Solar panel question....

[windstuffnow]

  I've added some solar panels to my system and just purchased 4 more used ones.  The used ones don't have diodes installed.

  Do they have to have diodes in them?  They will be wired paralell for 12V charging and going through a Xantrax C60 controller.  Doesn't the controller have blocking diodes in place so the batteries don't discharge into the panels at night?  That being the case there wouldn't be a need for the diodes in the panels... correct?

  What is the difference between "blocking" or "bypass" diodes as it relates to solar panels.  I've seen both mentioned but don't quite understand the difference.   One blocks the reversal of current and one blocks the current from other panels if one is shaded?

   As you can see I have little to no experience with solar panels.   I've had the first 3 on-line for about a month now but they don't deliver quite enough.

   Anyway any help is appreciated....  Oh, and if they do require diodes, where can the 6amp diodes be purchased?

Have Fun

Windstuff Ed

[TomW]

Ed;

I'll tell you what I found when I first got mine and in reading since then plus a bit of experience.

There is a school of thought that the blocking diodes [for stopping nitetime reverse current] are not really necessary. The reason being that the power loss through the diodes is more than the few microamps they may leak in darkness. I tested this theory on a pair of 1 foot square glass panels I got years ago. Reverse current into those was almost too low to measure with a cheap meter. Just a few milliamps.  So a few Ma. over say 12 hours compared to the loss of even 10 amps over 12 hours thru a diode [7 watts loss whenever charging compared to less than a watt loss in darkness]. Hope that makes sense. It is mostly from reading up on it and makes perfect sense. I do not use blocking diodes with my Trace C60.

My Siemens panels came with the bypass diodes so not sure if they are needed. My personal understanding from the reading is that unless you are in a situation where one or more are shaded "regularly" then you should use them but under most conditions of open sun they are not needed for essentially the same reason blocking diodes are not.

Of course I imagine the controller already has some isolation due to the FETs that it probably uses but cannot say for sure. Of course this may be one of those topics where there are as many opinions as people offering them. If in doubt a few tests would prove or disprove the loss numbers. There may be issues I am unaware of, also.

Cheers.

TomW

[DanB]

If I recall... my panels have blocking diodes (I understand to block current from flowing back into them) but they would only be used in the 24 volt configuration - not the 12 volt.  So I've always thought that they were used in higher volt systems... not at 12 volts.  Im not sure if thats true... of even if it were, why (cause it doesnt really make sense to me)

If it were me Ed... I'd just hook 'em up w/o the diodes, wait till it gets dark...and see if any current flows back into them.

[windstuffnow]

   I read an article by Sandia Labs and they pretty much said the same thing.  For the 12V system their pretty much not necessary.  They say its an option with a 24V system but manditory with 36 volts and above.  The bypass diodes are mostly for system that are wired in series for higher voltage.  

   Their going up tomorrow rain or shine!  That will put just over 600 watts in the sun. Now I won't have to be quite so conservative until my wind system is back in place.

   Unfortunately, these solar panels have used up a considerable amount of my project funds so I'll be skimping on some projects soon.

   As soon as I get this old aermotor tower rebuilt things will get back to normal... well as normal as it gets around here....

Thanks for the input all!!!

Lots of fun

Windstuff Ed

[camp185]

I am not sure of the size of your solar panels, but you can buy a 4 pack of 6 amp diodes from Radio Shack for $2.49.  They should be able to handle a 50 watt no problem.  Without the diodes I know power will try to go back to the panels, but unsure if your controller stops this or by how much.  You could always disconnect the panels one evening and see what kind of power comes out of your controller.

I have a cheapo controller, 7 amp max and the loss without diodes is .7 volts at .03ma so basically nothing.  I could shine a flashlight on the solar panel, and probably make up the difference.

Rob

[Opera House]

I always believed that blocking diodes weren't necessary till I got my Photowatt PW-750s. They are after all just big flat diodes in series!  Leakage was obvious at night and can vary by panel. I run straight into a battery by flipping a switch that bypasses the diode durring the day to help compensate for long line loss.  As voltage at the panel increases, the max current drops.  For me, no regulator is better till I design a max power point tracking one.  Probably 24V panel > 12V battery.

