Solar fuseing, confuseing

Ed_Gee wrote:
Your ( Red31 ) comments above were not clear to me. I repeat, you will NEVER see your panel's Isc amps going into a battery. That would violate Ohm's Law!

BPL13 - you are misusing the term Isc It has nothing to do with battery voltage. In the Solar Industry it is solely defined as short circuit amperage out of a Solar panel.

We all know that! You are perfectly right. However you are missing the whole point that you get (almost--ok?) Isc as your charging amps when using PWM as 12-12 or 24-24. BTDT. Measured it twice and cut once!

Try it yourself and see! :)

Your ( Red31 ) comments above were not clear to me. I repeat, you will NEVER see your panel's Isc amps going into a battery. That would violate Ohm's Law!

BPL13 - you are misusing the term Isc It has nothing to do with battery voltage. In the Solar Industry it is solely defined as short circuit amperage out of a Solar panel.

"By definition, Isc is short circuit current measured at the solar panel output. Zero resistance for a short circuit means you will see the maximum current the panel can provide. You will never see Isc with a battery hooked up in place of the short circuit because a battery has resistance which will limit the current provided by the panel."

Right, except in real life, the Isc you get on your ammeter depends on how much sun there is on the panel. If you are seeing "rated Isc" then IMO that means you are in STC or as good as.

On a 12v panel's IV curve, Isc is related to battery voltage, so with rising battery voltage Isc does come down a bit. Once you get over about 13.4v that starts to show.

With 12v PWM I have compared (many times!) the Isc at the disconnected panel with the amps I get showing on the Trimetric when I connect back up and they are the same (within meter accuracy), so it is "close enough for government work" for what I need to know.

(What drives me crazy is people saying you should get Imp doing that. They don't seem to know that Imp is strictly for MPPT work and has nothing to do with ordinary PWM solar, where the higher amp Isc is the "expected amps")
-----------

You can't trust panel specs very much either. I have a 100w panel, which I know from the "usual" ratings, should get about 6.2 amps Isc.
(it is all proportionate--a 130w will get 8.2, and a 120 will get 7.6)

On its label it says Voc 21.5, Vmp 17.1, Imp 5.8 (no Isc listed) and on its package it said output is 5.8a. In the sun it gets 6.2 amps like it is supposed to.

Then I got a pair of 100w panels made in India with these specs:
Voc 22.7, Isc 5.8, Vmp 18.8, Imp 5.3 So I wanted to put these three together but was worried about them not being close enough in voltage etc. Put the India ones in the sun and sure enough--6.2a Isc! Vocs were the same too, in the sun.

Putting all three in parallel aimed at sun, my PWM Solar30 controller said 18.3 amps to the battery (confirmed by Trimetric)

So I then ran all three together all last summer with my Tracer MPPT and was able to connect them in series or parallel no issues either way. The panels are your basic 100w panels that get 6.2 amps. Too easy! (I still only got 18ish amps using the MPPT though, due to panel heating loss with MPPT)

Ed_Gee wrote:
You will never see Isc with a battery hooked up in place of the short circuit because a battery has resistance which will limit the current provided by the panel.

Ya might see a current higher than Isc, the solar irradiance does exceed 1000 wh/m^2 at times! Some panels exceed spec also :D

I understand using Isc as the current from panel @ blue sky conditions, the actual battery charging current +/- sticker Isc value.

BFL13 wrote:

red31 wrote:
they did mean Voc for OFF, no matter what voltage Voc is, there is no power open circuit, no watts, no amps! (that's why I don't like using Isc for PWM ;), no power, watts, voltage).
I had assumed it continued to track (MPPT) but now get it that it can't sweep during on/off.

I use "Isc" because that's what I get at the battery. Take the Isc with panel disconnected. Could be any number depending on how much sun there is. Connect up, ta da!-- same amps as you got for Isc now charging the battery. I don't know what else to call it for the purists. We need a name for it.
. (System check)

By definition, Isc is short curcuit current measured at the solar panel output. Zero resistance for a short circuit means you will see the maximum current the panel can provide. You will never see Isc with a battery hooked up in place of the short circuit because a battery has resistance which will limit the current provided by the panel.

For the above graphs the afternoon sky was clear. There are house loads in addition to the battery charging. An attempt to not use additional house loads but notice the 6:08 PM house amp bump in the first graph.

Check the panel IV graphs. The amps are constant from 0V to the knee of the curve at Vmp. Since this spans bulk charging and Isc is part of the spec it's acceptable usage for PWM for me.

red31 wrote:
they did mean Voc for OFF, no matter what voltage Voc is, there is no power open circuit, no watts, no amps! (that's why I don't like using Isc for PWM ;), no power, watts, voltage).
I had assumed it continued to track (MPPT) but now get it that it can't sweep during on/off.

EDIT--nice graphs! Hard to interpret without knowing what the sunshine was doing later in the afternoon, which effect might get mixed in with the reducing output from the battery acceptance effect by then.
-------

I use "Isc" because that's what I get at the battery. Take the Isc with panel disconnected. Could be any number depending on how much sun there is. Connect up, ta da!-- same amps as you got for Isc now charging the battery. I don't know what else to call it for the purists. We need a name for it.

My Eco-Worthy MPPT controller has a feature where you can check how much MPPT is doing for you :) When you do, it shows the panel amps and that is the same as panel Isc at the time. So that is handy--you can check for Isc without disconnecting. (the controller must short the panel to get that number?) My old ASC controller operated all the time by shorting the panel to do its "Low frequency PWM" on off thing.

I check for Isc to tell whether I am getting rated STC sunlight so I can then tell if I am getting the output I should be. (System check)

And a recent Q capture. :)


Click For Full-Size Image.

MPPT algorithms can be complex and the manufacturers don't reveal a lot of details. This data was captured 6/27/15 Colorado Junction, CO mostly clear skies. 3 series panel specs: Vmp 91.74 Voc 112.35. Morningstar MPPT 60 controller.

Notice the decreasing amps especially at 5PM with little change in array voltages, perhaps the change from absorb to float. These values were logged by the controller.