Blocking Diode Question please:

[upnorthandpersonal]

Just as panels typically have the bypass diodes pre-installed, typically the SCC typically has the blocking diodes built in, it should say on its datasheet or manual.

This is true, but those serve a different purpose - to prevent power from flowing from your battery back to the panels. If you add multiple parallel strings to the same charge controller, the charge controller blocking diodes will prevent power from the battery from flowing back to the panels, but not from power flowing between the parallel strings of panels.

 

[Checkthisout]

This is true, but those serve a different purpose - to prevent power from flowing from your battery back to the panels. If you add multiple parallel strings to the same charge controller, the charge controller blocking diodes will prevent power from the battery from flowing back to the panels, but not from power flowing between the parallel strings of panels.

The MPPT will drop the voltage down to whatever results in highest wattage output from the array which will be at or below the voltage of the shaded string.

 

[upnorthandpersonal]

The MPPT will drop the voltage down to whatever results in highest wattage output from the array which will be at or below the voltage of the shaded string.

I'm not disputing that. Only that the blocking diodes in the MPPT are not the same as blocking diodes in parallel solar strings.

 

[Checkthisout]

I'm not disputing that. Only that the blocking diodes in the MPPT are not the same as blocking diodes in parallel solar strings.

Got it.

 

[timselectric]

I tested my array with and without blocking diodes. for two weeks each way. Same amount of sunlight (which actually took me a couple of months to accomplish). There was no noticeable difference. I believe that any benefit from the diodes was being eaten up by the heat loss at the diodes. So I decided that it was better to remove the extra failure points. Since I wasn't gaining anything in return.

 

[timselectric]

It's also worth noting that my array was very unbalanced. Two strings of ten and one string of nine, facing east. Six strings of ten and one string of nine, facing west. I would call it extreme testing. lol

 

[octal_ip]

It's also worth noting that my array was very unbalanced. Two strings of ten and one string of nine, facing east. Six strings of ten and one string of nine, facing west. I would call it extreme testing. lol

Extreme indeed, I'm amazed you didn't see a difference in that scenario.
A great example of the value of real-world testing.

 

[Cal]

This video shows it's better not using blocking diodes.

There's two panels: 24V, 100 W and 12V, 50 W panel. The 50W panel could be considered being a 100W panel with shaded conditions, producing just 50W.

Individually the panels produce:
24V, 100W panel outputs 5.85 A into a 12V battery, with Vmp = 34V
12V, 50W panel outputs 3.08 A into a 12V battery, with Vmp = 18V

When the two panels are in parallel they output 6.29 A, with Vmp = 20V

Conclusion:
If a blocking diode is used then output current is 5.85 A. Without a blocking diode output is 6.29 A. There's better performance without blocking diodes. This is a worst case test. If all panels are the 24V variety then the shaded panel Vmp will be significantly greater than 18V. Output current will then be significantly greater than 6.29 A.

 

[curiouscarbon]

to deal with this issue, using multiple MPPT instead of paralleling arrays facing different directions to one MPPT is the approach i plan to implement.

being able to see more granular data about PV performance is also a motivation for me

either way, parallel connecting them to one MPPT without supplemental blocking diodes also seems just fine..

thanks again everyone

 

[Bud Martin]

Help me out here.

Can't we test if we need these diodes by simply installing our system, then disconnecting the shaded side array and seeing what happens to the output at the charge controller?

Maybe make a little dc breaker and add a couple mc4 connectors and plug that in before sunup to use for testing purposes as a switch. Or just shut off the breakers to each string at the combiner box ?

Why would this be something you need to figure out during initial design instead of after install?

How often do you see blocking diodes in professional setups and what do UL listed blocking diodes look like?

I see them used in the Combiner box, see the schematic.

4-1 Solar Panel Combiner Box – 4 circuits to 1 (1 x 63A max output) – Watts247 Wholesale

watts247.com

 

[Checkthisout]

I see them used in the Combiner box.

4-1 Solar Panel Combiner Box – 4 circuits to 1 (1 x 63A max output) – Watts247 Wholesale

watts247.com

Ok

 

[meetyg]

Just for reference, Aliexpress is full of MC4 blocking diodes.
I use these from BlueSunSolar in Aliexpress:

https://www.aliexpress.com/item/32707078191.html

I don't remember what the voltage drop is on them, but they seem to be good quality.

I am also planning two series of panels facing different angles.

Rectifying diodes are usually used in PV applications due to thier high reverse voltage rating. Schottky diodes usually have a lower reverse voltage rating which could be problematic with high voltage PV strings.

 

[Cal]

In order to obtain best performance the series strings need to be balanced. They should all have similar Vmp. For example, there are panels on a east and west facing roof. The east facing panels should not be connected together to form series strings. Nor the west side. A series string should contain equal number of east and west panels. The series strings are now balanced due to shading or different panel angles to sun. This configuration will require longer connecting wires but performance will be significantly better.