[Adrian L]

My panels draw say 5ma per panel overnight without a diode or regulator....

If you have a solar controller/regulator it should do all the work of blocking reverse current for you unless it's a particularly ordinary one! So rule of thumb - if you have a decent solar controller/regulator, don't use the diodes in the panels, bypass them, stuff wasting more power than you have too, there is hardly one good reason to use them as I will explain:

If you live in a shadey area with lots of trees/objects in the way it still doesnt matter, if you are running a 12 volt system your panels are all paralleled, if one panel gets shaded over in a set it won't make a difference to the others output, so a bypass diode is completely unnecessary - if you are running a 24 volt system again it's a waste of time putting bypass diodes on the panel!  Solar panels basically freewheel when they aren't making power, they won't drag the rest down thankfully.

Some panels have bypass dioedes integrated in on individual cells, or small groups of cells, in this case they may help with shading, depending how bad the shading is.

I have made and sold a few of my own high efficiency solar regulators,  I use reversed N channel mosfets to block the reverse current at night, and during the day these turn on hard to be an excellent conductor = very little voltage drop = very little power wasted... Cheaper regulators use diodes to block reverse current and these are bad news, they are just as bad as using the diodes on the panel!  Trace C40, C60 etc, use the Mosfet way of blocking reverse current i believe.

Hope this helps,

Adrian L

Adiman on Otherpower IRC

[windstuffnow]

  Thanks Adrian,

    That seems to follow most of what I've been reading about them.  I put them up without the diodes and all seems to work fine.

Have Fun

Windstuff Ed

[richhagen]

Bypass diodes,

Solar Cells are essentially large flat P/N junctions, like a diode.  At high voltages, if a cell or cells are shaded, a voltage could develope across the shaded cell(s) greater than the reverse breakdown voltage of the cell (diode).  At the amperages that panels produce this would likely damage the cell.  I believe that this is why many manufacturers include bypass diodes as under these conditions they would conduct and prevent damage to the cells. The higher voltage the string, the more important they would be.  You wouldn't want a stray leaf landing on your panel on a sunny day and ruining it.

Richhagen

[ghurd]

Diodes- 2 parts...

Before you say it's wrong- get a panel, battery, diode and meter and try it both ways.

As far as blocking diodes go... If all the panels are the same, and you have a controller, they are using power doing something the controller already does.

If you don't have a controller (you should) the diode will save more than it wastes (I said wastes, not uses). Most panels are actually designed to 'operate' at about 17 volts. People seem to think the 'diode power used' is taking away that much from the battery, but it's not. Current to the battery is what counts. Say you panels are running and charging the battery, and the battery is at 12.5V. Add the diode at .7V and the panel is running at 13.2V. Check the manufacturers chart for your panel for the max power output current for the 12.5 and

13.2v. Its just a few milliamps, times 12.5v is just a few milliwatts difference the battery sees. Losses at night would have to be more. We checked, real life, on a hazy day and a low battery and could not get a measurable diff.  Some times it was higher with the diode. Must have been more variation in the cloud cover than it looked.

If you have 1 panel, and a controller, the diode justs wastes power to do something all modern controller already does.

If you have multiple panels, Especially if they are different, of if any 1 gets shade before another, you -may- want the diodes. Explanation- If the shaded panel is so shaded (thats a lot of shade!) that it can't maintain the voltage where the others are charging, current from the others can go back through the severely shaded panel instead of the batteries. I can't really see this happening in many instances, or at least not very long. My panels still put out over 12 volts open circuit very shortly after sundown. I said VERY shortly. They will kind of balance out with each other, each making the current it can, at that voltage, in the light each has got to work with.

If you have a 'self-regulating' panel connected with a regular panel, put a diode in the self regulator. They can only get to about 16v max, where the others can get past 20v, so in low light it will back feed through the lower V panel.

