What does the MPPT do in partial shading?

[Black-on-Plus]

Hi guys,

I am about to plant a solar system on my van (3S3P - 900W).
the air conditioning is unfortunately a bit higher than planned.
Presumably only 5min/day all panels will get sun at the same time.
It will almost always be one of the two "problem panels" in the partial shade.
Victron says the solar charge controller can handle and compensate for partial shading.
However, I am somewhat skeptical about this statement. Unfortunately I have no practical experience in this
Area. Does anyone have a similar setup? What is your guess when a panel is in the shade?
900W - 100W = 800W or
900W - 300W = 600W ? (of course only theoretical values)

or does someone have a better idea for the interconnection?
I am grateful for every answer!

Panel: 100WP
IMPP: 5.04A
ISK: 5.34
UMPP: 19.86V
UOC: 23.7V

Charge controller: SmartSolar MPPT 100/30 Victron Energy

Load: 24V / 400Ah LiFePo4

 

[Bobert]

Hi guys,

I am about to plant a solar system on my van (3S3P - 900W).
the air conditioning is unfortunately a bit higher than planned.
Presumably only 5min/day all panels will get sun at the same time.
It will almost always be one of the two "problem panels" in the partial shade.
Victron says the solar charge controller can handle and compensate for partial shading.
However, I am somewhat skeptical about this statement. Unfortunately I have no practical experience in this
Area. Does anyone have a similar setup? What is your guess when a panel is in the shade?
900W - 100W = 800W or
900W - 300W = 600W ? (of course only theoretical values)

or does someone have a better idea for the interconnection?
I am grateful for every answer!

Panel: 100WP
IMPP: 5.04A
ISK: 5.34
UMPP: 19.86V
UOC: 23.7V

Charge controller: SmartSolar MPPT 100/30 Victron Energy

Load: 24V / 400Ah LiFePo4

The results will vary depending upon how complete the shading is ( not all shade is created equal.) My guess is that if the majority of a panel is shaded the series string will produce about %10 of it’s rated capacity. 1 panel shaded your array will produce 450 watts if everything else is in full sun. This is how I came to my conclusion. 3 series strings at 300 watts each. First string shaded so 10% 30 watts and the balance 600 watts at %70 because they are oriented flat.total 450 watts

 

[time2roll]

Consider getting two (three?) controllers so each can handle a separate string. That way a single panel with shade only affects the single string.

If the rack can be elevated the problem could mostly be resolved and avoid shade.

 

[Samsonite801]

I've never tried it myself yet, but some people might say to put in blocking diodes on each of the strings right before they combine (which go to a common MPPT controller input), so if voltage drops lower on one string from shading, it can't backfeed onto the shaded string and pull down the voltage on the other parallel strings on the circuit. You could try that first maybe.

Something like this to be simple:

https://www.amazon.com/dp/B0825HS2VQ/

Diodes can introduce a bit of voltage drop, some people say to use Schottky diodes for lower loss.

This site below, says an 'active diode' can be even better than traditional diode solution (starting under the 'Active solutions' section):

https://www.digikey.com/en/articles/active-bypass-diodes-improve-solar-panel-efficiency-and-performance#:~:text=The%20Schottky%20bypass%20diodes%20used,strings%20is%20shaded%20or%20damaged.

And then try to arrange each series string of 3s in a way that any one of the sets of 3 doesn't get partially shaded...

But as time2roll said, separate MPPT chargers is a sure-fire method to isolate strings, costs a bit more maybe though...

 

[pollenface]

I'm a fan of using multiple arrays with their own charge controllers. I currently have 2 but planning for 4.

 

[Black-on-Plus]

Thank you for your answers and inspiration!
I think the best solution would actually be 3 separate arrays with 3 charge controllers.
Which connection would you prefer? 3x3 parallel or 3x3 series connected panels?
What is the best way for the MPPT to compensate for partial shading?

 

[Black-on-Plus]

Ok, I think the question is actually nonsense ?
I have to connect 3x3 in series because my Batpack has >24V. I assume, however, that if one panel fails, there will still be 200W on this string. and on the other two strings 300W each. so a total of 800W. ?

 

[efficientPV]

Everyone's experience may vary. I operate 6 strings in parallel in extreme shade and I tried three charge controllers because I had them, and it was a disaster. One or two charge controllers would contribute nothing thinking they were in float. I went back to a single charge controller. Unless they are designed to communicate with each other I would never do it again. Sure, when the battery is in deep discharge they will all contribute.

 

[time2roll]

Ok, I think the question is actually nonsense ?
I have to connect 3x3 in series because my Batpack has >24V. I assume, however, that if one panel fails, there will still be 200W on this string. and on the other two strings 300W each. so a total of 800W. ?

In a perfect world maybe. More realistic is to de-rate everything 20%.

 

[Goboatingnow]

Everyone's experience may vary. I operate 6 strings in parallel in extreme shade and I tried three charge controllers because I had them, and it was a disaster. One or two charge controllers would contribute nothing thinking they were in float. I went back to a single charge controller. Unless they are designed to communicate with each other I would never do it again. Sure, when the battery is in deep discharge they will all contribute.

well this isn't the case
if the batteries are low, then every solar controller will contribute current to their max ( unless you have the settings wrong)
as the absorption phase starts each controller will contribute current as long as the battery voltage is below their max absorption voltage cutoff.
then as each charge controller reaches its designated absorption it will detect a low charge current and change to float mode, It doesn't matter which ones go first in float mode as the battery has been charged to full anyway

There's no downside, synchronised charger is " nice ", but its not needed

 

[RKDowner]

Ok, I think the question is actually nonsense ?
I have to connect 3x3 in series because my Batpack has >24V. I assume, however, that if one panel fails, there will still be 200W on this string. and on the other two strings 300W each. so a total of 800W. ?

I would assume that if one panel fails it would break the circuit and you wouldn't get anything out of that string of three.

One of us is wrong, and it's probably me , but I'd really like to know the answer. Edit: I just saw the dates in this thread, I'll probably not get an answer.