Do I need a blocking diode??

simagicsimagic Solar Expert Posts: 100 ✭✭✭✭
If I intend on running (2) 100w panels ( identical specs) in parallel, do i need ( what someone, somewhere said) a device called a blocking diode. Also, If one is in the shade and one is in the sun, will the shady one take juice/energy from the sunny one if they are in parallel???

Comments

  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Do I need a blocking diode??

    The answer to both is "no".

    You do need a charge controller, however.

    A blocking diode prevents the battery from discharging through the panel at night, if there is no charge controller. As a rule they are only found on small systems without controllers.

    If one of the panels is partly shaded its contribution to charge will diminish but it will not draw from the other panel; they are semi-conductors. The only case where this can be a problem is with panels that are mismatched where the current output from one can exceed the maximum of the other. In such a case fusing the lower power panel according to its Isc should prevent it being damaged if the situation arises.
  • simagicsimagic Solar Expert Posts: 100 ✭✭✭✭
    Re: Do I need a blocking diode??

    Thank you......Phew, I thought I'd have to start learning all about diodes now. Also, my logic tells me one thing, but when it comes to all this series, parallel mumbo jumbo,I can't be 100% sure. Here's the thing. Presently I have a 10amp circuit breaker/fuse coming from my one 100w panel going to my charge controller. If I now add the other 100w panel (in parallel) as I intend, do I need to increase that 10a fuse to a higher number such as 15a or 20a. Again, logic tells me yes, but a confirmation would be great. Thank you. Also (again), on my last post about car wax, I see 5 stars next to the post. What's that mean??
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Do I need a blocking diode??
    simagic wrote: »
    Thank you......Phew, I thought I'd have to start learning all about diodes now. Also, my logic tells me one thing, but when it comes to all this series, parallel mumbo jumbo,I can't be 100% sure. Here's the thing. Presently I have a 10amp circuit breaker/fuse coming from my one 100w panel going to my charge controller. If I now add the other 100w panel (in parallel) as I intend, do I need to increase that 10a fuse to a higher number such as 15a or 20a. Again, logic tells me yes, but a confirmation would be great. Thank you. Also (again), on my last post about car wax, I see 5 stars next to the post. What's that mean??

    Look at the Isc on the panels. It's going to be about 6 to 7 Amps maximum. The Imp will be between 5 and 6. So the most these panels together could put out is around 13 Amps. Really a 15 Amp fuse/breaker would handle it. Make sure it is a DC breaker, not an AC breaker. Mostly it won't do anything as the panels can't supply more than their Isc. But if something should go horribly wrong and the controller allows the batteries to back-feed a huge amount of current into dark panels ... Cheap insurance, fuses. Breakers make nice disconnects. Your choice.

    The stars on threads are ratings according to how good others think the topic is. At the top of the page just below the thread title on the right side are pull-down menus. The fifth one over says "Rate This Thread".
  • nielniel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: Do I need a blocking diode??

    "If one of the panels is partly shaded its contribution to charge will diminish but it will not draw from the other panel; they are semi-conductors. The only case where this can be a problem is with panels that are mismatched where the current output from one can exceed the maximum of the other. In such a case fusing the lower power panel according to its Isc should prevent it being damaged if the situation arises."

    i have to point out here that a shaded pv will make it imbalanced and act like another pv of a different rating. this does put the shaded pv as a potential load by the higher outputting pv, but is offset by the voltage differential. now what happens at night in full darkness is a full differential from any other power source and the pv output is 0v and 0a and does allow the full potential of voltage pressed to it to leak based on the design of the semiconductors, aka pvs. what this means is that a pv that is shaded (low illumination) will dissipate a tad of current being fed from the fully illuminated pv, but would be too small to worry about.
  • inetdoginetdog Solar Expert Posts: 3,123 ✭✭✭✭
    Re: Do I need a blocking diode??
    niel wrote: »
    i have to point out here that a shaded pv will make it imbalanced and act like another pv of a different rating. this does put the shaded pv as a potential load by the higher outputting pv, but is offset by the voltage differential.

