to 48v battery if you think you may ever reuse it in a different system. Costs s but more $351.05

You should have a breaker or switch to “turn the sun off” between the panels and the SCC. And if the SCC is any distance from the battery/bus bar the wire going to the SCC should be fused.

Good Luck - any questions?

 

[hazed]

With eight of those panels
Voc 20.5v
Vmp 17.6
Imp 6.03

With 4s2p that will be:
Voc 82v
Vmp 70.4
Imp 12.06
That will work with both a Victron 100/50 and a 150/60

With 2s4p
Voc 41v
Vmp 35.2v
Imp 24.12a
That will work with both a Victron 100/50 and a 150/60. You do need the verify that you have 10 awg wire from the roof to the SCC. You will also need to fuse each string.

100/50
At 800w your system will be slightly over paneled with the 100/50 (it can max produce 700w), but your panels are flat - so you will very seldom see 700w (My array1 has 800w into a 100/50 and I can only recall four or five times in 4 years that I have seen 700w). You will need #6awg or #4 awg from the SCC to the batteries/bus bar. This one is a little cheaper $184.45

150/60
This SCC will capture the occasion bump into 700+ watts, You will need #4 awg from SCC to batteries/bus bar. This SCC can connect to 48v battery if you think you may ever reuse it in a different system. Costs s but more $351.05

You should have a breaker or switch to “turn the sun off” between the panels and the SCC. And if the SCC is any distance from the battery/bus bar the wire going to the SCC should be fused.

Good Luck - any questions?

I think to keep it simple, 4s2p wiring. Provided I have equal sun exposure to all the panels, will the batteries all charge at the same rate under 4s2p and 2s4p? I will likely pull larger wire to the controller now and while I am doing it, and it probably won't hurt to have done, if it gets wired differently down the road. In respect to shade, I don't have an overly big rv, so if one area is slightly shaded, it won't be long until another area is too as the day goes by. The better half like sunny candy places anyhow. Do you prefer to hook your panels together for rigidity or space them a bit, in case one needs replaced?

 

[hazed]

Well, first question is "Is shading an issue where you go?" In my case I live in trees so I ended up going 4p through a combiner box so I can keep each panel running independent from each other, but it was no big deal for me to run 8awg wire down the size to my distribution box. With 8 panels up top you can either go 4s2p or 2s4p quite easily. Being mounted flat on a roof you're always going to take a hit in production because unless you're on the equator at solstice, the sun is shining at an angle on the panels instead of straight on.

You can see my process and what I did on my camper in my other thread. It might be informative.

OK, so let's do some napkin math with rough numbers:

100w panels are usually in the region of 22Voc, 20Vmp and 5a, so we'll use those numbers for the guesstimates. The rule of thumb for 12v systems is you need 10a of SCC per 100w of panel, so you're going to need somewhere in the 60-80a range. 60 would probably be fine which are pretty available.

8s: 176Voc, 160Vmp, 5a is pretty light load so 16awg would work, 14awg would be better. This lets you re-use the wire coming through the walls without having to re-fish anything. Down sides are that you'll need a really expensive MPPT to take advantage of that high voltage, and if a single panel gets shaded your whole system is nerfed to the lowest panel's production. Works fine in the desert, pretty worthless anywhere else.

4s2p: 88Voc, 80Vmp, 10a so 14awg would be fine, 12awg would be better. You Might have that size already in the wall so you can re-use that wire. No combiner box needed, just a Y-splitter which makes wiring easy. Pretty much any half decent brand of MPPT controller will work well with those specs. Downside is that if a single panel gets shaded by a branch or anything, half your panels are nerfed and not producing squat.

2s4p: 44Voc, 40Vmp, 20a so you'll need at least 12awg wire, 10awg or 8awg preferred. You'll need a combiner box or a stack of fuses (which introduces multiple points of failure) which adds about $100 or so. Again, any half decent brand of MPPT will play nicely with those voltage and amperages. The plus side is if a single panel gets shaded, 3/4 of your strings are still plugging along just fine. The down side is the requirement for an extra piece of hardware (the combiner box or fuses) and the extra cost of the thicker wire.

