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Looking to please get confirmation on whether one should add or subtract the STC from the PV's max temperature when calculating Vmin.
On the Victron MPPT calculator, it seems to use:
V_min = Voc * (1 + (PVmax_temp + STC) * (-V temp. coeff/100))
However everywhere else i research, it tells me to not add but subtract it as follows:
V_min = Voc * (1 + (PVmax_temp - STC) * (-V temp. coeff/100))
Hi @Harlov
That's because you have to compute the delta from STC and maximum temperature is usually bigger than STC, so (Max - STC). So second one is OK.
My 2 cents....
Humans are used to think in positive terms.
For example, the distance between you and the wall is 2 meters, you'll never think of it being -(minus)2 meters.
So, compute the delta in modulus, meaning "how much from 25 deg.C to maximum of 60 deg.C".
Human nature will tell you that it's 35 deg.C temperature difference. Notice, no sign.
Apply the temperature correction, also in modulus, meaning ignore any sign. Example: for a -0.25%/*C will be in fact: 0.25*35 = 8.75%
Compute the correction for VoC (suppose VoC = 47V). Again all positive: 8.75% * 47V = 4.11V
Now, because we know that the hotter the air (compared to 25*C STC reference) , the smaller the VoC will be, we will simply substract: 47 - 4.11 = 42.89V
If reversed, the colder the air (compared to 25*C), the bigger the VoC, then we will add.
Simply as that...
Alex
Yeah so basically this whole rabbit hole started when i entered specs of a JA solar panel and went on to choose 2 panels in series.
It won't show me any MPPT's because its calculating the min voltage at 45.8V, which based
((37.91 * (1+ (85+25) * (-0.35/100)) *2) - 0.8) = 45.83V
Is how's its getting there.
Only when i increase to 3 panels does it recommend the 150-70 MTTP, as it only now sees the Voc as vBat+5 (54.6 +5 = 59.6V). The same equation it uses again checks out to give 69.2V
((37.91 * (1+ (85+25) * (-0.35/100)) *3) - 0.8) = 69.14V
However if I were to use the (85-25) equation (which you and my online research says is correct), then it should actually add up to 59.09V with 2 panels and the MPPT 100/20-48V should be recommended. Correct? Although it is technically under the vBat+5 (54.6 +5 = 59.6V) by 0.5V for startup, but only if the panel hits its max 85deg C, so I'm sure it safe to assume the panel wont be at 85 deg when starting up the MPPT.
((37.91 * (1+ (85-25) * (-0.35/100)) *2) - 0.8) = 59.09V
Obviously with 3 panels this increases to 89.04V:
((37.91 * (1+ (85-25) * (-0.35/100)) *3) - 0.8) = 89.04V
=======================================================
In terms of MAX voltage at -30 deg C, the
((37.91 * (1+ (-30-25) * (-0.35/100)) *2)) = 90.42
Matches what it has with 2 panels, as well as the 135V it has when choosing 3
((37.91 * (1+ (-30-25) * (-0.35/100)) *3)) = 135.62
So these aren't the issue. Its the conflict between the MIN voltage equations.
Hi @Guy Stewart (Victron Community Manager)
Can you pretty please check with Victron engineers if there's a problem here or it's us?
Thank you,
Alex
This gets confusing. Too much info.
The delta between STC and min temp is 55C.
So the voltage increase at min temp is
Voc at STC x 55 x 0.35%
Note I dropped the two minus signs as they cancel out.
I'm too lazy to go through Victron Vs web calcs.
It would be easier if the temps were in Kelvins. Then there's no need to worry about adding or subtracting STC depending on whether the lowest temp is above or below zero.
