Blocking diodes for paralleled panels?

[Axel_G]

Originally Posted by missourisailor

A year or so ago, I did the exact same test that Paul did, with the exact same results.

Two Sun Power 100 watt flexible panels.
500watt, wire round potentiometer for a load.
Several VOM’s and two, cheap Ah counters.
NO added diodes!

The outputs were pretty much close to equal, whether the panels were paralleled with one fully shaded. Or with just a single panel connected to the load.

What I want to know is if anyone else has done an output test using 1:1 controller/panel configuration.

I did a quick and dirty test following the above test using some old BlueSky MPPT controllers that came off a clients system.
The results were not what I expected. In a 1:1 set up, the output was less (about 2~3%) than with paralleled panels driving one controller.

Not anywhere close to what I’ve been told by the manufactures and so called “internet experts”.

Just wondering if I fubarred the test some how.

Thinking about it that could have been expected. MPPT controllers are switching voltage converters. These have a maximum efficiency close to the max. load, the efficiency goes down at lower loads due to switching losses. So one MPPT operated near its max. load will have a better efficiency then two MPPTs operated at lower loads.

[noelex 77]

Originally Posted by Paul Elliott

I then disconnected everything and hooked up a variable power supply to one completely shaded panel. There was no significant back-current until I exceeded Voc. I might be able to dig up my measurement data if anyone cares.

So with these two panels anyway, there is no need for blocking diodes.

This is typical. Blocking diodes are only very rarely a good idea.

[noelex 77]

Originally Posted by missourisailor

The results were not what I expected. In a 1:1 set up, the output was less (about 2~3%) than with paralleled panels driving one controller

The power gain with multiple controllers is produced because each panel is tracked independently and therefore more accurately.

I suspect in your test the panel was so shaded that even with better tracking the shaded panel was not capable of producing any power.

MPPT controllers do have significant self consumption when tracking. You need to be careful that this self consumption does not exceed the gains. This can happen especially with smaller panels and large older controllers, and will always happen in some circumstances.

Tests on the various solar panel configurations are difficult and time consuming. I admire anyone who is willing to take this trouble. Even a couple of data points are useful, but there is no one method of connecting solar panels that is best in all conditions. The key is to maximise the total output in all the various conditions that are common on boat.

[missourisailor]

Originally Posted by noelex 77

MPPT controllers do have significant self consumption when tracking. You need to be careful that this self consumption does not exceed the gains. This can happen especially with smaller panels and large older controllers, and will always happen in some circumstances.

This is a fact that is not stressed often enough!
And something that did not dawn on me until well after my "quick and dirty" tests were done.

But beings my testing procedure were done "spur of the moment", it is very likely my methods added to the power loss.

I was in the 1:1 camp, but now I have evolved to "maybe simple is better".

With all of this in mind, I have decided to just install all of the panels in parallel, driving the original BlueSky 3000 controller and see how this set up works.
If the power production is satisfactory, them I'm good.
I'm not going to sweat a couple of percent loss.


THe take away and back to the OP question, it is supper easy to add blocking diodes to an existing system. So install your panels with out the diodes and see what happens. If you you have doubts about production, plug in a diode in each panels pos lead and see if it that makes a difference.

But as has been stated many time during this thread, I don't think you will see much, if any difference.

[noelex 77]

Originally Posted by missourisailor

I'm not going to sweat a couple of percent loss.

Good point. Most solar panel discussion is about optimising the set up, for example the best type of charge controller, how it should be wired etc. Many of these options only make a small difference to the total output.

This thread is a prime example. I would suggest that adding blocking diodes will reduce the overall output and it is not something I would advocate, but the maximum theoretical loss will only be the voltage drop of the diode divided by the voltage drop of the panel. So if a Schottky diode is used the maximum loss will be approximately 0.35/17 or about 2% . This is in the worst case, ignoring any gains, and using low voltage panels.

For someone like myself cruising and deriving nearly all power from solar, small differences such as this are worth fussing over, but it is easy to wrongly gain the impression from threads such as this that incorrect wiring will have far more serious consequences than is the reality.

[CatNewBee]

The blocking diodes are meant for protection like a fuse, if one cell has a specific faulure (hotspot). otherwise they do not much.

It is like home. I mean a circuit breaker has a voltage drop and can get warm, but you would not optimize your circuit at home to save 1 or 2 watts by removing all the breakers and risk a fire...

[TLSparks]

Blocking or as I would call them Isolation diodes do to good things. I have 4 large cells with individual cables running back to my regulator. The cables run in my radar dome an other stainless pipes. With a moving boat, I had a concern of a cable fraying and shorting out all cells.


2nd since the cells are on port starboard on the cockpit and aft on the arch there is usually shading going on. The diodes make sure the side with the most sun and voltage is always being used and the lower cells are not dragging the stronger panel(s) down.



Seems to have worked well all through Mexico as we always had charged batteries.

Kind regards
Terry

[KP44]

Originally Posted by KTP

They are like $500 (for the Kid). I know that is only half a boat unit but still...

If you're going to stay with Kid, then stay. I buy good controllers for well under 100$US. (More like $50 for a 12v 40 amp controller.) Most good controllers already have the blocking diode, which is there to prevent your panels from radiating power into the dark night sky and draining your batteries.

But if you need blocking diodes, get good Schottky Diodes that are rated above your expected current needs. They will get hot while working because all lost power is lost as heat. Schottky have less voltage drop than most other diodes.

If one side array of panels is really not up to 17v because it is not in direct sun, or because there is a line or shroud shadow, then a separate controller makes sense.

[Mike Banks]

I used two controllers and put their outputs in parallel. I never had any problems doing it this way.

[Paul Elliott]

Originally Posted by TLSparks

The diodes make sure the side with the most sun and voltage is always being used and the lower cells are not dragging the stronger panel(s) down.

The measurements I've taken (described up-thread somewhere) show that a shaded panel will not drag down an unshaded one -- assuming similar panel design.

[Tikka]

NO, no diodes need it, they are allready build in the panel.
Always use one controller, make sure , controller can handle current.

never ever use 2 controllers connected parallel to one bat. bank.

blocking diodes are used quite opposite, when charging 2 independet batterie banks from one solar system.
Solar system can consist multiple panels with one controller, diodes are connected between controller and bat. bank.

[skipgundlach]

Heh.

The consensus seems pretty strong for no blocking diode.

Yet, as my controller's service guy was the one who strongly suggested I put them in my system (2 panels, paralleled, into one controller), at the $5 bux shipped I had to pay for them, I did it.

I don't have the means to effectively test the results, and don't disagree/contest those who say they're not needed. But absent a really good reason why they are detrimental, given that it came from the horse' mouth (the folks who make the controller), I'm leavin' 'em

[john61ct]

Originally Posted by Tikka

never ever use 2 controllers connected parallel to one bat. bank

Absolutely false, bad advice

Done all the time, no problem IRL.

[skipgundlach]

Originally Posted by john61ct

Absolutely false, bad advice

Done all the time, no problem IRL.

I don't have that setup, but modern controllers often have the ability to tandem to handle larger loads.

Perhaps one here who has such could post their info...

[Paul Elliott]

Originally Posted by skipgundlach

But absent a really good reason why they are detrimental, given that it came from the horse' mouth (the folks who make the controller), I'm leavin' 'em

With a "12V panel" (about 17V Vmp), a blocking diode will dissipate about 3% of the available power. If 3% is important to you, take out the diodes. Remember that we are overjoyed if we get an extra 10% by using an MPPT controller.

If you have higher-voltage panels, or a series-string of panels, then the blocking diode loss will be proportionally less.