Big panels back feed a lot at night, but bigger panels should have a regulator anyway. They are cheaper than the first ruined battery.

If anyone is still worried, get the Shottkey (SP?) diodes. 0.5v drop instead of 0.7v. That's about 30%. Radio Shack doesn't have them.

Part 2- bypass diodes.

These give an alternate route for current to pass, if there section of the panel can't keep up with the rest. In series ALL the current made goes through the whole setup. If there is a section that can't keep up, the others have to force it through, and thats bad.

Many 12 volt panels are actually 2x 6 volt panels wired in series. A bypass diode won't do any good. If it needs to pass through the diode (.7v), add the typical absolute highest V the other 6v panel makes (maybe 10.5v) OPEN circuit (no current flowing at all) and you get 11.2. Really with full sun, and a little bit of current, you could get 9V, and 0.7 for 9.7. Thats not enough to charge a 'dead' battery (about 11.9) at all, even an abused battery at 10.0V.

For higher voltage systems, like 48V, sure that would help. Maybe even 24v (9x3=27,+0.7= 27.7v compared to 25v for charging.

Many companies put them in there just because people have read the word and ask about it. (yes sells more than no, but so does shiney stuff) And now with so many grid tie (higher than 12V) systems going in, its a hair eaisier for the installers.

[gatu]

As I understand it, bypass diodes are installed, particularly for high power systems, so as to prevent shaded cells from burning out. When shaded, PV cells have a high resistance, so can turn into a resistive heater dissipating all the energy collected by their string of cells.  

[ghurd]

The PV cells are in a series string. There can be no more current in the string than any cell. Like the the Weakest Link in a chain.

So.

Most '12V' panels rate about 20v. And most of the BIGGER ones are really 2 of the 6v panels, in series. If one panel is shaded to say 2v (out of 10)and the other has NO current, say 10v, then you get 12.0v, enough to almost put a charge into a 12v battery (11.9 is considered DEAD by most manufacturers). But that really gets the battery nothing, because as soon as current is drawn from the PV system, the V drops below 11.9v, or below the battery V.

In a higher V system, say 24v, you have 4 at 6v panels, or 3 at 10 v MAX, I said MAX V voltage panels and 1 at zero v. This means you have 3 non-shaded panels at 10 volts peak, being 30 volts, minus the 0.7v for the diode. Thats enough to do a little charging at the 28.8v you need for a 24v system.

If you have a 48v system, you have 8 panels X 6v = 48vmax. 48-10v(shaded cells- or single shaded cell in a string)=38v.

Subtract the 0.7v diode = 37v. This is enough to still do 'some' charging after you figure off the v drop for current flow.  A 12v battery needs about 14v and a 24 volt battery needs about 28v. To do any charging at all.

So by pass diode could make a differance with a 24v system.

The differance gets bigger at higher battery V. Bypass diode would possibally make a differance in a 48v system. No chance in a 12v system.

Either way, a single cell can't carry more current than any other link in the chain.

If shading could burn up a cell or a chain, if would need high currents to do it. More than a decent chain could make.

Counter-point

I saw some panels that had diode accross each and every cell. Each cell can make 0.4v under any load, but the diode needs 0.7v before it bypassesanything? In theory in could work, but not reality to any effect.

They were gov't surplus.

[jimjjnn]

Bypass diodes also prevent a shaded or faulty solar panel from being fed current from an any other panels in a string. You have lower output but not as low as a panel being fed from other panels.

[PV Power]

Having a bypass diode on each cell allows a partially shaded panel to still contribute from those cells not shaded. Of course, if enough cells were shaded, the voltage developed would still not be enough for the panel to contribute. It would still contribute most of it's power if only a few cells were shaded.

Like was already explained, having bypass diodes on sets of cells, like every 12 cells, really only helps on higher voltage strings.

Blocking diodes are more for preventing nighttime losses than anything else. If the voltage drop bothers anyone, they can anways use germanium diodes instead of silicon diodes. Germanium has a voltage drop of .2 volt compared to .7 volt in silicon.

Bob