diy solar

diy solar

Series String Wiring Question

Snoobler, thank you for the correction, I did a fair bit of research before writing up that post, and asked for review, only took 6 months to learn of what sounds like a pretty foundational misunderstanding :rolleyes: (every time I think I've wrapped my head around something I find out its more complicated than I thought :)). Would you be willing to comment in that thread with a slightly more concise and clear explanation of what you wrote above, ideally with a simple example.

A couple followup questions:
1. Do you know of a resource where we could read up on this.
2. Is your correction specifically related to MPPT controllers, is the linked article accurate 'pre charge controller', but inaccurate once an MPPT algorithm is added to the mix?
 
I think we are missing the point here a bit guys, All panels are exactly the same. Same model, voltage and amperage. I’m just asking about using 1 less panel on the last string, that’s all.
I'm not sure there is a meaningful distinction here. My basic understanding is that in most respects, series-strings of panels in parallel are conceptually similar to single panels in parallel (which are essentially just a bunch of solar cells in series, with a few bypass diodes).

Please correct me if I'm wrong but from the charge controllers point of view I think a difference in voltage between paralleled strings and a difference in voltage between paralleled panels would look essentially the same.

Forget the SCC, wiring, breaker and so on.
From the sound of it you can't forget the SCC, I think the MPPT controller is the central component of Snoobler's correction/explanation.

I guess I can change the orientation of those 3 panels, like facing them North so that they provide power during the morning hours, but I would like to the know the science behind it. Snobbler is on the right path.

If its already been covered and I missed it I apologize, but why are you so set on moving to 4S-4S-4S-3S as opposed to 3S5P? What do you hope to gain beyond ?

I Imagine the ideal solutions would be either:
1. Stick to 3S5P
2. Buy another panel and go with 4S4P
3. Buy a second small controller, and use 4S3P on the main controller, and 3S on the secondary controller.
 
Snoobler, thank you for the correction, I did a fair bit of research before writing up that post, and asked for review, only took 6 months to learn of what sounds like a pretty foundational misunderstanding :rolleyes: (every time I think I've wrapped my head around something I find out its more complicated than I thought :)). Would you be willing to comment in that thread with a slightly more concise and clear explanation of what you wrote above, ideally with a simple example.

A couple followup questions:
1. Do you know of a resource where we could read up on this.
2. Is your correction specifically related to MPPT controllers, is the linked article accurate 'pre charge controller', but inaccurate once an MPPT algorithm is added to the mix?

1. No, but I deal with dissimilar parallel voltages almost daily. The blocking diode in the lower voltage panel should not inhibit the higher voltage string, but I might be wrong (many many years since my circuits class). If I can find the specs on my blocking diodes in my panels and confirm I wont' pop it, I'll try an experiment to confirm a 2S string in parallel with a single panel will still show 2S Voc at the leads.
2. The blanket statement is wrong in all circumstances (using the article's example):
  • 17V in parallel with 19V will not pull everything down to 17V with MPPT. MPPT will find the sweet spot which will be between 17 and 19V because they overlap in the VoC-Vmp ranges.
  • 17V and 19V panels becomes even more irrelevant with PWM as battery voltage drives the power, and you're getting big losses anyway. If you have 17V and 19V panels on a battery charging at 14.4V, all panels are pulled down to 14.4V, so it doesn't matter that they have different Vmp.
I routinely put two voltage sources in parallel:

Source 1: 250VDC
Source 2: 0-400VDC with blocking diode connected to source 1 terminals

VM on both sources

Source two is connected to source 1 with a blocking diode to prevent backflow from source 1 to source 2 (analogous to the OP's two strings).

Source two slowly raises its voltage. Both VMs show the correct voltage for the two sources. Current will only flow from source two to source one once source 2 voltage exceeds source 1.

