Blocking Diodes

Rick D

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Jun 14, 2008
7,131
Hunter Legend 40.5 Shoreline Marina Long Beach CA
So, reading the manual for my new solar panels, I see that if they are wired in parallel, they insist you install a blocking diode in each. Well that is for the two 100 watt that I purchased. There are also four 50 watt that are wired in parallel.
So, when I put on the four 50's I did some research and thought I found that the current drop from a blocking diode was such that it was pretty much a wash for shading. So, on those four 50's which are arranged two on each side aft, I installed individual switches instead and switch them off whenever a panel becomes consistently shaded. Doing the same for the two 100's which will be on a bimini top isn't as easy.
Bottom line... does anyone have any applicable knowledge here that would help me figure out the best path here? I should probably have all six the same.
 
Jun 6, 2006
6,990
currently boatless wishing Harrington Harbor North, MD
Hey Rick
hows about a manufacture and model or a link to the manual so we all know what you are talking about. Or a picture of what you are trying to do.
The diodes used to "fix" partial shading should have already been installed inside the panel as they have to be placed every 3 or 4 cells to bypass them in the parallel cell matrix. don't think of diodes as current blockers for this bypass purposes, they are voltage drops. Blocking diodes are used to prevent nighttime back flow of current from the batteries through the panels. Your controller may already have them in it so you need to check that.
good luck
 

Rick D

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Jun 14, 2008
7,131
Hunter Legend 40.5 Shoreline Marina Long Beach CA
Hey Rick
hows about a manufacture and model or a link to the manual so we all know what you are talking about. Or a picture of what you are trying to do.
The diodes used to "fix" partial shading should have already been installed inside the panel as they have to be placed every 3 or 4 cells to bypass them in the parallel cell matrix. don't think of diodes as current blockers for this bypass purposes, they are voltage drops. Blocking diodes are used to prevent nighttime back flow of current from the batteries through the panels. Your controller may already have them in it so you need to check that.
good luck
Hi, Bill. I'm sure they have internal diodes. And, I'm sure my controller has a back feed block too. That this is referring to is the cannibalizing of one panel's feed to another when there are multiple panels wired in parallel. I have six. Here's the quote
Wherever possible, it is highly recommended that each Solar Panel has its own dedicated MPPT controller. This
will ensure the maximum performance possible even if one panel becomes compromised by partial shading.
If two or more panels are wired in parallel, blocking diodes should be installed in the positive lead of each panel.
 
Apr 26, 2015
660
S2 26 Mid On Trailer
Wherever possible, it is highly recommended that each Solar Panel has its own dedicated MPPT controller. This
will ensure the maximum performance possible even if one panel becomes compromised by partial shading.
If two or more panels are wired in parallel, blocking diodes should be installed in the positive lead of each panel.
I think you would still need one master to determine the state of the unit being charged.
The good thing about diodes is they are there when "you are not" to flip the switch. You are already headed down a worm hole mixing panels as far as losses go. I like to drink beer and let the electronics do their thing. :waycool:
I've been in the solar business for about 20 years but still learning so enlighten me on the problem with the minuscule drop across a diode. rickssolar@gmail.com
Don't take this the wrong way, I love solar. Lived in an RV (1400 watts) off grid for 12 years and hate the electric bills our "current" :))) house generates and how the state (AZ) screwed us, over grid tied solar.
 
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Likes: Rick D
Jun 6, 2006
6,990
currently boatless wishing Harrington Harbor North, MD
Ok, blocking diodes in this case are keeping a (example) fully lit panel from acting like a voltage source and forcing current backward through a partially lit panel. Would only happen when the battery voltage is above the partially lit panel voltage. think of solar panels as batteries charging each other. You want to direct the charge to the actual batteries and not waste it on "charging" the partially lit panel.
As for the MPPT controller on each panel, I'd have to agree. A partially lit panel will have a different MMP voltage then one that is fully lit.
FWIW a boost converter (google it) will raise the voltage of the panels output to a level that can charge the battery in most illuminations. A MPPT controller has a boost converter in it that checks for the MPP and sets the output voltage at that level. The charging issue is the MPP voltage may be below both the battery being charged and the other panel's output voltage resulting in current being force through the less lit panel
 
