Panel voltage

[memilanuk]

Looking to order four 200 watt panels for the roof of our RV (truck camper). Using Rich Solar's panels as an example, if they are the same physical size, but available in "12v" or "24v" models... is there a compelling reason to go one way or the other? I'm planning to keep the rest of the DC system @ 12v.

 

[Mcgivor]

The answer depends on the controller being used, with PWM 12V nominal panels would be required, MPPT on the other hand needs higher voltage so 24V nominal panels would be used.

 

[chrisski]

If you have an MPPT SCC, if you set them up in series, the 12 volt is probably better. If you set them up in parallel the 24 volt is probably better.

The “12 volt” is actually 18 - 22 volts volts. I am doing a 12 volt to 24 volt upgrade and I have 3 in series on my roof. I can see up to 66 volts on those three panels. Had I put three 24 volt panels which actually push 33 volts, I may exceed the 100 volts of the controller.

On a separate charge controller, I also have two more ”12 volt” panels in series. These also put out 18-22 volts and are great when one panel is shaded, I get way more output from these panels in paralllel with one shaded then when one of my series panels is shaded. Series and shading kills output.

I’m now up grading to 24 volts. My series panels will perform exactly as before. My parallel panels will not have enough volts out to charge batteries. TO charge a 24 volt battery, you need around 29 volts for the MPPT to work. I have to rewire the parallel panels and put them in series.

If you use a PWM controller, ensure whichever panel you purchase matches the voltage input for The SCC. The PWM has a very narrow range of voltages.

 

[memilanuk]

I have no plans for a PWM controller. I was looking more along the lines of a Victron 100/50... but I am trying to avoid the $$$ of jumping up to a 150/85 if I don't absolutely have to.

I figured with the 12v panels, running 4S might be skirting a tad close (4 x Voc of 24.3 = 97.2vdc @ STC) to the upper voltage limit if the outside temps drop significantly. Configuring them for 2S2P would appear to solve that... but the same panels in the "24v" config running 2S2P would be @ 2 x Voc of 45.4 = 90.8vdc @ STC. Putting four 24v panels in parallel would exceed the capacity of the wiring in place from the roof gland down to the battery.

 

[chrisski]

I have no plans for a PWM controller. I was looking more along the lines of a Victron 100/50... but I am trying to avoid the $$$ of jumping up to a 150/85 if I don't absolutely have to.

I figured with the 12v panels, running 4S might be skirting a tad close (4 x Voc of 24.3 = 97.2vdc @ STC) to the upper voltage limit if the outside temps drop significantly. Configuring them for 2S2P would appear to solve that... but the same panels in the "24v" config running 2S2P would be @ 2 x Voc of 45.4 = 90.8vdc @ STC. Putting four 24v panels in parallel would exceed the capacity of the wiring in place from the roof gland down to the battery.

Sounds like you have a good handle on things. If you do decide on the 2S2P config for 12 volt panels, then you can always go up to 3S2P.

I have four Victron SCCs now. 3 For the RV. Two are on the roof, The 100/50 for a 600 watt 3S2P, 100/30 for a 350 watt set up, and 100/30 for a 400 watt portable panel. Better planning could have got that to One SCC for the roof, and an additional one for the portable panels. The last SCC is a 75/15 for a portable generator set up.

One plus for the 150 volt Victrons is they can be upgraded to a 48 volt system. If You see yourself going to 48 volts, that maybe a good choice. My 24 volt upgrade would be a 48 volt upgrade, but in part my 100 volt SCCs only charge 12 or 24 volt batteries, not 48.

 

[memilanuk]

Sounds like you have a good handle on things. If you do decide on the 2S2P config for 12 volt panels, then you can always go up to 3S2P.

I have four Victron SCCs now. 3 For the RV. Two are on the roof, The 100/50 for a 600 watt 3S2P, 100/30 for a 350 watt set up, and 100/30 for a 400 watt portable panel. Better planning could have got that to One SCC for the roof, and an additional one for the portable panels. The last SCC is a 75/15 for a portable generator set up.

One plus for the 150 volt Victrons is they can be upgraded to a 48 volt system. If You see yourself going to 48 volts, that maybe a good choice. My 24 volt upgrade would be a 48 volt upgrade, but in part my 100 volt SCCs only charge 12 or 24 volt batteries, not 48.

Highly doubt I'm going to go 48v on a truck camper. Probably not going to bother with 24v, as the distances are short and the loads not that large. Yes, there will be some 4/0 involved... but that's less of a PITA than the alternatives. Similarly, four 200w panels is all I can fit on the roof, unless I start going nuts and building an elevated superstructure to go over the roof vents, air conditioner, etc.

I've toyed with the idea of having separate 100/30 controllers, one each for a 2S string. I've seen a few mentions in videos here and there that cite it as a method for reducing the effects of shading, enhancing redundancy (reduce single point of failure), etc. In theory, if I were to apply a correction factor for cold temps to the panel voltages mentioned above, either the 4S 12v or 2S2P 24v panel configs could easily exceed 100vdc if I were to do some winter camping (assuming that I got a rare clear-n-bright day). At that point... the 12v panels in 2S2P to a 100/50 controller would still be under the limits, and two 100/30 controllers running two separate 2S strings would still be cheaper than a single 150/85 controller... but I'd have to figure out how to re-do the wiring on the roof on down, from one pair of MC4 connectors in a roof entry gland box, to some kind of combiner box with separate bus bars - or a second roof gland box

I do have a separate ground-deply Renogy 100w panel with it's own 'onboard' PWM controller that I run out on a 30' cable to catch some sun when we are parked under trees, etc. I'd thought about putting a separate MPPT 75/15 controller in the truck to do the voltage control much closer to the DC bus, rather than out at the wrong end of 30 feet of #10 wire.

 

[chrisski]

but I'd have to figure out how to re-do the wiring on the roof on down,

The going through the roof was the thing I researched the most prior to doing it. The next time I do it will be so much easier. I left extra room for growth and I've filled that up. The next thing I drill will be something to improve cell phone range.

 

[RCinFLA]

Be careful with '12v' panels. Many are using 30 or 32 series cells which do not have enough overhead voltage to run some MPPT controllers with one panel or paralleled panels. The low cell count panels are intended for PWM controllers, unless you run them in series to get voltage up.