Yes. I'm describing a battery charger. :)

In other words, in a 4S string in parallel with a 3S string, the 3S is isolated from the array by the blocking diode in the panels because Voc of the 4S is above Voc of the 3S. In fact, Vmp of the 4S is above Voc of the 3S, so if the MPPT never lowers the voltage of the array to below the Voc of the 3S string, they are isolated from the circuit and not part of the array.

Even simpler terms... 4S in parallel with 3S AREN'T in parallel even if the way you've connected them says so. The blocking diodes in the 3S array keep it isolated from the circuit when voltage is above 3S Voc.
 
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Well I don’t have the panels here but in the past I put a panel with something like 48v and 5 amps in series with a panel that was something like 24 v and 2 amps. I routinely used that combination to charge a 48v nominal battery and the current was limited to around 2 amps like the article says. Of course the MPPT put out a little more than 2 amps when it dropped the higher combined voltage down to 48v nominal but it wasn’t significant.

What am I missing?
 
Well I don’t have the panels here but in the past I put a panel with something like 48v and 5 amps in series with a panel that was something like 24 v and 2 amps. I routinely used that combination to charge a 48v nominal battery and the current was limited to around 2 amps like the article says. Of course the MPPT put out a little more than 2 amps when it dropped the higher combined voltage down to 48v nominal but it wasn’t significant.

What am I missing?
I think Snoobler's issue is with the other half of the article (concerning parallel panels of different voltages).

The article in question is not the only place I've read that panels in parallel are pulled down by the lowest voltage panel, so I think further clarification is necessary. This seems like the type of thing @SolarQueen may have tested in one of her videos, I'm gonna see if I can find anything relevant.
 
Funny enough, before I even had a chance to look for a video, I randomly stumbled across a post from her on another forum asking about the precise issue we are discussing.

SolarQueen said:
I made a series of demo videos showing what happens when you wire mismatched solar panels in various configurations. I'm now trying to explain the "why" behind what we saw. I thought some of you folks here would be able to give me the mathematical or scientific reasoning. I'm cool with the bottle neck that was created with trying to wire two panels with different current in series. What I'm trying to explain is why when two different voltage solar panels are wired in parallel, the voltage from the higher voltage panel was pulled down to the lower voltage panel.
I knew I came to the right place. Thanks for your terrific answer. I did several different versions of this demo, with varying differences between the panels. The one I'm trying to explain is a 12V 100W and a 24V 200W, both with 5.56Imp. Wired in series with an MPPT was pretty much the sum of the individual outputs, but wired in parallel the voltage output was just about the same as the 12V alone (34.7V and 18.1V in parallel equaled 20.9V.) It was just as expected, the lower voltage dragged the higher voltage down, but the why was tricky. I think I will just stick with the "what" happens and not complicate it with the "why". The geek in me is trying to over complicate it.

Also, one of the answers to her question, is pretty similar to what Snoobler said I think:
GoldDigger said:
The solar cell (and the combination of cells in series to make a panel) can be modeled as a diode plus a voltage source (powered by sunlight) which has a maximum voltage almost independent of light level and a maximum current which is proportional to the light level.
With no load the panel produces the open circuit voltage, Voc, but no current and so no power.
With a short circuit the panel produces the short circuit current, Isc, but no voltage and so no power.
In between those two extremes is a point where the panel produces its maximum power, Vmp times Imp.
If you put two panels with different voltages in parallel and try to maximize the combined power output you will drag the voltage of the higher voltage panel down below Vmp and get less than its maximum contribution. if the two panels are close enough in voltage the operating point will be somewhere between the low Vmp and the high Vmp and the total power will be more than either panel alone but less than the sum of the two at their individual optimum.
Not sure which videos of hers would be most relevant, but some of these may provide some examples
 
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Funny enough, before I even had a chance to look for a video, I randomly stumbled across a post from her on another forum asking about the precise issue we are discussing.




Also, one of the answers to her question, is pretty similar to what Snoobler said I think:


Not sure which videos of hers would be most relevant, but some of these may provide some examples

I have no idea about the why it happens but what happens is very close to what the cited article calls for.