Jun 6, 2006
6,990
currently boatless wishing Harrington Harbor North, MD
FWIW a boost converter with fixed 14.4 volts output connected to a PWM controller will probably give more Ah out of a set of panels than the same panels with a MPPT controller on each panel. The MPPT controllers get confused as they both try to charge the same battery. A way to spend a lot of money and still have "I have MPPT on all my panels" bragging rights is to not try to charge the same batteries. break the bank into two banks and charge each separately using diodes, big ones so the whole bank sill still act together when discharging.
Honestly if you are looking to spend a lot of money for more AH a solar tracking system will up the output by over 30% over the course of a day. MPPT, PWM, and boost converters can't come close to that. Seems intercepting more sunlight is a better solution in all cases.
 
Jun 6, 2006
6,990
currently boatless wishing Harrington Harbor North, MD
And before it gets said.
boost converters are simply a coil of wire, a diode and a cap that you can make in an hour. Takes longer to figure it out than to build one.
solar tracking does not require gps, clocks and a computer. It can be done with six $2.00 opamps, 2-12 volt motors 2 H-bridges and a hand full of LEDs (sensors) and resistors (opamp controls).
 

Rick D

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Jun 14, 2008
7,131
Hunter Legend 40.5 Shoreline Marina Long Beach CA
LeeandRick: I don't recall the solar website I was reading in which the allegation that the blocking diodes cannibalized more current than they protected was in. I guess I was alarmed enough to go to the manual switches at the time, some years past. I defer to your experience and will install the diodes. Bill, thanks for the points. I have an MPPT controller. I'll have to digest the boost converter suggestion you made. As far as tracking, I have the four 50's mounted where the aft upper lifelines used to be; I had Garhauer make up some s/s tubing to replace the wire and mount my panels there. I made up adjustable telescoping braces that attach to the toe rails so that each panel can be adjusted to the sun angle during the day. Since my (vacation) mooring is located where the sun passes port to starboard, I don't have to worry about the fore/aft plane.
 

walt

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Jun 1, 2007
3,511
Macgregor 26S Hobie TI Ridgway Colorado
FYI, if you use diodes in the solar system, I would use a type called "schottky". I have not bought these but Amazon has a model that would seem like a good choice https://www.amazon.com/AKOAK-Schott...qid=1511388441&sr=8-1&keywords=schottky+diode

If you put a diode in series with a solar panel, the MPPT controller sees one diode drop voltage less than without the diode. A regular PN diode at that current might drop .75 volts which is 4.1 percent of 18 volts. So by having the diode in series, you loose 4.1 percent of the power each panel puts out (assuming MPPT).

Schottky diodes have a lower forward bias voltage drop than a regular diode and might be about .4 volts at a solar panel current. .4 volts is only 2.2 percent loss of what is available vs no diode.

There is no damage danger if you just parallel the panels without diodes. The problem happens if one panel is shaded and another stronger panel biases the voltage higher than the shaded panel operates at. A solar panel that has a voltage higher than the voltage it would create can actually draw current rather than provide current but I have heard its only about 5 percent of the panel rated current.

So... if every panel has a regular PN type diode, every panel loses about 4% all the time. If no panels have diodes and one is partially shaded, the shaded panel only might remove about 5% of what another single panel outputs.

If you are worried about it, just get the shottky diodes in the link above.

FYI, the numbers above only apply to MPPT controller. PWM is a different story. Since PWM doesnt take advantage of all the voltage available, its also much less sensitive to things like diode voltage drops.
 
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Rick D

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Jun 14, 2008
7,131
Hunter Legend 40.5 Shoreline Marina Long Beach CA
Thanks, Walt. I gotta think on that a bit. I did order just the diodes you suggested, the Shottky ones. So, it sounds like I'm only messing around with a nominal loss because of shading in the first place without the diodes, and a nominal loss of current but all the time with them. So, maybe as LeeandRick suggested, I am over-thinking this.
 

walt

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Jun 1, 2007
3,511
Macgregor 26S Hobie TI Ridgway Colorado
That about sums up what I think also.. The 5% number that I used for what a solar panel would drain when over biased was also from some web site I saw a long time ago on why you need a series diode at night. But in this case, you are putting 12 volts (battery voltage) on the panel at night when it would normally be generating zero volts. The parasitic draw likely looks at least somewhat resistive - ie, the drain is somewhat proportional to voltage. In the case of partial shading, the difference between a panel in full sun and a partial shaded panel would probably be less than 12 volts - and the the drain would be less than 5%.

