Over paneling

[Horsefly]

What you measure is certainly correct, at least at that very moment in time. ?

There are at least two things people use the spec voltages for. You would use the Vmp (voltage at max power) along with Imp (current at max power) to size your PV array, wire, and components for what your system needs are. The second thing is to use the Voc (open circuit voltage) to make sure however you plan to hook up your panels will not burn out your charge controller.

If you read the rest of the thread, you'll find discussion of how the voltage of the panels goes up as the temperature goes down, and so you need to account for that, not just counting on the measurement you take.

 

[Fripi]

Very interesting post, I have a maybe completely stupid question regarding this voltage limit:

• Lets say my limit is 200v
• I have for example 10 panels in series, their total voltage of 250v, exceeding the limit

Does anything change if 1/2 panels are facing south and 1/2 facing west, so that they are never 100% active at the same time of the day ?

 

[wattmatters]

Does anything change if

No. They will still present too high a voltage potential.

In any case you should not have such a string arrangement. If you have strings across two orientations, they should be of the same size and connected in parallel (or each connected to their own MPPT), and not in series.

 

[PVorBust]

Maximum Voltage Calculator​

Solar panel open circuit voltage (Voc)

45.9

Number of solar panels wired in series

3

~

Maximum open circuit voltage of your solar array:

= 148.7V

Sound about right?

Almost.
45.9 * 3 = 138.7 VOC

This gives you an acceptable margin for 150V input device.

 

[PVorBust]

Mine always read about 65 volts with
180w panels x 6
In series.

Are you sure?
65V is very low for 6 panels in series - that's less than 11V per panel.

 

[Fripi]

No. They will still present too high a voltage potential.

In any case you should not have such a string arrangement. If you have strings across two orientations, they should be of the same size and connected in parallel (or each connected to their own MPPT), and not in series.

ok so in my example, what would be good is 2 series of 5 panels, each their orientation, and those 2 series in parallel, is that called 5s2p
this way the volt doesn't add up and stays at 1/2 (125 based on imaginary numbers of my example) and amp will double ?

How does it react if the 2 series doesn't have the same voltage, like 4 panels in one serie and 6 in the other, will the voltage be the one of the biggest serie ? (not sure i'm clear in that question)

 

[Horsefly]

Almost.
45.9 * 3 = 138.7 VOC

This gives you an acceptable margin for 150V input device.

Be careful there. You are ignoring the impact of temperature on Voc. The message you replied to shows that the calculated total voltage at the coldest anticipated temperature comes out to 148.7V, which is very close to the 150V max. Still may be OK, if the minimum temperature is correct.

 

[PVorBust]

Understood, but when Florida has low temps, is there is much sun?
1.3V is an uncomfortably slim margin, I'll grant you, but will it ever happen?

 

[Daddy Tanuki]

Be careful there. You are ignoring the impact of temperature on Voc. The message you replied to shows that the calculated total voltage at the coldest anticipated temperature comes out to 148.7V, which is very close to the 150V max. Still may be OK, if the minimum temperature is correct.

to piggy back on this, my system using the morning star calcs gives me a 138.2 volt system at 75f. at -5 my coldest in the morning (not record coldest I might add) I am at about 145 volt so what horsefly is suggesting is valid as recorded by an actual system in use gambling is dangerous for your SCC or your AIO. a little colder and i could be screwed.

 

[Daddy Tanuki]

Understood, but when Florida has low temps, is there is much sun?
1.3V is an uncomfortably slim margin, I'll grant you, but will it ever happen?

a little mist to cool the panels and a breeze, coupled with the sun suddenly coming out... it could happen and the question is how mucha re you willing to risk for compnents?

 

[Horsefly]

Understood, but when Florida has low temps, is there is much sun?
1.3V is an uncomfortably slim margin, I'll grant you, but will it ever happen?

Doesn't matter much if there is really bright sun. The idea of hyper-Voc - that will potentially damage a SCC - is with no current produced by the PV array. It is truly an open-circuit voltage. I would not take the risk.

Edit: Saw that @ken morgan was replying as I was about to post. So I should just say.... what he said!

 

[PVorBust]

Fair enough.
My gear isn't exposed to unpredictable swings like that, so I could live with a 10V margin.

Everyone should take advice as guidance only and take into account their local conditions.

Personally I've networked all my SCC's, so I've been able monitor "my" real world conditions.

 

[timselectric]

It only has to go badly once.
Then you get to buy the replacement.

 

[wattmatters]

ok so in my example, what would be good is 2 series of 5 panels, each their orientation, and those 2 series in parallel, is that called 5s2p
this way the volt doesn't add up and stays at 1/2 (125 based on imaginary numbers of my example) and amp will double ?

If a panel has a nominal operating voltage of 25 V then a 5S2P array will be 125 V nominal. The current from each string will be added together. It won't be exactly double one string as you would expect each string to generate different amounts of power given their different orientation.

How does it react if the 2 series doesn't have the same voltage, like 4 panels in one serie and 6 in the other, will the voltage be the one of the biggest serie ? (not sure i'm clear in that question)

The MPPT will operate at a voltage which is sub-optimal for both strings.

