Blocking diode with parallel solar panels

[noelex 77]

Originally Posted by tanglewood

The specific test I ran was to apply a variable voltage to a shaded panel using an external power supply, the power supply simulating an unshaded panel. As I increased the voltage, there was very little current flow until it reached about 80% of the rated voltage, then it started to flow. Beyond that, current increased as voltage increased.

This test is worth doing. Could you please quantify the current? The value around and just below the panel’s Vmp would be most applicable. Finally, what brand and type of panels were these?

Thanks.

[tanglewood]

Originally Posted by noelex 77

This test is worth doing. Could you please quantify the current? The value around and just below the panel’s Vmp would be most applicable. Finally, what brand and type of panels were these?

Thanks.

The test panel was a Solarex MSX120. It's probably 20 years old, and I don't know if they are even in business anymore. Voc = 21.3, Vmp=17, Isc = 7.7, Imp=6.9 HOWEVER it's wired at it's junction boxes as 2S2P groups of 18 cell, and I tested against one of the groups. So the operative numbers are half, or Voc=10.7, Vmp=8.5, iSC=3.8, Imp=3.4. A forklift gouged the back of the panel leaving only two of the four cell groups usable. But the junction box gives access to all the groups individually. I ran the test separately on each of the functioning cell groups and got the same results.



The cell group started conducting at about 8V, so a bit below Vmp. And at 10V the current hit the power supply's limit of 6A. The panel has a fuse requirement of 20A (10A for each cell group), so I expect if you kept increasing voltage it would keep drawing current until it melted down.

[tanglewood]

Originally Posted by Martkimwat

Modern Schottky diodes only drop the voltage about 0.5 volts but they don't affect the current so as you correctly say, modern panels are less likely to be so much affected, I believe you are incorrect in saying larger panels will be less affected since the diode does not affect the current which is what a larger panel will give you more of.

From what I've seen, larger panels tend towards higher voltage, not higher current. There will be some variation, of course, but the 360W panels I'm working with are 60V and 6A (approx), and the 120W panel that I did testing on is 20V, 7A. The overall trend in solar I think is towards higher voltage.


But the loss in a blocking diode is straight forward as the voltage drop (.5V is a good baseline) times the current. So for a 6A panel it's about 3W, regardless of the panel's voltage. For a 360W panel, I think a <1% loss to a diode is well worth it on a boat where some random shading is highly probable at all times. Even for the 120W panel, a 2.5% loss is probably a worthwhile trade off.

[s/v Jedi]

This is why the “series or parallel” questions are flawed. One needs to decide array setup on shading characteristics. If you have two panels and normally one gets some shading while later only the other one gets shading, they need separate (smaller) controllers. This is the setup with two panels on a bimini at each side of the boom.
When you can fit 4 panels on the bimini, with 2 on each side of the boom, then you still need 2 controllers and only now the question on series or parallel is valid for the two in each group. Here, series can be more efficient or even required to get the voltage high enough.

[tanglewood]

Originally Posted by s/v Jedi

This is why the “series or parallel” questions are flawed. One needs to decide array setup on shading characteristics. If you have two panels and normally one gets some shading while later only the other one gets shading, they need separate (smaller) controllers. This is the setup with two panels on a bimini at each side of the boom.
When you can fit 4 panels on the bimini, with 2 on each side of the boom, then you still need 2 controllers and only now the question on series or parallel is valid for the two in each group. Here, series can be more efficient or even required to get the voltage high enough.

Agreed. For every complex question there is a simple answer... and it's wrong.

[noelex 77]

Thanks for the detail Tanglewood.

With a specified Vmp of 8.5v the actual working Vmp after temperature compensation is likely to be around 8v so this is the voltage where you would be concerned about back feeding. I note that you indicate that the current only began around this voltage. Voltages in the range 7.8-8.5v would be seen in practice so the current at these voltages is the most relevant. However, the high current you are seeing at higher voltages is not typical. I suspect the panel is defective, possibly from the forklift damage, or perhaps the bypass diodes have seen better days and are breaking down as the the voltage is increased.

