# Discrepancies between Temp coefficients of Vmpp and Pmax

herodotus
Registered Users Posts:

**40**✭✭
Hey all,

This is something that has been puzzling me for some time, while attempting to relate the observed performance of my array with the manufacturer's specifications. It seems like it is common for many panels to have a significant difference in the temperature coefficient of Vmpp and Pmax. In my case, Pmax drops by 0.45% for every degree C, but Vmpp apparently only drops by 0.35%. Looking around at other specifications for other panels, this sort of difference seems common, if not quite universal.

But this set my wondering, how is this the case? Unless I'm misunderstanding something, it seems to me that there are only two possible explanations:

Cheers,

This is something that has been puzzling me for some time, while attempting to relate the observed performance of my array with the manufacturer's specifications. It seems like it is common for many panels to have a significant difference in the temperature coefficient of Vmpp and Pmax. In my case, Pmax drops by 0.45% for every degree C, but Vmpp apparently only drops by 0.35%. Looking around at other specifications for other panels, this sort of difference seems common, if not quite universal.

But this set my wondering, how is this the case? Unless I'm misunderstanding something, it seems to me that there are only two possible explanations:

- An entirely different measuring methodology is used for these two coefficients, so no reasoning about their relationship is possible. This has been suggested to me before as an explanation, but it beggars belief slightly. I get that output measurements aren't always conducted in the most informative or realistic of ways, but it would seem astonishing that the measurement of two such closely related variables would be done in a way that makes it impossible to use them together - particularly given that it seems to be a more or less consistent pattern across many manufacturers.
- Impp must also typically have a meaningfully negative temperature coefficient. This seems like the more likely explanation, but it contradicts what I thought I knew about the relationship between current and temperature. The temperature coefficient of Isc (which is often the only current coefficient quoted) is
*positive*rather than negative; and from graphs that I have looked at, the shape of the curve generally doesn't change with temperature - it just gets shifted to the left on the voltage axis as the temperature rises.

Cheers,

Outback VFX3048E, Outback FM80, Outback FNDC, Outback Mate3, Outback Hub4, 15x Munchen 250W panels (5x3), 8 x Rolls S605 (48V system)

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## Comments

4,781✭✭✭✭40✭✭4,781✭✭✭✭20✭✭Is this based on ambient temp? How would you measure cell temp?

It is likely that an array will not have uniform cell temps.

The hotter they get the greater the cell temp... and Vmp mismatching possibly.

With 3 strings... 1 string that responds more to high temps would pull dow power more than Vmp.

Then there is a question of the MPPT tracking? If the tracking is catching up to the real Vmp when it is getting warmer Vmp will stay higher then.

I saw this on an Astronergy data sheet.

Pmp coef. = -.469%/C

Vmp coef. = -.463%/C

Have you checked this with a single module?

-MStar Applications EE

40✭✭28,315adminhttp://forum.solar-electric.com/discussion/5458/two-strings-in-parallel-with-unequal-string-voltages

It does have an example of where the Pmp is more temperature sensitive than Vmp would suggest (Imp temperature sensitively is the "opposite" of Vmp--Imp goes up (slightly) with increasing temperature)... Not a big difference, but it does confuse me a bit.

-Bill

6✭✭please note that the coefficients in practically all the panel specifications I ever saw are not provided for the same opperation point:

the coefficient for current I is normally given for short circuit (indicated by SC)

the coefficient for voltage V is normally given for open circuit (indicated by OC)

the coefficient for power P is normally given for maximum power point (indicated by MPP)

These coefficients are not provided to perform a calculation of IV curves at non STC condidtions, but they are provided for the layout of inverters or battery controllers.

Three factors are predominantly to be considered:

- Max current permissive: This is the short circuit current ISC of the pv array. And this increases when reducing the temperature. Thus the ISC at the lowest temperture possible needs to be considered.
- Min voltage required to run the controller / inverter. Since the voltage reduces with increasing temperature one has to choose the number of panels high enough to remain even at very high temperture above the lowest operation limit of the controller/ inverter. Otherwise the mpp tracking of the controller/ inverter will be compromised.

- Max voltage that the controller/ inverter can take: One has to choose the number of panels low enough that at even the lowest tempertures possible the open circuit voltage (not the mpp voltage) will be below the max voltage of the controller/ inverter. Otherwise the capacities in the DC part of the controller/ inverter will blow.

Please note that the coefficients are given in % of the specified value. And teh coefficient of current is two magnitudes smaller than for voltage and power.Hope this can help

20✭✭-MStar Applications EE