The switches on the panels.. hmm.. I dont think that is at all necessary. I doubt anyone has actually measured what bad things happen if you dont use diodes.. and any reason is somewhat speculation. It would be a pain in the axx to do a really good experiment where you really only changed one variable.

Put the Schottky diodes in... not a big deal. You will have peace of mind that you are only losing about 2 percent and a partial shaded panel gets switched out of the parallel array. Dont put the Schottky diodes in.. also lilkely not that big of a deal but I dont think anyone really knows how much you lose. I think it would be overall small but am just guessing. I just would not use regular PN junction diodes.
 
Aug 22, 2017
1,609
Hunter 26.5 West Palm Beach
Your average garden variety silicone diode has a forward voltage drop of about .7v. It varies slightly based on specific device, load, temperature, etc, but .7v is the thumbnail sketch figure that engineers normally use when laying out a circuit. Shoddy diodes (Schottky) can range from about .3 to .5v of forward drop, but there is no free lunch here. The reduction in voltage drop across the half junction comes at a price. As the forward voltage drop improves, you usually loose PIV rating, reverse leakage current usually increases & they often become more susceptible to thermal runaway. Silicone diodes are generally a lot more robust in those areas. Leakage in common silicone diodes is often around 5 or 10 micro-amps.

I wasn't able to google up a spec sheet for an akoak XLL0026, so I can't quantify the cost of the performance trade off in this specific case.

If your panel puts out 16-18vdc & your controller knocks that down to 14.5vdc for the battery, then the loss of an extra .7 vdc at the panel is probably not going to be seen by the battery. The controller probably has a .7v minimum drop itself. If that is the case, then I would expect that any primary voltage from the panel that is above 14.5+.7+.7 would be seen as the same by battery that is being charged. If the diode was after the controller, that would be a horse of a different color. If the panel puts out less than 16vdc (shaded) then you might see a decrease in performance from the added diode.

If the controllers are dumb voltage regulators, then paralleling outputs of multiple controllers is probably OK. If they are some type of smart controller that pings the battery to determine it's condition before sending out a calibrated PWM charging barrage, Then putting the outputs in parallel is probably a risky proposition at best.

If you can get some spec's on the XLL0026 & you can take some voltage readings at the panel outputs & at the controller outputs, & you can measure leakage current on a fully shaded panel, then we stand a chance of making a reasonably educated guess as to which type of diode might be a better choice in this application. My gut tells me to put a silicone diode after each panel & feed them all into one controller, but without more information to work with, that is little more than a wild guess.
 
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Apr 26, 2015
660
S2 26 Mid On Trailer
Rick D, just so you know that Walt is not some BS forum expert, he is an EE with numerous patents and over thinks things for all of us. Good guy.
After years of going down the road of trying to make everything as perfect as possible, most current per watt to the battery, etc, I'm down to the KISS principal and adding 10% more wattage than I think I need. After years of pushing MPPT controllers I bought a PWM last week and a better oriented mount. If my load increases I'll dig one of the old MPPTs out of a box in my garage.
It appears that JimInPB has done some serious research also.
 

walt

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Jun 1, 2007
3,511
Macgregor 26S Hobie TI Ridgway Colorado
The reduction in voltage drop across the half junction comes at a price. As the forward voltage drop improves, you usually loose PIV rating, reverse leakage current usually increases & they often become more susceptible to thermal runaway.
You will need to explain more what thermal runaway means for this specific solar panel case.. if you dont mind. In this case for around a 100 watt panel (about 5.7 amps), we are using a Schottky diode rated for 15 amps.