8p: 22Voc, 20Vmp, 40a, so 8awg really good silicone wire, or 6awg thicc boy wire. You'll have to fish expensive thick wire through the walls, you'll need an 8 position combiner box ($$$!!) but if a single panel gets shaded you're other 7 are plugging right along. May have problems reaching starting voltage in cloudy weather. Just don't.

3s/4s for 7 panels: you'll have to run another set of wires through the roof and get a 2nd SCC involved because you can't get 7 panels to parallel up. At that point you have 2 arrays, a 3s and a completely separate 4s array with their own wires and their own controllers. That's why everyone is trying to get you into that 8th panel.


OK, those are how the different configurations work out with each other using example math. Does that help make sense?

Could you tell me, if all sun exposure to the panels is the same, do lifepo4 batteries charge at the same rate regardless of how the panels are wired? Can they only accept so much in regards to volts, amps and watts through the mppt to the batteries? If not, I may consider wiring for more volts vs amps or the opposite, if so.

 

[Rocketman]

Could you tell me, if all sun exposure to the panels is the same, do lifepo4 batteries charge at the same rate regardless of how the panels are wired? Can they only accept so much in regards to volts, amps and watts through the mppt to the batteries? If not, I may consider wiring for more volts vs amps or the opposite, if so.

The mppt will take “whatever*” comes in and convert it to the proper volts and appropriate amps.

To the mppt it doesn’t matter if the 600w that is arriving comes as 20v and 30a or 80v and 7.5a either way it will output 14.4v @ 41.6a.

There can be some differences with shading vs higher voltage and lower amp loses - but that is site dependent.

* “Whatever” is 5v above the battery voltage to start the process. Higher series starts earlier &goes later - but there is few amps, and higher volts = lower amps = lower voltage drop - lost power. More panels in parallel sometimes means better power in various shade conditions.

 

[hazed]

re transfer switches.

You will need a transfer switch for the inverter to power stuff in the RV.

I have seen four different approaches.

#1 inverter only 120v outlets. You run new outlets - these outlets are powered by inverter only, no other outlets are powered. (My dad did this, needed inverter power at night near the bed - worked for him and his needs).

#2 transfer switch built into inverter. My Victron Multiplus inverter/charger has this. This opens up lots of other options too - power assist,etc. everything just works…

My favorite is#2 with my Victron Multiplus- but your needs may be different than mine.

Do you know of an AIO inverter charger that has an input from the transfer switch for shore or genny power and an input for alternator charging with a BIMM, so it helps charge going down the road? Then, I could eliminate the existing controller and power everything through the AIO, or do you suggest a better route?

 

[Rednecktek]

Do you know of an AIO inverter charger that has an input from the transfer switch for shore or genny power and an input for alternator charging with a BIMM, so it helps charge going down the road? Then, I could eliminate the existing controller and power everything through the AIO, or do you suggest a better route?

Never heard of such a beast, you're probably going to need an independent alternator charging source to make that happen.

 

[hazed]

Never heard of such a beast, you're probably going to need an independent alternator charging source to make that happen.

Ok. I have a BIMM on it now that charges from the alternator now, I believe, and probably through the factory controller. So, I will have so scratch the head and see if that will work, where that's wired through and may have to utilize a DC to DC charger. As always, thank you to all have chimed in with your knowledge.

 

[hazed]

Never heard of such a beast, you're probably going to need an independent alternator charging source to make that happen.

Ok, as I slowly chip away at my project, I now have 800w of solar (8×100) wired 4s2p. Fortunately, my factory wire coming in from the panels to the controller is 10awg.

My next head scratcher is this. I have 4 12v 280ah lifepo4 batteries being delivered shortly and each comes with a 250bms built in. Since my rv is currently wired for 12v, I was looking to wire the 4 batteries in parallel.

Now comes the wiring question and it appears the "halfway" method may work. Since I will have to split the batteries up, due to space restrictions, I will have a pair on each side of the rv in a storage area.

It sounds like I want to have equal lengths round trip to the inverter to keep the wear on the batteries the same. If I am not pulling more than 20-30 amps on a regular basis, how important is equal length wiring on Li with a BMS on each? Can I oversize wire for less resistance? And, does the spacing distance between each pair of batteries have any affect on charging equally?