This is a measurement that is worth doing if you are planning to connect multiple panels to a solar controller. The results do vary with different types of panels, but normally the potential backfeed current is very tiny and installing a blocking diode results in a (small) net loss of power (providing your solar controller disconnects the panels at night. Nearly (but not quite all) modern solar controllers will perform this function so with a suitable solar controller normally blocking diodes are counterproductive.

If anyone can report other measurements, that would be great.

[tanglewood]

Originally Posted by noelex 77

Thanks for the detail Tanglewood.

With a specified Vmp of 8.5v the actual working Vmp after temperature compensation is likely to be around 8v so this is the voltage where you would be concerned about back feeding. I note that you indicate that the current only began around this voltage. Voltages in the range 7.8-8.5v would be seen in practice so the current at these voltages is the most relevant. However, the high current you are seeing at higher voltages is not typical. I suspect the panel is defective, possibly from the forklift damage, or perhaps the bypass diodes have seen better days and are breaking down as the the voltage is increased.

This is a measurement that is worth doing if you are planning to connect multiple panels to a solar controller. The results do vary with different types of panels, but normally the potential backfeed current is very tiny and installing a blocking diode results in a (small) net loss of power (providing your solar controller disconnects the panels at night. Nearly (but not quite all) modern solar controllers will perform this function so with a suitable solar controller normally blocking diodes are counterproductive.

If anyone can report other measurements, that would be great.

You have nailed the fundamental question which is how much back-feed current is there, and under what conditions.


What we do know if that blocking diodes originally came into play when panels were direct connected to batteries, and overnight the batteries were draining back through the panels. Blocking diodes were the solution, and are now a part of every charge controller.


So we know there is some drain back, but how much, and under what condition? In other words, do we care?


I'm pretty sure the portions of the panel that I'm using are fine. The damage is very visible, as are the cells and their wiring. But I agree more confirming data would be very valuable.


To that end, I think today we will get some data from the actual boat with the four 360W panels. The test will be to measure the current for each panel, and see if it goes negative on a shaded panel. If anyone else can run a similar check, that would be great.


I've been trying to find info on the electrical characteristics of solar cells when shaded, and have only been partially successful. Most info focuses on forward biasing a shaded cell, in which case it conducts and is why panels all include bypass diodes. I did find one lecture on youtube that made it clear that a shaded cell will be a load to a parallel unshaded cell, but it wasn't quantified as much as I'd like.

[CatNewBee]

I use reverse current protection diodes (Schottky string diodes) on my 5 parallel panels installation. Makes sense, alternative would be to add dedicated string fuses per panel.

[noelex 77]

Originally Posted by tanglewood

Blocking diodes were the solution, and are now a part of every charge controller.

Most of the better charge controllers don’t actually incorporate a blocking diode. The associated voltage loss would hurt their efficiency. Instead they disconnect the solar panels from the batteries when the software determines that the potential solar panel current would produce less power than self consumption of the controller. Normally this is done with MOSFETs or relays rather than diodes. At this stage the solar controller goes into “sleep” mode and disconnects the panels from the batteries while still monitoring the solar panel voltage.

The electrical effect is much the same, but it does create some other consequences, for example the controller needs to “wake up” at the right point as the event is software controlled rather than automatic as would be with a simple diode.

Originally Posted by tanglewood

So we know there is some drain back, but how much, and under what condition? In other words, do we care?

We should care. Utilising solar on a boat needs an efficient set up. It is particularly important to have the physical installation working as well as is practical. It is not unusual to hear of people installing blocking diodes reasoning that they must help, but I think in most cases they are actually decreasing the net output and introducing another failure point. However, it would be great to see more data so the issue can be definitively resolved.

Strangely, when panels are connected in series, very few people fit additional bypass diodes around each panel. These have much more potential to improve output. It would be nice to see some practical measurements on this option as well.

[noelex 77]

Originally Posted by CatNewBee

I use reverse current protection diodes (Schottky string diodes) on my 5 parallel panels installation. Makes sense, alternative would be to add dedicated string fuses per panel.