As seen in post #3 in the thread you quoted here:


35v plus 18v in parallel put out 21v.
 
There are two claims to the article:

Current in a series string is limited to the lowest current panel - true.
Voltages in parallel panels are limited to the lowest voltage panel - not true.
 
There are two claims to the article:

Current in a series string is limited to the lowest current panel - true.
Voltages in parallel panels are limited to the lowest voltage panel - not true.

So if we diluted the specificity of claim #2,
From:
The voltage of a parallel connected PV array is limited to the lowest voltage panel
To:
The voltage of a parallel connected PV array is pulled down by the lowest voltage panel

can we reconcile the different explanations/observations?
 
That's more accurate, but with a discrepancy of a whole panel's worth of voltage, that whole portion of the array may behave as though its not there.

Definitely true for deviations where Vmp and Voc values overlap.
 
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That's more accurate, but with a discrepancy of a whole panel's worth of voltage, that whole portion of the array may behave as though its not there.
So essentially, in searching for the optimal MPP for the whole array, the MPPT controller may find--if the difference in voltage between strings is too great--that to maximize power, choosing an MPP that is too high for the 3S string (and renders it non-productive) but maximizes the productiveness of the 4S strings is the least worst option?
Definitely true for deviations where Vmp and Voc values overlap.
So, if the Voc of the 3S string, is below the Vmp of the total 15 panel array(?) the 3S string would contribute nothing as it could not reach a high enough voltage at any point on its I/V curve?

However if the difference was more modest and there was overlap, all panels could contribute at reduced power, total array Vmp would fall somewhere between the 3S and 4S strings, the 4S strings would be operating below its Vmp, and the 3S string would be operating below its Imp?
 
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If I can find the specs on my blocking diodes in my panels and confirm I wont' pop it, I'll try an experiment to confirm a 2S string in parallel with a single panel will still show 2S Voc at the leads.
No blocking diodes in the panels. Most have bypass diodes. If you add blocking diode, it would be one per string, in series with the string.
PV panels are a string of forward-biased diodes. Bypass diodes are oriented the other direction, each wire across 1/2 or 1/3 of the cells.
Cells let the voltage rise to the knee of the curve in full sun and no load. Try to impose higher voltage and current shoots through the roof, but voltage hardly increases.

If you wire 4S || 4S || 4S || 3S, voltage will rise to 3x Voc of one panel, hardly any higher. The string of three will carry it's own Isc through the cells, plus current from the other three strings according to their IV curve at 3/4 of Voc. That may well be sufficient to damage the cells and/or interconnects. Looks from the data sheets of my panels, pretty close to 4x Isc goes through the panel, 30% above its fuse rating. I don't think that would cause immediate failure but would age the panels more rapidly. Those were SunPower. For my Sharp panels, current would be 2.2x fuse rating.
 
Well then, I'm completely wrong. Thanks for the clarification. I thought panels had blocking diodes in addition to bypass diodes.
 
I've never seen a panel with a blocking diode factory fitted and even asked anyone on the forum that knew of any to post links to same in a thread where it was claimed there were panels like that, but IIRC there were no responses beyond my own picture of a panel that did not have one.
 
The article in question is not the only place I've read that panels in parallel are pulled down by the lowest voltage panel, so I think further clarification is necessary. This seems like the type of thing @SolarQueen may have tested in one of her videos, I'm gonna see if I can find anything relevant.
Someone else posted the playlist where I test different amps and volts in different configurations. I think this one is most relevant to the question, same amps, different volts.
 
I've never seen a panel with a blocking diode factory fitted and even asked anyone on the forum that knew of any to post links to same in a thread where it was claimed there were panels like that, but IIRC there were no responses beyond my own picture of a panel that did not have one.

It's the SCC that has the blocking diodes. I must have moved that to the panel in my head.

What a dummy.
 
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