Renewable Energy Questions/Discussion > Solar (heating or electric)

Testing PV. I(sc) I(mp), V(oc), V(mp) !! It's all so confusing!

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rossw:
This is a subject that was thrown about in IRC a while ago.
There seems to be a lot of confusion, particularly to the newcommers, about what these things mean and how to measure them.

Couple of basic points to start.
Watts. Watts are the basic unit most panels are sold by. "100 watts" or "215 watts" or whatever.
From there on, it gets just more and more confusing...

So... lets look at a typical panel.


Maximum power: 100 watts.
Open circuit voltage: 22V  Short circuit current: 6.14A.
Anyone knows "Power = volts times amps" - so 22V * 6.14A = 135W.  Huh??

The answer is, that the "Open circuit" voltage is with no load. As you apply load, the voltage will fall.
And the "Short circuit current" of 6.14A is the maximum current the panel can deliver into a dead short.
It can't provide any POWER in that circumstance of course, because a load resistance of 0 ohms means no work done!

The "maximum power point" is the sweet-spot where the voltage hasn't dropped too far, AND the current is as high as we can manage while delivering a reasonable voltage. The product of the two produces the "maximum usable power". And for this module, that's about 17.5 volts and 5.72 amps (100 watts).

If you were to take a large variable resistor and an amp meter in series and connect them across your solar module under perfect illumination (square on to full sun), you would expect to be able to vary the current from not much (depends on the maximum resistance), up to a little over 6 amps.

If you also connected a voltmeter across the panel while you did this, you would see the voltage change from almost zero (when you measure maximum amps), up to close to 22V.

Simply measuring open circuit volts is almost meaningless.
Measuring maximum (short circuit) current can tell you a number of things about your panels however. If you only get a few milliamps from a large panel like this (but you DO get full open circuit voltage) almost certainly means a cracked, broken or burned conductor in the panel, or a high-resistance connection.

ghurd:
This pertains to standard solar controllers.

For real life guesstimates, I use:

About 10% less than Isc for peak amps into the battery.
However, if the Imp is higher than that, I use Imp.

The actual peak charging amps into a battery is higher than Imp, even if the user is not there to see it when it happens.

For the panel shown, my real life guesstimate would be 5.72A.
The battery would be between 11.9V and 14.4V usually, under standard conditions.
The actual watts of output would be between 68W (11.9V x 5.72A), and 82W (14.4V x 5.72W).

Under good conditions, it is not uncommon to see more charging amps than Imp.

Even if the panel was making Isc, into a 11.9V (0% charge remaining), the output would only be 73W.
(11.9V x 6.14A = 73W)

G-
(PS- "Operatine"?)  :D

rossw:

--- Quote from: ghurd on March 06, 2012, 07:39:41 pm ---This pertains to standard solar controllers.

For real life guesstimates, I use:

About 10% less than Isc for peak amps into the battery.

--- End quote ---

Hard to say. I agree the quoted figures are often "difficult to believe". However these ones I have, I *REGULARLY* see stated power. I will admit, I'm using MPPT charge controller, 6 modules in series for each array and nominally 100V DC from the arrays, to batteries at (nominally) 48V (around 50-53V charging)


--- Quote ---Under good conditions, it is not uncommon to see more charging amps than Imp.

--- End quote ---
That would usually be at lower voltages, where current may be more than Imp, but volts are less than Vmp and power into batteries is below rated watts.


--- Quote ---Even if the panel was making Isc, into a 11.9V (0% charge remaining), the output would only be 73W.
(11.9V x 6.14A = 73W)

--- End quote ---
Yes, which is why I've said time and again, MPPT makes a lot of sense, because you can't ever get anything like full panel output into batteries without it some sort of voltage stepdown.


--- Quote ---(PS- "Operatine"?)  :D

--- End quote ---

Chinglish at its finest :)

rossw:
Just to prove the point: it's currently 13:40. Daylight savings is in force, so it's just after solar noon here.

My batteries are in a reasonable state of charge. Controller had dropped back to "Absorb" mode.
I isolated all except 2 arrays, so I could make those two deliver the most power they could, and took the following two photographs for you.

Here, two arrays, 6 of the above photographed modules each, in series.


And here is the charge controller display:


That was taken with zero setup, or waiting for any special event - just walked in, flicked 4 breakers off, snap, snap, flicked the breakers back on and came here to post it.

Edit: typo. Also not sure why the CC shows 14.2A when the analog meters show about 6.5A ea.
Analog meters are not particularly accurate, but should be within 5% I'd have guessed. I might go check with clamp meter..

rossw:
Well, the analog meters may be showing off...

sorry for the shaky-cam pics. Trying to get stable readings with gusty wind making the charge controller keep backing off as battery volts change, is tricky!

I took a picture of the CC screen and hit HOLD on the clamp meter at the same time.

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