 

[Checkthisout]

In order to obtain best performance the series strings need to be balanced. They should all have similar Vmp. For example, there are panels on a east and west facing roof. The east facing panels should not be connected together to form series strings. Nor the west side. A series string should contain equal number of east and west panels. The series strings are now balanced due to shading or different panel angles to sun. This configuration will require longer connecting wires but performance will be significantly better.

I would think this would be the worst way as you're forcing the bypass diodes in the shaded panels to do a lot of work and basically adding a restriction for current traveling from the sunny panels.

How is that more efficient than having all the panels in one parralell string in full sun/shade?

 

[timselectric]

In order to obtain best performance the series strings need to be balanced. They should all have similar Vmp. For example, there are panels on a east and west facing roof. The east facing panels should not be connected together to form series strings. Nor the west side. A series string should contain equal number of east and west panels. The series strings are now balanced due to shading or different panel angles to sun. This configuration will require longer connecting wires but performance will be significantly better.

That's not going to work very well.
All panels in a string need to face the same direction. In order to see the same amount of sunlight.

 

[meetyg]

In order to obtain best performance the series strings need to be balanced. They should all have similar Vmp. For example, there are panels on a east and west facing roof. The east facing panels should not be connected together to form series strings. Nor the west side. A series string should contain equal number of east and west panels. The series strings are now balanced due to shading or different panel angles to sun. This configuration will require longer connecting wires but performance will be significantly better.

I don't get it either...

 

[Cal]

I would think this would be the worst way as you're forcing the bypass diodes in the shaded panels to do a lot of work and basically adding a restriction for current traveling from the sunny panels.

How is that more efficient than having all the panels in one parralell string in full sun/shade?

You don't seem to understand the repercussions of having large Vmp differences in series strings. See post #48. The 50W panels could be on the east side while the 100W panels are on the west side. When combining (in parallel) the east strings and west strings you'll see a large discrepancy in Vmp. The parallel configuration will produce slightly more current than just the west side themselves. This is not efficient because there's large differences in Vmp. If you combine east and west panels in a series string then all the series strings will have similar Vmp. Solar harvest will be significantly higher.

Rule number 1 when connecting panels in parallel: panels (or series strings) must have equal Vmp for best performance.

 

[Checkthisout]

You don't seem to understand the repercussions of having large Vmp differences in series strings. See post #48. The 50W panels could be on the east side while the 100W panels are on the west side. When combining (in parallel) the east strings and west strings you'll see a large discrepancy in Vmp. The parallel configuration will produce slightly more current than just the west side themselves. This is not efficient because there's large differences in Vmp. If you combine east and west panels in a series string then all the series strings will have similar Vmp. Solar harvest will be significantly higher.

Rule number 1 when connecting panels in parallel: panels (or series strings) must have equal Vmp for best performance.

Well, ok.

So what would be the difference in output between two like panels, one operating at Vmp and the other being pulled down to almost short circuit current?

Edit: And which setup would be better if our panels didn't have bypass diodes?

 

[timselectric]

You don't seem to understand the repercussions of having large Vmp differences in series strings. See post #48. The 50W panels could be on the east side while the 100W panels are on the west side. When combining (in parallel) the east strings and west strings you'll see a large discrepancy in Vmp. The parallel configuration will produce slightly more current than just the west side themselves. This is not efficient because there's large differences in Vmp. If you combine east and west panels in a series string then all the series strings will have similar Vmp. Solar harvest will be significantly higher.

Rule number 1 when connecting panels in parallel: panels (or series strings) must have equal Vmp for best performance.

Panels facing different directions and put in series with each other is the worst possible thing to do.
All that does is turn the bypass diodes into heaters.
I think that you are mixing two different things together.

 

[Cal]

Panels facing different directions and put in series with each other is the worst possible thing to do.
All that does is turn the bypass diodes into heaters.
I think that you are mixing two different things together.

Let me take you back to school. This video will explain it.

We got two 100W panels. A 12V and a 24V panel. By themselves the 12V panel puts 5.82A (Vmp = 20.2V) into the battery and the 24V panel puts 5.69A (Vmp=33.1V) into the battery. Since they are both 100W panels, current into the battery is about the same (as it should be).

Now the million dollar question is which configuration is better, parallel or series when combining them. Even though panel currents are extremely dissimilar (5.82A vs. 2.78A) the series configuration is significantly better (25.8%) than a parallel configuration (8.81A vs. 7.0A into the battery).

The reason for better performance with series is because the parallel configuration has unequal Vmp (20.2V vs. 33.1V). What's rule #1 when connecting parallel panels (or connecting series strings)?

If you have to connect possibly large Vmp deviation series strings in parallel then first configure the series strings so that Vmp's are similar. It's way worse for parallel panels to have unequal Vmp than for a series string to have unequal current.

Since the series configuration output current is greater than one panel by itself (8.81A vs. 5.82A) we can conclude the bypass diodes are not be conducting in the 24V panel! I'm not mixing two different things together (as you say). You don't fully understand solar.