Schottky diodes do have a little higher leakage current but it doesnt matter in this application. At night, it is still the controller that takes care of panel leakage. The diode that is in series with the panel doesnt even get biased at night - because of the controller. The only time there is any reverse bias is when a panel gets partial shading.. and the reverse current is still likely way less than 1 ma under almost all conditions. Say the diode gets 10 volts bias accross it under partial shading. That is only 10 thousands of a watt. Its in the noise..

Then compare the power loss for a 100 watt panel between a regular diode and a schottky diode. Say the panel is putting out 5.7 amps. The power loss because of voltage drop with a PN diode is 4.27 watts (Vdrop = .75). But the power loss with the Schottky diode is 1.71 watts (Vdrop = .3). The Schottky in the system results in way lower power loss.

You will notice that the Schottky diodes in the link before said they were for a solar application. But I also did not see a detailed spec for that specific diode.
 
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Sumner

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Jan 31, 2009
5,254
Macgregor & Endeavour 26S and 37 Utah's Canyon Country
...I'm down to the KISS principal and adding 10% more wattage than I think I need......
:plus::plus:, but do use MPPT controllers, but don't expect more than 8-10% increase over the others most of the time if that much. Out 2 weeks recently with a fairly large fridge and had plenty of power all the time,

Sumner
=============================================================
1300 miles to The Bahamas and Back in the Mac...
Endeavour 37 Mods...
MacGregor 26-S Mods...
Mac Trips to Utah, Idaho, Wyoming, Canada, Florida, Bahamas
 

Rick D

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Jun 14, 2008
7,131
Hunter Legend 40.5 Shoreline Marina Long Beach CA
Whew. I am in awe of the expertise here. I think what I am chasing is likely a micro issue. It's just like anything else on a boat, you want it to be efficient. I want to thank everyone who contributed for the terrific detail and advise. It was more than I hoped to get and I hope this is found in the future by others having the same question.

To fill in the blanks here, I use the temporarily-mounted solar panels when cruising outside of marina territory, which isn't customary is SoCal since there are few anchorages that are not restrictive. If I am doing three or so days at sea, I simply use the high-output alternator for charging twice a day. Where I use the solar is on my Catalina mooring. I had 266 watts total with the four 50's on the rail and three 22's on the bimini (I have had them for about two decades!). I sold off the three 22's and replaced them with two semi-flexible 100's for the bimini. The 266 watt configuration gave me about two days of (summer) running the two hogs, the refrig & freezer (two compressors) and otherwise modest electrical with fresh batteries (400 amp hours). I figure the addition of fresh panels and +134 watts will give me significantly longer time between Honda 2000 runs. And, I don't want to add wind, even though that mooring location is suited for it.

So, thanks again all!
 
Aug 22, 2017
1,609
Hunter 26.5 West Palm Beach
You will need to explain more what thermal runaway means for this specific solar panel case.. if you dont mind. In this case for around a 100 watt panel (about 5.7 amps), we are using a Schottky diode rated for 15 amps.

Schottky diodes do have a little higher leakage current but it doesnt matter in this application. At night, it is still the controller that takes care of panel leakage. The diode that is in series with the panel doesnt even get biased at night - because of the controller. The only time there is any reverse bias is when a panel gets partial shading.. and the reverse current is still likely way less than 1 ma under almost all conditions. Say the diode gets 10 volts bias accross it under partial shading. That is only 10 thousands of a watt. Its in the noise..

Then compare the power loss for a 100 watt panel between a regular diode and a schottky diode. Say the panel is putting out 5.7 amps. The power loss because of voltage drop with a PN diode is 4.27 watts (Vdrop = .75). But the power loss with the Schottky diode is 1.71 watts (Vdrop = .3). The Schottky in the system results in way lower power loss.

You will notice that the Schottky diodes in the link before said they were for a solar application. But I also did not see a detailed spec for that specific diode.
Thermal runaway was just listed as a general concern. Since I do not have the specific parameters of the circuit in question, I can’t determine if it is likely to be a problem here or not.

I was unaware that the controller also acted as a rectifier. That changes the playing field substantially & attenuates some of my concerns.

