Solar Panel Voc-MPPT?

Somehow I thought Voc could only be taken with the panel disconnected. I thought there is a "panel voltage" you can read when it is connected and Vmp was a possible panel voltage.

Can Voc being higher or lower than its rating at 25C be used as a predictor of panel watts at that time?

BFL13 wrote:

Does the full Isc really indicate full irradiance for panel power?

If I read ken correctly, no. Please correct if wrong but the way I understand it, you could get full Isc as long as you have enough voltage spread.

Hi BFL13,

You would "see" voc at the output terminals on the panels. You would "see" it at the input on the controller less any line losses.

Where would you ever "see" any panel voltage when using an MPPT controller? You do see it with PWM because it is the same as battery voltage, at least with 12v panels. anyway---

If the "voltage at maximum power" is 30v and you see that you are getting the 1000 irradiance (or are you?) by the fact you see rated Isc, --

--does that mean you will get full power at 30v (where I is 7.67, so power is rated at 230w) despite the measured Voc being only 34.5? (at 15C too---doesn't Voc go up when it is colder than 25C?)

Does the full Isc really indicate full irradiance for panel power?
Temp coefficients are:
P minus 0.45%/degreeC
V minus 0.34%/degreeC
I plus 0.06%/degreeC

This is one of those items you must trust the quality of the controller.

BFL13 wrote:
Wondering if there is any relationship between measured Voc and MPPT performance with a 24v panel.

My 12v panel has a Voc rating of 21.9 and Isc of 8.2, but I often see 8.2ish when Voc is only 20.3ish. That's fine, I just want that Isc. ( I know they have different coefficients from 25C)

But now with MPPT and a 24v panel I am seeing the same sort of thing.
Panel is rated Voc 37v and Isc 8.3a In the sun trying it out I got 34.5 Voc (varies -did see 36 at one point) and Isc of 8.24a

Normally, I would be happy at seeing that Isc, but now I am wondering if that 34.5 Voc instead of 37 rated would show up as lower amps to the battery via the MPPT.

(Vmp is 30v and Imp is 7.7a)

Is it still the case with MPPT that if you are seeing the Isc at the panel, all is well, or with MPPT does Voc matter now, or what?

Thanks.

I wish I had measured the VOC while camping up there to put what else I saw in perspective.

Yes VOC has a impact on MPPT since the controller is converting excess volts to amps. I can't explain it all techy like but going from a mile high to 10,000 ft can show it.

I see a big jump in PWM (ISC) up there from 9a to 14a. In the early morning I see double the MPPT amps that I would see in Denver. The smallest increase is in max MPPT is amps at mid day when I expected the most. There I saw less than a 2a boost over the max in Denver.

What I believe is happening is at some point I hit the max VOC of the panel and in Denver I was close to it already. The boost in the morning is because the voltage came up faster than in Denver and that's when I need the power anyways. I have no idea why I'm seeing such a jump in PWM and I'm not complaining.

I doubt that even at 10.000 ft I'll ever see the rated VOC for either of my panels.

BFL13 wrote:
Wondering if there is any relationship between measured Voc and MPPT performance with a 24v panel.

My 12v panel has a Voc rating of 21.9 and Isc of 8.2, but I often see 8.2ish when Voc is only 20.3ish. That's fine, I just want that Isc. ( I know they have different coefficients from 25C)

But now with MPPT and a 24v panel I am seeing the same sort of thing.
Panel is rated Voc 37v and Isc 8.3a In the sun trying it out I got 34.5 Voc (varies -did see 36 at one point) and Isc of 8.24a

Normally, I would be happy at seeing that Isc, but now I am wondering if that 34.5 Voc instead of 37 rated would show up as lower amps to the battery via the MPPT.

(Vmp is 30v and Imp is 7.7a)

Is it still the case with MPPT that if you are seeing the Isc at the panel, all is well, or with MPPT does Voc matter now, or what?

Thanks.

Solar panels will produce usable voltage/power from their Voc all the way down to battery voltage and then they don't produce usable power anymore.

So higher voltage panels have a wider power margin that an MPPT controller can take advantage of.

If you assume the controller is ideal, then power in equals power out - lets keep this analysis simple.

Example: Power in equals 150W when V = 30V and I = 5A. Power out equals 150W when the battery voltage equals 13.3V which means the current has increased to 11.29A.

The above example is one benefit of the MPPT controller - higher panel voltage and less losses.

The other advantage is the panel voltage will be able to produce power between the 30V value all the way down to the 13.3V battery voltage, so it has a voltage margin of 16.7V which gives the panel a wide power range into the lower light levels.

If a PWM panel only produced 17V for the same irradiance, it would only have about a 3.7V margin and would cut out much sooner as the light intensity diminished - much less power capability.

Good luck with your quest...

:)

BFL,

I think that if you think in terms of Watts instead, it will make more sense.

Steve