This is a good point. Almost everyone installs a fuse between the batteries and the solar controller, as they should do, but many do not realise that in some installations fuse protection is also needed on the solar panel side of the wiring.

Fortunately, this additional protection is only needed where you have a string of three or more panels. Many marine installations do not meet this requirement, especially with the current trend to fit one controller per panel.

If fuse protection is required it can be argued that blocking diodes can be used to provide the same protection. Unfortunately, as I understand the situation, most licensing authorities (anyone know what the ABYC say ?) do not agree that diodes alone (without fuse protection) are adequate. I assume their reasoning is a diode could fail in such a way that would not be apparent and the protection would be lost, but I have never heard this spelled out.

[rha9]

Seems pretty well agreed that bypass diodes are part of the design of solar panels now, but what is the typical configuration? As one article put it, “diodes cost money and complicate the wiring”, so perhaps cheap panels don’t have as many and so will not perform as well under shading conditions? Anyone have any experience with this? I’ve never seen info on bypass diodes included in panel specs.

[TeddyDiver]

Originally Posted by rha9

Seems pretty well agreed that bypass diodes are part of the design of solar panels now, but what is the typical configuration? As one article put it, “diodes cost money and complicate the wiring”, so perhaps cheap panels don’t have as many and so will not perform as well under shading conditions? Anyone have any experience with this? I’ve never seen info on bypass diodes included in panel specs.

A couple of years ago here in CF was a video of a test on shading solar panels with to solarpanels both in series and paralled configuration. They were without bypass diodes as even small shading dropped the output of the panel close to zero..

[noelex 77]

Originally Posted by rha9

Seems pretty well agreed that bypass diodes are part of the design of solar panels now, but what is the typical configuration? As one article put it, “diodes cost money and complicate the wiring”, so perhaps cheap panels don’t have as many and so will not perform as well under shading conditions? Anyone have any experience with this? I’ve never seen info on bypass diodes included in panel specs.

Bypass diodes are primarily installed to prevent internal damage of the solar cells. All reputable solar panels incorporate bypass diodes. The number depends on the characteristics of the particular solar cell, around 3 is typical. This information should be found in the technical description of the panel.

More bypass diodes slightly help the shade tolerance, but the difference is not great. Other factors tend to be more important when deciding on the best panels for your application. It is not (usually) feasible to add extra internal bypass diodes, but if connecting panels in series an extra bypass diode around the whole panel is worth considering.

[s/v Jedi]

Originally Posted by TeddyDiver

A couple of years ago here in CF was a video of a test on shading solar panels with to solarpanels both in series and paralled configuration. They were without bypass diodes as even small shading dropped the output of the panel close to zero..

It’s even worse: the whole array loses power, not just the shaded panel.

Let’s say we have two panels and each has 60 cells in series at 0.5V per cell for 30V and 3 bypass diodes. This means each panel has 3 strings of cells in series with a diode parallel to the string. So that is 20 cells per diode which is 10V.

Let’s connect these two panels in series. We get 60V. If a panel gets partial shading on one string, output drops to 50V, on two strings 40V etc. so we lose 1/6th of power for each shaded string. Without diodes, or failed diodes, the whole array becomes high resistance and output drops dramatically.

Now we put these in parallel: at any partial shading, the bypass diode(s) trigger and the panel voltage lowers but as it is parallel to a full voltage panel, it’s can’t output any current anymore and array output drops by 50%

In reality there are more losses. For series there are diode losses, meaning that instead of 10V you loose 10.3V. For parallel, without diodes, the unshaded panels pump their output into the shaded panels as well as the controller.

Always have diodes and make sure they work. It’s easy to check controller statistics to see if anything changes significantly.

[s/v Jedi]

Someone asked me:

Wouldn’t blocking diodes in each parallel panel mitigate the shading loss in that configuration?

Correct, however most panels don’t have blocking diodes anymore as they count on the solar controller not to discharge the battery during the night.