Mismatched Panels - 2 Panels Parallel to 1 in Series

[eXodus]

Hello,

I got an RV with 2x 72V 435W Panels and found space to add another 445W panel.

Constrains:
Unable to find another of the 72V panels - those are like unicorns.
MPPT max voltage of 150V and I don't necessary want to add another MPPT controller since my Growatt has plenty of capacity left.



So I came up with this idea. Please tell me if that is possible or not?

When I do my math correctly - 72V + 41V = 114V and the total amps are limited through the lowest panel in the series. that would be 11A instead of (6A+ 6A) 12A

114V * 11A = 1254W usable of 1315W theoretical - doesn't sound to bad or? Am I missing something?

 

[Symbioquine]

Your math is right and it is possible, but it could be useful/interesting to consider some additional details...

Safety: It probably isn't a problem, but double-check the max series fuse rating for the 72V panels and the maximum series voltage for the 41V panel. (These are probably numbers like 15A and 600V respectively so you'll likely be fine, but for the sake of completeness worth checking.)

Performance: You probably want to look at the peak power numbers (current=Imp and voltage=Vmp) on the panels or - even better - the power output curves. Depending how well those line up, they'll tell you something about how much of the time it will behave as your math above describes. If the curves are very different shapes (just voltage vs current and/or also including temperature coefficients), you might never get very close to that theoretical performance. Hint: You might need WebPlotDigitalizer to take the graphs from the panel datasheets and overlay them.

 

[eXodus]

Safety: It probably isn't a problem, but double-check the max series fuse rating for the 72V panels and the maximum series voltage for the 41V panel. (These are probably numbers like 15A and 600V respectively so you'll likely be fine, but for the sake of completeness worth checking.)

Max. System Voltage 1500 V UL & 1000 V IEC
Maximum Series Fuse 15 A

Both Panel types have 1000V / 15A ratings

Performance: You probably want to look at the peak power numbers (current=Imp and voltage=Vmp) on the panels or - even better - the power output curves

the numbers I quoted are the Vmpp and lmp like you suggested. Thanks for checking thought !

the 435W:
Vmpp 72V
Voc 85.7V

the 445
Vmpp: 41V
Voc: 49V

I have no idea where I can find the power curves.
What do you think could happen? that the one panel is going to restrict more? Isn't the bypass diode taking care of that?

 

[Symbioquine]

I have no idea where I can find the power curves.

They might be in the datasheet for each panel.

What do you think could happen? that the one panel is going to restrict more? Isn't the bypass diode taking care of that?

No, what I'm talking about is probably unrelated to the function of the bypass diodes.

Just that it isn't necessarily accurate to assume that the Isc/Vmp points for different panels will occur at the same irradiance/temperature values. If the curves are very different shapes then the composite curve for the series/parallel array you're talking about might be (somewhat) lower than one would expect just looking at the Isc/Vmp numbers.

I wouldn't worry about it too much, you can probably just test it out and see if the performance is enough to justify the effort of mounting the extra panel. (My guess is that it will be.)

 

[Rednecktek]

Wouldn't you end up with 41v @ 6a?

 

[Tecnodave]

A better solution would be 2 standard off the shelf 72 cell panels (36 volts ea. @MPP) wired in series and then put in parallel with your 72 volt panels. In that case you would not be limiting either string (s) of panels. The voltages would be the same but each string would operate at their own current.

 

[eXodus]

A better solution would be 2 standard off the shelf 72 cell panels (36 volts ea. @MPP) wired in series and then put in parallel with your 72 volt panels. In that case you would not be limiting either string (s) of panels. The voltages would be the same but each string would operate at their own current.

I still got two 34V 315W panels sitting in my garage leftover from another project. Do you think that would be close enough? 68V vmpp vs 72V?

Not sure If I can fit them on the roof. But I might be able to find some smaller panels with a similar voltage.

 

[Tecnodave]

There is a small mismatch but the percent lost would be in the 1-2% range. Not enough to worry about.

 

[eXodus]

There is a small mismatch but the percent lost would be in the 1-2% range. Not enough to worry about.

yeah I don't have enough roof real estate for another two residential panels.
The space I'm working with is 60x80 inches. The best for coverage I could come up with - 3x 200W panels

Those are 58.3 x 26.4 (x 3 = 79.2 inches)

If I put 3 side by side, I end up with 600W added capacity. vs one residential panel which could be at 400w.

The 200w panels have 20V Vmpp x3 = 60V Which is a bit to far away from 72V from my main system to put them in parallel.

In series - I would be ending up with 60V + 72v = 132V but 85V + 75V Voc = 160V exceeding my Growatts rating there....

Need a second MPPT for this configuration...

 

[TimE]

Why don't you just put a seperate MPPT on the single 41v panel? Way more efficiency

 

[eXodus]

Why don't you just put a seperate MPPT on the single 41v panel? Way more efficiency

Poking another hole in the roof, running another 30ft of wire behind cabinets and having capacity left at my current MPPT.

I even have another MPPT laying around from different project.

So the only thing holding me back is laziness and my relentless driving for simplicity and for efficiency. (two MPPT, double standby draw)

 

[eXodus]

Found another panel

410 Watt Solarever Solar​

Model #	SE-182*91-410M-108N
Watts	410W
VOC	37.12V
ISC	13.08A

Panel Dimensions

67.87L x 44.64W x 1.37H"

72V + 32V vmp = 104V
x12A = 1248W

From a maximum for 445w + 445w + 410W = 1300w

so very close to using the maximum of the panels with my current Inverter.

 

[sunshine_eggo]

I like that a lot better as you have a lot more Voc margin. Should be good down to about -18°C

 

[eXodus]

Just bought a used 400W panel.
Q.Peak Duo. BLK ML-GL10+

VOC: 45.30V
VMPP: 37.13
ISC: 11.14A

The total System will be:
VOC: 136V (nice under the 145V limitation)
VMPP: 109V (under the 115V MPPT limitation)

Total Power: 109V * 11A = ~1200W

The panels are total at 1270W - so I'm leaving 70W peak power on the table in exchange to not use a second MPPT - which should compensate in standby draw.

 

[grizzzman]

yeah I don't have enough roof real estate for another two residential panels.
The space I'm working with is 60x80 inches. The best for coverage I could come up with - 3x 200W panels

Those are 58.3 x 26.4 (x 3 = 79.2 inches)

If I put 3 side by side, I end up with 600W added capacity. vs one residential panel which could be at 400w.

The 200w panels have 20V Vmpp x3 = 60V Which is a bit to far away from 72V from my main system to put them in parallel.

In series - I would be ending up with 60V + 72v = 132V but 85V + 75V Voc = 160V exceeding my Growatts rating there....

Need a second MPPT for this configuration...

There is always room for more panels.

 

[eXodus]

There is always room for more panels.

I saw that video and already drew up some ideas:

https://diysolarforum.com/threads/removable-panels-on-side-of-rv.69721/post-923465.

But I ended up going up a different route now, since I'm going to move soon - I need a trailer for my home base stuff.

Building a Cargo-Solar Trailer now - you can fit much more solar on one of those. Nice square box with nothing on the roof.

Solar Cargo Trailer Built - System Check Please

Currently planning a 7x14 or 7x16 Trailer - for towing behind my Class A RV to move to a new property. There we are going to built a small house. During construction this trailer should also server as base of operations to support the whole thing. Storing tools. Washing machine. Etc...

diysolarforum.com

3680W on the trailer, plus the 1200W on the RV. Almost 5KW... sounds like a bit overkill now that I write it down. But it's going to power the building site, and my future home. So should be fine