Have a look at these examples of testing different combinations:

How to wire mismatched solar panels. Wire in parallel or in series?

Video series testing wiring mismatched solar panels in parallel vs. wiring in series to maximize solar PV output with an MPPT charge controller.

www.altestore.com

 

[PVorBust]

It only has to go badly once.
Then you get to buy the replacement.

I understand your point, but please give an exact number you consider acceptable (you don't accept mine).
I have justified what is acceptable in my operating condition (and why).

Lowest expected temperature
32°F
Temperature coefficient of Voc
-0.32%/°C
-
Calculate
Maximum open circuit voltage of your solar array:
= 148.7V

Could you show me the maths for this please?
Making a bunch of assumptions, my numbers come out at 146.7V allowing for a 0°C min temp.

 

[wattmatters]

Could you show me the maths for this please?

I'm guessing it's from a calculator which does the maths for you.

But in essence, just work out the temperature differential between the Voc at STC and the lowest expected temperature and multiply by the Temperature coefficient and the Voc at STC and add to the Voc at STC.

e.g. say
Voc at STC is 138.7 V (STC is 25°C)
Temp coefficient is -0.32% / °C

If lowest expected temp is 0°C (STC is 25°C)
Then we have
(0°C - 25°C) * -0.32%/°C * 138.7 V + 138.7 V
= 11.1 V + 138.7 V
= 149.8 V

If lowest expected temp is -10°C then:
(-10°C - 25°C) * -0.32%/°C * 138.7 V + 138.7 V
= 15.5 V + 138.7 V
= 154.2 V

 

[PVorBust]

If lowest expected temp is 0°C (STC is 25°C)
Then we have
(0°C - 25°C) * -0.32%/°C * 138.7 V + 138.7 V
= 11.1 V + 138.7 V
= 149.8 V

That's what I tried to do (but my windows calc added 8 instead of 8%).
Without knowing anything about the SCC, I agree that's too close unless you are confident you'll never see the cold temps and decent sun at the same time.

For interest, my 250W panels do not reach claimed VOC. The ISC is as claimed and power out is nominally as expected

 

[timselectric]

I understand your point, but please give an exact number you consider acceptable (you don't accept mine).
I have justified what is acceptable in my operating condition (and why).

I like to leave at least a 15% cushion, after calculated temperature coefficient. In the right conditions it can drift higher than expected. Last February my system seemed to defy the math a few times. So, I lowered each string by one panel. It's not worth the gamble to me. Pushing it to within one or two volts is just asking for trouble in my opinion.

 

[Horsefly]

Could you show me the maths for this please?

I showed the equation in post #27 above:

https://diysolarforum.com/threads/over-paneling.38030/post-590951

and demonstrated the calculation using that equation in post #35:

https://diysolarforum.com/threads/over-paneling.38030/post-592229

 

[Fripi]

I looked at your equations, also videos about mismatched panels and some calculators, now I try to put it al together to understand well
sorry if this is somehow redundant ?

I tried to find all info about the hardware but so much info I got lost about what is the most relevant with Vmp, Voc, Vmax...
The situation is on 2 balconies, one south, one west, that's why I go with extra light panels:

Hardware :

• EcoFlow Delta Pro : 1600W, 150V, 15A
• 1 x EcoFlow 400W - TKVoltage -(0.33+/-0.03)%/k
• 6 x PiE AIR superLIGHT HC 200W - Voltage temperature coefficient -0.33%/°C

Record lowest temp all time in my region : -13 °C

1. Serie 1 : 1 ecoflow 400 + 2 Pie Air 200
   W: 400 + 200 + 200 = 800W
   Vmp: 41 + 18 + 18 = 77V
   Voc: 48 + 22 + 22 = 92v
   Vmax: 54 + 24.8 + 24.8 = 103.6V
   Isc: 11 + 11.7 + 11.7 = 11A (keep the lowest)
   Imp: 9.8 + 11.1 + 11.1 = 9.8A (keep the lowest)
   
   
2. Serie 2 : 4 Pie Air 200
   W: 4 x 200 = 800W
   Vmp: 4 x 18 = 72V
   Voc: 4 x 22 = 88V
   Vmax: 4 x 24.8 = 99.2V
   Isc: 4 x 11.7 = 11.7A
   Imp: 4 x 11.1 = 11.1A
   
   
3. S1+S2 in Parrallel
   W: 800 + 800 = 1600W
   Vmp: 77 + 72 = 72V (keep the lowest)
   Voc: 92 + 88 = 88V (keep the lowest)
   Vmax: 103.6 + 99.2 = 99.2V
   Isc: 11 + 11.7 = 22.7A (only 15A will be used)
   Imp: 9.8 + 11.1 = 9.8A

In this case V seems ok below 150, A exceed the 15 limit some will be lost
and I will only get in best case 88V x 15A = 1320W

To get closer to the 1600 I could add 2 extra Pie Air 200, one to each serie to get to 110V and 1650W
9 panels in total, from point of view of W and A it's over panneling but V is ok so no risk ?

 

[Tym]

Are you sure?
65V is very low for 6 panels in series - that's less than 11V per panel.

Sorry. 3 panels in series and 3 panels in parallel.