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

[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"?) 

[rossw]

This pertains to standard solar controllers.

For real life guesstimates, I use:

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

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)

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

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.

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)

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.

(PS- "Operatine"?) 

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.

[ghurd]

We are on the same page. 
(when I said "This pertains to standard solar controllers", I meant PWM)

The vast majority of my emails related to solar are for arrays under 250W.  For those people, the money for a MPPT controller would almost always be better off spent on more PV watts and a nice less expensive PWM controller, at least here in the US.

When it get up to 1KW, like yours, MPPT at todays pricing is a no brainer.

Do you have any idea why the meters are reading so much different?
Maybe try loosing, wiggling, retightening the shunt terminals?  You knew that.
G-

[Wolvenar]

I have a 720 watt array now, not yet setup, but at this point I only have a standard Xantrex C-60 charge controller. Its that or some home brew setup.. I think for now the charge controller is the easiest for me to get this system doing something. Maybe in the future I will get a MPPT, as I get more panels I will be able to move these to a better solar location. With a MPPT I can raise the voltage so I would have less losses in the longer lines needed because of higher voltages.
This is another good reason for, and what my main interest for a MPPT is, not eeking out a couple percent boost of  output at this point

[rossw]

Do you have any idea why the meters are reading so much different?
Maybe try loosing, wiggling, retightening the shunt terminals?  You knew that.

No, I'm puzzled. I *THOUGHT* they were pretty much on the money.
They have internal shunts, not much I can do wrong!

[rossw]

I'm puzzled. I *THOUGHT* they were pretty much on the money.

Even more puzzled now.





Can only surmise that conditions had changed in the few seconds between the first and second photos.
It's quite possible - wind has been quite gusty, it's making the charge controllers job more difficult for sure.

[ksouers]

... wind has been quite gusty, it's making the charge controllers job more difficult for sure.

Ok, I'm confused. I thought this was solar.

How does the wind affect the output?


Kevin

[rossw]

... wind has been quite gusty, it's making the charge controllers job more difficult for sure.

Ok, I'm confused. I thought this was solar.

How does the wind affect the output?

Because I, like many off-grid dwellers, have a hybrid system.

My PV goes through one charge controller onto the batteries.
My turbine goes through another controller also onto the batteries.
My propane genset goes through a third controller (actually my main inverter), to the same batteries.

If there is input from more than one source, they tend to interfere with each other.

Today at time my turbine was putting its full output into my batteries, which were already virtually fully charged, so the solar controller was backing off what it was trying to put in.

Make sense now?

[ksouers]

Because I, like many off-grid dwellers, have a hybrid system.

Ah, OK. Got it now.
Thanks Ross.


Kevin

[Rover]

I have the same affliction as Ross, multiple controllers ( different manufacturers) + mini wind. (700W Solar (soon to be 900W)).

Since this was a growing system, I have a smaller controller for some panels, and a larger one for the bigger array).

Bad news is the absorption set points don't match up (actually float, etc don't match and the little guy is not adjustable)... so my little guy shuts down as my larger one rises to a higher set point.

Really not a big deal for me as my batteries are usually fully charged, and if not it will take more from the smaller set until the larger set takes over and basically shut down the smaller one.

Rover

[ghurd]

I have the same affliction as Ross, multiple controllers ( different manufacturers) + mini wind. (700W Solar (soon to be 900W)).

Since this was a growing system, I have a smaller controller for some panels, and a larger one for the bigger array).

Bad news is the absorption set points don't match up (actually float, etc don't match and the little guy is not adjustable)... so my little guy shuts down as my larger one rises to a higher set point.

Really not a big deal for me as my batteries are usually fully charged, and if not it will take more from the smaller set until the larger set takes over and basically shut down the smaller one.

Rover

Rover,
You know it, but it is a good topic to touch on...

Not a problem usually, since the Smaller array shuts down when the Larger one is close to tapering down the amps.
When the smaller array shuts down, the larger array keeps going or increases the current.
The end result is about the same.

"A man with 2 watches never knows the correct time.
A man with 1 watch always knows the correct time"
(or something like that...)

The simplest things can mess up the calculations.
G-

[rossw]

The simplest things can mess up the calculations.

Or, in my case - the photographs