I’m not understanding how you are calculating power loss due to the diode drop. I am not seeing how power loss is a linear quantity based on a fixed voltage drop before the controller. If the voltage drop was after the controller, I would see the % loss.

With the voltage drop before the controller, I only see a loss when the minimum drop of the controller + the drop of the rectifier + the desired output voltage is more than the primary voltage from the panel.

Lets say that the controller puts out 14vdc for round numbers. Lets say that the rectifier drops 1vdc for round numbers. If you put in 18vdc from the panel, then the controller sees 17 after the diode & puts out 14. In this case the controller drops 3 volts. Lets say that the panel puts out 17vdc. In this case, the controller sees primary of 16 & still puts out 14, so it drops 2 volts. In either case, 14vdc is put out. Lets eliminate the diode & say that the panel puts out 18 volts. In this case, the controller creates a 4 volt drop to still put out 14. I don’t see a change in current. The secondary voltage is regulated at the same point regardless of how much extra primary voltage is provided by the panel. Therefore the wattage output after the controller would be the same. No?

I don’t see a difference between a 3 volt drop across the controller or a 2 volt drop across the controller + a 1 volt drop across a rectifier. The controller varies it’s drop until the primary voltage is too low for it to put out what it wants to put out. I don't see a power output loss until the voltage from the panel drops below probably about 16v due to shading.

Am I missing something? Perhaps I don’t understand how the controller works? Perhaps I assume that the panel puts out more voltage than it actually does?
 
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walt

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Jun 1, 2007
3,511
Macgregor 26S Hobie TI Ridgway Colorado
Im speculating somewhere here..

There are two main type of controllers we see sold. PWM and MPPT.

PWM is the simpler type and it regulates current by modulating a switch between the panel and the battery. But when the controller is trying to pass full power, the best it can do is connect the solar panel directly to the battery. The solar panel has nearly constant current over a lot of its operating range so the solar panel simply supplies its current - but at the battery voltage. The power delivered is that current * battery voltage. If you insert a diode in series with the panel and the PWM charger is trying for max power, the panel voltage will be the diode drop plus the battery voltage. The power going into the battery is still the same since that power is still battery voltage * panel current. Adding the diode for this particular parallel case didnt cause any power loss but the diode also really has no value.

Ive been assuming that Rick has MPPT.. I have been watching my Victron MPPT conroller lately and at night the panel voltage is zero. As the sun starts to come up, the panel voltage rises until it hits 5 volts above the battery voltage and then the MPPT switcher kicks on. The circuit basically adjust the impedance to the panel and using magnetic switching (some what like a switching power supply), converts the power it gets from the panel to power to the battery. The actual battery voltage doesnt matter except for when the controller wants to cut back on current.

The controller will find the max power point by doing an impedance to the panel sweep. During the sweep, the controller is measuring current and voltage so can determine power. It finds the max power point and uses that as a basic for the next sweep. This sweep can be fast.. I think one controller I looked at was 15 hz. One controller I looked at a long time ago just did a sweep every 90 seconds. Actual maximum power point does not change quickly (how fast do clouds move) but you can have a fast partial shade like a fluttering flag.

So with the MPPT controller where it will set an impedance to get the max power point, any voltage loss in line between the panel and the controller directly correlates to a power loss. Example, in one case a MPPT controller biased a panel at 18 volts and the current is 5.7 amps. Put a .75 volt diode in series and now the controller still sees 5.7 amps but .75 volts lower than before and the power loss in the diode is simply VI or .75*5.7 amps.

Also with the MPPT controller. The voltage loss in the wires between the panel and controller because of wire resistance also directly correlates to power loss of the solar panel output. If you calculate the wire size for 3% voltage loss, the wire is of course safe to use but you also lost 3% of the power the panel outputs.
 
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Aug 22, 2017
1,609
Hunter 26.5 West Palm Beach
Some of what you say makes sense to me & some does not. I have a pretty good background in various flavors of switching power supplies, but not so much in MPPT. I'll probably be back with a few questions after I do a little reading & improve my understanding of how MPPT systems work. I may have made some poor assumptions regarding the characteristics of that particular little black box. That may be the origin of the differences in our perspectives.

Thank you for the detailed dissertation.