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Fra avatar image
Fra asked

How to pair 2 non-identical solar panels

Hello, for some years I have owned a 300w monocrystalline panel with mppt smartsolar 30A regulator on my rv where I live.

Now I would like to increase the power of the photovoltaic.

I'm undecided whether to replace the current panel with a 450 watt half cut ... But. I'd rather add another 300/400 watt to the current.

What do I have to respect to couple 2 non-identical panels?

What happens if one has the MPP voltage at 32 volts and the other at 34v and I put them in parallel?

Or what if one has the max current at 9A and the other at 10A and I put them in series?

Thank you

Solar Panelsolarsolar sizing
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johanndo avatar image johanndo commented ·
You may need a second controller, yours cannot handle such a big array.
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2 Answers
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pwfarnell answered ·

To put panels in parallel they should have the same or close max power voltage Vmp. If they do not match, the combined panels will settle somewhere between one or the other and total power will be down perhaps only on one panel. The closer the Vmp values, the lower the loss in power.

To put panels in series they should have the same or close max power current Imp. If they do not match then the combined Imp will be limited to the the Imp of the lower rated panel.

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Fra avatar image Fra commented ·
Hi thank you very clear answer ... So I could add in series to my 300w panel with Vmpp 32v and max current 9A, a caravan panel at Vmpp 18v and always 9A of max current?

Or add in parallel a half cut panel with Vmpp 34v?

How will the overall output be in your opinion with a parallel between 300w Vmpp 32.7v and a 450w half cut with Vmpp 34v? Is it an acceptable compromise?

Is it necessary, or advantageous, to interpose a diode between the 2 panels in one of these configurations?

In any case the Smartsolar 30A will limit the maximum power to 12v to 440 watts ... So in the future maybe, but MAYBE, I will switch to a 24v system so that I can use up to 880 watts with the smartsolar ... I will have to change inverters and add a 24/12 converter for the utilities of the camper, but the advantages are many in terms of power, cable savings, in the perspective of using a small air conditioner with solar power ....

Thank you
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pwfarnell avatar image pwfarnell Fra commented ·

characteristic-curve-of-the-solar-panel.pngIf you look at a typical solar panel curve, as the voltage reduces below Vmp the current does not vary much so if you are running at 32V instead of 34V the power will be reduced by 32/34 which is 94%, or a 6% loss on that panel. As to diodes, it depends on if the panels have them built in.

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characteristic-curve-of-the-solar-panel.png (116.5 KiB)
Fra avatar image Fra pwfarnell commented · 
5c650c607bec3 (2).pdfTaleSun_300M_2p.pdfYes I know these curves quite well ... In your opinion what is the most advantageous connection between these 2 panels attached in pdf on the smartsolar 30A ?? The 300w is my current one .. While the half cut 340w would be the one to add.
Also would you opt more for these two, or to directly replace the 300 watt with a single 460 watt half cut?
Thank you
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5c650c607bec3-2.pdf (2.5 MiB)
talesun-300m-2p.pdf (284.6 KiB)
Fra avatar image Fra Fra commented ·
Sorry the datasheet of just solar 340w is wrong... In fact is a half cut type
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Fra avatar image Fra pwfarnell commented ·

Sorry, I rephrase the question:

My idea is to buy a 340w half cut panel to add to the current 300w monocrystalline.
The panels in question can collect these technical data:

340w:
Vmpp = 34.4 v
Imax = 9.9 A

300 W:
Vmpp = 32.9v
Imax = 9.12

The coefficients for temperature of Vmpp, Imax and pmax are very very similar between the 2.

Which link is most beneficial?

Series or parallel? I now that mppt controllers conversion efficiency drop if the input voltage is much higher than the output voltage. For example, having 60v panels and 12v batteries has a worse conversion efficiency (something like 1 or 2%) than having 30v panels and 12v batteries.

Should a diode be placed between the 2 in case of shading of one of the 2?

Is a voltage of 60v (series) in any way dangerous in case of contact with the hand that closes the circuit?

I add that the 300w panel has almost 5 years of use and the real Vmpp is between 26.5 and 31v depending on the temperature.
And that obviously its performance compared to the technical data sheet is about 95% due to age
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pwfarnell avatar image pwfarnell Fra commented ·
I have no idea if parallel or series would be better, connect them both ways and see if there is a difference. Higher voltage has lower power losses in the cables, makes up for the lower conversion efficiency.
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pepe1395 avatar image
pepe1395 answered ·

Buy another SmartSolar, that’s it.

Ciao!

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Fra avatar image Fra commented · 
Thanks, but this is not a profitable option for me ...
Rather in the future I might switch to 24v batteries ...
Or replace the current 300w panel with a 450w on the smartsolar and use the old 300w panel with a cheaper but good quality mppt solar controller like Epever ...
Buying a second smartsolar plus a new panel is too expensive for me.
Rather I invest the money in a 24/12 converter to be able to use my 30A smartsolar up to 880 watts of output ....
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