combining 12V and 24v panels?

[vanlifeftw]

Hi folks!
Thanks Will and company for making such an awesome community! I wanted to give my questions some context and talk about my project in general. I'm self-building a Ford Transit high roof (non-extended) camper. I have 16 x 304Ah factory direct Eve cells which I intend to assemble into 2 x 24V batteries. (I probably have another post coming about BMS selection). My idea was to have "full solar" on the roof covering my fans. I ended up buying 4 x 24V 200W Rich Solar panels. I decided to go with a strategy I learned about on the fordtransitusa forum, where the fan covers are removed and the solar panels become the thing that keeps rain out. Later I realized that the rear roof support constrained my fan placment very specifically to the point that I can really only fit 3 of the 4 Rich 200W panels mounted. DANG IT. I see maybe 2 ways to recover from this and pack as much solar on the roof as possible.

1) find a > 200W panel that's very close to 58.7 in by ~ 40 in. This has not been a fruitful search. Very open to suggestions!

2) try to incorporate one of Rich Solar's slim 12V 100W panels to give me a little more power. I contacted support about mixing these. They were not encouraging, but I think the way I asked the question prevented me form getting a yes. Also I'm sure it's in their best interest to practice CYA.

SO on to the question. Can I wire my panels like this? Please forgive the Fritizing diagram. The power rails of the breadboard represent a combiner inside the van. The resistor represents system load. I don't think I can put a 12V and 24V in series because the Max Power Current spec of both panels exceeds the Maximum Series Fuse Rating spec of the 100W slim. Also attaching the specs of both panels. I planning on using Victron Energy BlueSolar MPPT 150|35. Thanks for your time!!

 

[sunshine_eggo]

The 12V and 24V panels shown can be placed in parallel. Their Vmp is very close.

You appear to have the 12V in parallel and the 24V in series. No. This will not work.

You could series the 12V and series the 24V and then place those two strings in parallel.

The 12V and 24V must not be in the same series string as the 12V panels will cut the power output of the 24V panels in half.


These two scenarios would work:

 

[vanlifeftw]

Hey @sunshine_eggo, thanks for taking the time. Unfortunately, I am confused by your message. I like the first of your proposed scenarios the most. But doesn't this statement contradict the diagram?

The 12V and 24V must not be in the same series string

I don't know a lot obviously or I wouldn't be here, but brother, this diagram is showing 12V and 24V panels in series. (and 2 series strings wired in parallel) Can you help clarify?




I was under the impression that the combined current of these panels would blow the fuse on the 12V panel - in addition to your voltage drop point. Oh, wait. I think they might get averaged instead of summed.

 

[sunshine_eggo]

Hey @sunshine_eggo, thanks for taking the time. Unfortunately, I am confused by your message. I like the first of your proposed scenarios the most. But doesn't this statement contradict the diagram?


I don't know a lot obviously or I wouldn't be here, but brother, this diagram is showing 12V and 24V panels in series. (and 2 series strings wired in parallel) Can you help clarify?




I was under the impression that the combined current of these panels would blow the fuse on the 12V panel - in addition to your voltage drop point. Oh, wait. I think they might get averaged instead of summed.

I'm sorry. I must have been half asleep in the middle of the afternoon. I identified the panels on the left as 100W and the panels on the right as 200W. I missed several other details.

1) This will create other complications. I'm confused why you could fit another panel even larger than the one that won't fit. Are you considering replacing all panels with something else? Maybe I catch on down below in 3).

2)

Both the 100W and 200W panels you have linked are 12V panels.

If you place a 100W panel in series with a 200W panel, you will cripple the 200W panel to perform closer to 100W, i.e., the 200W would operate at the current of the 100W panel. You add voltage in series and current is restricted to the LOWEST panel in the string. You do not add current.

3) Other options:

Place the 3 12V 200W panels you can fit in series as your 150/35 MPPT can more than handle it.
Find another higher power 60 or 72 cell panel with a highly similar Imp and place in series with your two 12V 200W panels.
Find another higher power 24V panel with comparable Vmp (40.8V) and place in parallel with your 2S 12V 200W panels.

 

[vanlifeftw]

Dang it, sorry I put the wrong info for the 200W panels - they are 24V and I acutally have 4 of them on hand. Everything I wrote I believe is accurate though. I'll reattach the correct info-graphics for both panels. The 100W 12v Slim is just one I was considering - to use the smaller available space I now find myself with. 4 x Rich 200w 24V panels will fit fine on top of regular length Ford Transit. BUT if you decide that the panels need to adequately cover roof fans that are spaced as far apart as possible, that brings the outermost panels in just enough to crowd out one panel. Leaving me with 600W instead of 800W. I was hoping I could - for not an insane amount of money - buy my way out of this situation with either by adding ONE Rich 100W 12V Slim panel to the existing 3 Rich 200W 24V panels. OR find a really big possibly ~350W panel that might fit something close to ~58.7 in by ~ 40 in that I would use with only 2 x 200W Rich 24V panels. Sorry that wasn't clear earlier. I think this idea may be a bust as I'd really like it not to be over 60in wide and that's a big ask apparently.

