MPPT voltage diagram - why the small dip at high noon - just curious

Finally I've got the first strings installed and had a nice 100% sunny and cloud free day behind me (max. ambient shade temp was about 33°C). Here are the diagrams:






I wonder why the voltage dips (so much) while the current gets higher. I know about Ohm's law but my wire distance between the MPPT and the string is by far not long enough to be able to explain this dip of about 20V.

I've done the following short calculation:
Distance is 26m which results in a wire length of L=52m. The resistivity of copper is ρ=1.68E-8 Ωm and my cross section with 10AWG is A=6E-6 m2.
This results in about 0.145Ω for the wire resistance and with 11Amps (max. current at high noon)I would loose a maximum of 1.6V on the wire.

I ask myself just out for curiosity if this is typical behavior for MPPT chargers?
What are the reasons for this 10% voltage dip?
Maybe losses inside the MPPT?
Weakness of the MPP tracking algorithm?


Some specs:
- used MPPT's are integrated in EG4-6500EX AIO inverters
- bi-facial mono half cut cell
- string has 5 modules in series

STC:
Pmax=460W
Voc=50.8V
Isc=11.5A
Vmpp=42.4V
Impp=10.86A
Efficiency=21.2%

NMOT (43±2°C):
Pmax=341W
Voc=47.5V
Isc=9.27A
Vmpp=39.6V
Impp=8.62A

TcP=-0.36%/°C
TcVoc=-0.26%/°C
TcIsc=+0.043%/°C

I think I found the answer by myself. It's the Voc temperature coefficient which explains the voltage drop very accurate.
At about 60°C PV module I will loose about 9.2% voltage which is about 18.4V and together with the wire resistance loss of about 1.6V it will perfect explain the 20V dip.
60°C sounds realistic in direct sun with an ambient of 33°C (shade) temperature.

I was a bit hasty with my question and could have thought of it myself beforehand...

I don't know if the dip is typical, but I would not call it surprising either. The MPPT is constantly adjusting the current to find the best power point. As the sun get to the best angle the best power point is a lower voltage and a higher current.

FilterGuy said:

I don't know if the dip is typical, but I would not call it surprising either. The MPPT is constantly adjusting the current to find the best power point. As the sun get to the best angle the best power point is a lower voltage and a higher current.

Click to expand...

This was my first idea also that it has to do with the tracking mechanism... I'm sure it will have an influence but my feeling is that the temperature coefficient may explain the most part of the voltage dip(see my 2nd post) ... but not 100% sure.

fmeili1 said:

This was my first idea also that it has to do with the tracking mechanism... I'm sure it will have an influence but my feeling is that the temperature coefficient may explain the most part of the voltage dip(see my 2nd post) ... but not 100% sure.

Click to expand...

Its going to be both. The characteristics of the panel change with the sun angle and the temperature, The MPPT is constantly adjusting to get the most it can from the panel.

BTW: That nice bell curve of the power tells us the MPPT is probably doing a good job tracking the powerpoint.

FilterGuy said:

Its going to be both. The characteristics of the panel change with the sun angle and the temperature, The MPPT is constantly adjusting to get the most it can from the panel.

Click to expand...

I'm pretty sure you're right.

FilterGuy said:

BTW: That nice bell curve of the power tells us the MPPT is probably doing a good job tracking the powerpoint.

Click to expand...

Yes, I didn't expect such a perfect curve (even in our perfect weather condition here in the desert). My expectation with the integrated MPPT controllers inside the EG4-6500EX were not very high (because of some other quality issues with this AIO model like unstable output aka LED pulsing). But surprisingly the MPPT's seems to be pretty good.
Until now I have 2 out of 8 strings installed and I hope this will be the same good tracking on all MPPT's... (by end of next week I will know).

Update:
Maybe it's because of this nearly perfect condition and the tracking quality may change under rapidly changing cloud conditions, the quality may be behind some high quality competitors like Victron... but I can't tell.
For now, I'm happy with the result and because these weather pattern is usual for there for >280 days per year. So I don't worry too much.

Looks normal to me.