question

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

@Alex Pescaru thanks for the detailed reply. yes definitely easier to think in positives than negatives.

I am however still struggling to see why the output of two equations differ so much.

Here are the specs Im working with so maybe if i input what im doing, you can better identify the issue.

• Voc = 37.91V
• V temp. coeff (%) = -0.35%
• PV Temp (MIN) = -30 deg C
• PV Temp (MAX) = 85 deg C
• Panels in Series = 2
• Voltage Drop 0.8V

01: As per Victron calculator:

• V_min = ((37.91 * (1+ (85+25) * (-0.35/100)) *2) - 0.8)
• V_min = 45.83

01: As per Online research:

• V_min = ((37.91 * (1+ (85-25) * (-0.35/100)) *2) - 0.8)
• V_min = 59.09

So as you can see one is vastly different to the other, so confsued as to which is correct and/or where my mistake is?

many thanks

@Alex Pescaru

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

@Alex Pescaru while we wait for the engineers reply, I was up till 2am this morning going through another method for V_min

=============================================

V_max using min temp (working as intended)

=============================================

Firstly, to calculate the V_max using the Voc value, with the "T_min-STC" formula has been fine and working as intended, matching the Victron calculator everytime.

=============================================

V_min using max temp (the confusion)

=============================================

1. However, for V_min, you can either use the Voc value, with the "T_max+STC" or the Vmpp value (with what i have in the image). While not an exact match they are only decimal values apart.

• V_min using Vmpp = 78.41
• V_min using Voc & "T_max+STC" = 78.52

=============================================

2. Then the other alternative (which GPT, and your thoughts were), was to use Voc value, with the "T_max-STC". However as we saw, this is quite far above the other two values at 94.16

• V_min using Voc & "T_max-STC" = 94.16

=============================================

3. Then there is the Victron calculator's output, giving 74.30, which i actually don't know how its getting to this output.

• V_min using Unknown = 74.30

I found this new conflict based on changing the Temperature Coefficient of Voc (%/°C) to -0.275 from yesterday's -0.35, as this was actually the panels Temperature Coefficient of Pmax (%/°C). This was simply an input error on my part.

But what this revealed, was that while using -0.35, it made it seem that the vic cal was also using Voc & "T_max+STC" formula, when its actually not.

When inputting -0.275 it now introduced a third possible equation into the mix.

Note these are all based on 3 panels in series. Here is the cal with the values filled in:

https://mppt.victronenergy.com#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

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.