Solar charging with some shade

BFL13 wrote:
what about the OP's 48v set-up and grid tie inverters?

I don't have grid tie inverters.

I agree with your description.

What happens if one panel is totally shaded? There have been tests where now the string has significant reduced current. To run this test, all you have to do is point all panels but one towards the sun. Rotate the other panel 45 degrees away from the sun to simulate shading.

CA Traveler wrote:

Simple example: My array is producing 5A at 90V and 5*90/14=32A which the battery is accepting. A shadow actives 2 bypass diodes of the 9 and now the controller finds a new maximum power point at 5A and 90*7/9V and the output is now 5*90*7/9/14=25A to the 14V battery.

2oldman wrote:

CA Traveler wrote:
I have no idea what the two of you are discussing ....

Neither do I.

Don't try! I don't know either, brain has fried.

So moving right along, nothing to see here folks, what about the OP's 48v set-up and grid tie inverters? It looks like they have stick house inverters that don't even use a solar controller? So why does the OP need his new 48v inverter and his controller, now that he has gone 48v?

CA Traveler wrote:
I have no idea what the two of you are discussing ....

Neither do I.

That doesn't seem right to me. (current staying the same) Shading does reduce the current and it also reduces the voltage a little. I have some numbers in my notes somewhere from when I was playing with a towel on parts of my panel.

Imp is just what you get when you divide input watts by the Vmp. I see it as a result not a cause. (Maybe that is backwards though) I was wondering where the input watts (VA) came from in the first place and how it is affected by shading one panel.

I don't really like the current bottleneck explanation, because Imp happens after the array, not amongst the array where a bottleneck might exist at one panel. I am thinking it is two different sets of current. After a while it will come clear as these things do, but there is no reason for you guys to get all tangled up.

BFL13 wrote:
What I meant by "fixed" was being the same before and after connecting the controller. (if there is any wattage before connecting) Voc and Isc do change with insolation so they are also "dynamic"

No wattage.

Voc*0A = 0W

Also for a short circuit test.

0V*Isc = 0W

Both of these tests are useful for basic panel testing. With good sun both the open circuit voltage and short circuit amps will be near the rated values.

westend wrote:

But who says that is the same amps amount as what the Imp is when you divide the array output watts by the controller's Vmp?

You're trying to tie together two measurements?
The current produced by the panel array will be, at best, slightly more than the controller output because of the controller's less than total efficiency...and... by the amount of draw being dictated by the battery. In the case of shading, the current output of the panel array will be less and therefore, the controller's output would be less.

Does that explain a discrepancy?

I have no idea what the two of you are discussing but perhaps I missed something.

Let's go back to basics: Serial matched panels with bypass diodes and MPPT. The current output from the panels is dependent upon the amount of sun and can vary from 0 to the panels Imp. Shadows that activate bypass diodes reduce the voltage and don't effect the current from the panels.

So with the same current and reduced voltage the controller seeks a new maximum power point - that is maximum watts for those conditions. Do not confuse this with the panels rated Imp.

It then converts this input power to the output power by reducing the voltage and increasing the amps. I've observed 49A from my Imp rated panels of 8.2A. Yes the wiring and controller losses are a small factor. The battery acceptance for charging is a big factor on the actual controller output.

Simple example: My array is producing 5A at 90V and 5*90/14=32A which the battery is accepting. A shadow actives 2 bypass diodes of the 9 and now the controller finds a new maximum power point at 5A and 90*7/9V and the output is now 5*90*7/9/14=25A to the 14V battery.

What I meant by "fixed" was being the same before and after connecting the controller. (if there is any wattage before connecting) Voc and Isc do change with insolation so they are also "dynamic"

Now I forget what the question was :) Never mind this, the OP needs to get up there and shuffle all his panels. :)

Once you connect, the controller picks its Vmp and you get an actual flowing Imp, which depends on the wattage. If wattage is fixed, and Voc and Vmp are different, then Isc and Imp will be different amounts. But before you connect there is no wattage because no current?

In this statement lies the answer for understanding the relationships of power produced and how it's effected by shading. When the series string is shaded the wattage is not "fixed" but at a dynamic level due to the shading.

You can toss around all the solar nomenclature but the output of the shaded array will be diminished, especially so if the array is a series string.

I hope I'm helping with the chicken and the egg but I get a headache from theoretical things, occasionally. Come on over and talk to my kid about whether photons, which have no mass, are effected by gravity. :E
Instant headache on that.

Thanks. Yes, before the controller is attached, the array has no current, just an Isc. However, you can measure Voc and Isc and those are affected by shading. But there is no current flowing so how can there be an Isc current bottleneck for a string at the shaded panel?

Once you connect, the controller picks its Vmp and you get an actual flowing Imp, which depends on the wattage. If wattage is fixed, and Voc and Vmp are different, then Isc and Imp will be different amounts. But before you connect there is no wattage because no current?

Anyway, never mind. I am having a chicken and egg which came first problem or something, which I will eventually figure out. No need for anybody else to get tangled up in my confusion. :)