Now that the constraints are properly communicated - I hope let's start over: If you had 3 Rich x 200W 24V panels, is there an optimal configuration one could make them work with one Rich 100W 12V slim panel?

 

[MichaelK]

Since the panels all have close Imp's, you could wire the four of them in series. In series, the amp flow gets reduced to the max of the lowest rated panel, which in your case would be the 100Slim at 5.13A.

If you wired just the three 200W panels together you'd get (37.6V + 37.6V + 37.6V) X 5.32A = 600.1W

If you wired the three, plus the 100W together you'd get (37.6V + 37.6V + 37.6V +19.5V) X 5.13A =678.7W

So, you'd get an extra 78W by wiring that 100W panel in series.

Keep in mind that these are pen and paper calculations. What happens in the real-world is always slightly different.

 

[sunshine_eggo]

Since the panels all have close Imp's, you could wire the four of them in series. In series, the amp flow gets reduced to the max of the lowest rated panel, which in your case would be the 100Slim at 5.13A.

If you wired just the three 200W panels together you'd get (37.6V + 37.6V + 37.6V) X 5.32A = 600.1W

If you wired the three, plus the 100W together you'd get (37.6V + 37.6V + 37.6V +19.5V) X 5.13A =678.7W

So, you'd get an extra 78W by wiring that 100W panel in series.

Keep in mind that these are pen and paper calculations. What happens in the real-world is always slightly different.

+1

Need an MPPT with a 150Voc limit. Will be good down to -8°C. Below that, and you'll pop the 150Voc limit and likely the MPPT.

 

[vanlifeftw]

Ok! Thanks, y'all. This really helpful. Re: cold temps. I hadn't considered the efficiency boost as potential problem - is there a standard curve for figuring out what voltage a panel might output at a given temp? Or would the manufacturer provide this? I am full time, and -8C ain't s$!7 around here. On the low end, I could easily face -21C.

 

[MichaelK]

Ok! Thanks, y'all. This really helpful. Re: cold temps. I hadn't considered the efficiency boost as potential problem - is there a standard curve for figuring out what voltage a panel might output at a given temp? Or would the manufacturer provide this? I am full time, and -8C ain't s$!7 around here. On the low end, I could easily face -21C.

I like to use Midnight Solar's string calculator for more accurate numbers. Here's a link to it. Just plug in your values, and what the coldest winter low could ever be and you'll get it.

MidNite Solar - Classic Sizing Tool.

MidNite Solar is the industry leader and manufacture of quality Renewable Energy System electrical components and E-Panels.

www.midnitesolar.com

 

[sunshine_eggo]

Ok! Thanks, y'all. This really helpful. Re: cold temps. I hadn't considered the efficiency boost as potential problem - is there a standard curve for figuring out what voltage a panel might output at a given temp? Or would the manufacturer provide this? I am full time, and -8C ain't s$!7 around here. On the low end, I could easily face -21C.

A conservative Voc temp coefficient is -0.4%/°C, i.e., most panels are better than that. If one is not available from the datasheet, it's what I use.

Formula for determining limit:

25°C + (Voc limit - series Voc)/(series Voc)/Temp coefficient

Example:

25°C + (150V - 135V)/135V/-0.4%/°C
25°C + (15V/135V)/-0.004/°C
25°C + (.111)/-.004/°C
25°C + (-27.7)°C = -2.7°C

Thus at -2.7°C, the series Voc value will equal the Voc limit.

 

[vanlifeftw]

Awesome, thanks. Could you please double check my math for 3 X 200W 24V panels?

25°C + (150V - 112.8V)/112.8V/-.004/°C
25°C + (37.2V/112.8V)/-.004/°C
25°C + (0.329787234042553)/-.004/°C
25°C + -82.4°C = -57.4°C

 

[sunshine_eggo]

Awesome, thanks. Could you please double check my math for 3 X 200W 24V panels?

25°C + (150V - 112.8V)/112.8V/-.004/°C
25°C + (37.2V/112.8V)/-.004/°C
25°C + (0.329787234042553)/-.004/°C
25°C + -82.4°C = -57.4°C

Yep. Hope it never gets that cold where you live. That's the Earth trying to kill you.

If you find that your panel's actual temperature coefficient is higher (-0.33%), you have even more safety margin.