    Quite true, and the detailed result will be that the Voc of the panel should remain almost unchanged, but the Vmp will go down and the Imp will be very much reduced. The Isc might appear unchanged if enough of the panel is still in full sun.
    If only part of the panel is shaded and the bypass diodes come into play, it may appear to have a much lower Voc, but only because the meter is really drawing some current.
    The variation in the Vmp that a shaded panel makes in a series string is one strong argument for using more than one MPPT CC (or more than one MPPT input if the CC has two or more) when you have two or more strings with different shade patterns.
    SMA SB 3000, old BP panels.
  • nielniel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: Do I need a blocking diode??

    "If only part of the panel is shaded and the bypass diodes come into play, it may appear to have a much lower Voc, but only because the meter is really drawing some current."

    no, it will literally have a lower voc as well as a lowered vmp as sections will be bypassed by the bypass diodes. there are many variables and scenarios that come into play, but what i said generally holds true that it may not be worth worrying about. the only way to insure a pv won't dissipate power fed back into it would be by using blocking diodes and that does have a power loss at all times as it is the forward voltage drop of the diode multiplied by the current passed equals the watts lost.
  • inetdoginetdog Solar Expert Posts: 3,123 ✭✭✭✭
    Re: Do I need a blocking diode??
    niel wrote: »
    "If only part of the panel is shaded and the bypass diodes come into play, it may appear to have a much lower Voc, but only because the meter is really drawing some current."

    no, it will literally have a lower voc as well as a lowered vmp as sections will be bypassed by the bypass diodes. there are many variables and scenarios that come into play, but what i said generally holds true that it may not be worth worrying about. the only way to insure a pv won't dissipate power fed back into it would be by using blocking diodes and that does have a power loss at all times as it is the forward voltage drop of the diode multiplied by the current passed equals the watts lost.

    Since Voc means Open Circuit, the bypass diodes will not be doing anything. And unless the shading on the panel section which is NOT being bypassed is so complete that is actually is showing Voc of zero because of internal shunt resistance (less than one candle at 20 feet maybe??), then the Voc of those cells will still be very close to their Voc under full light.

    If the shaded panel is part of a series string which is forcing current through it, then the V(not really O.C.) of the panel will go down because some or all of the bypass diodes will be conducting, but that is not, by definition, Voc.

    I agree that the end result either way is that blocking diodes are not needed to protect one string from another, and that the power lost to the forward voltage drop of the blocking diode is high enough that if it were due to wire resistance, we would call it unacceptable. :-)
    SMA SB 3000, old BP panels.
  • nielniel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: Do I need a blocking diode??

    "I agree that the end result either way is that blocking diodes are not needed to protect one string from another, and that the power lost to the forward voltage drop of the blocking diode is high enough that if it were due to wire resistance, we would call it unacceptable. :-)"

    yes, for the purpose of this thread we do agree.

    as to the voltage open circuit (voc) if those diodes are activated the whole section across those diodes are bypassed and their place and output if any is now moot. it's akin to 4 identical pvs in series and shorting across one pv. you would now have the voc and vmp of the 3 pvs in series at the same current specs. internally done in a module by a diode is the same result.
  • simagicsimagic Solar Expert Posts: 100 ✭✭✭✭
    Re: Do I need a blocking diode??

    So after I read post 2, I was happy. then I read ( but really didn't understand what I was reading), posts 5 thru 9...Soooooooo. Is adding another identical 100w panel to another 100w identical panel by connecting in parallel and going to the same charge controller good to do or not good to do. ??
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Do I need a blocking diode??
    simagic wrote: »
    So after I read post 2, I was happy. then I read ( but really didn't understand what I was reading), posts 5 thru 9...Soooooooo. Is adding another identical 100w panel to another 100w identical panel by connecting in parallel and going to the same charge controller good to do or not good to do. ??

    Don't worry about it.
    Two panels side-by-side aren't likely to have any drastic shading issues vis-a-vis one vs. the other. If there is a problem of shading the panels aren't in the right location. Or you need to cut a tree down. Or a phone pole. Or your neighbour's house. :p
  • simagicsimagic Solar Expert Posts: 100 ✭✭✭✭
    Re: Do I need a blocking diode??

    Thanks, BUT..... they will be out on my deck. I was thinking one would aim at the 9am to noon sun and the other would aim at the noon to 4pm sum..???Problem???
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Do I need a blocking diode??
    simagic wrote: »
    Thanks, BUT..... they will be out on my deck. I was thinking one would aim at the 9am to noon sun and the other would aim at the noon to 4pm sum..???Problem???

    Nope.
    Lots of people do similar things using the PWM type charge controller. It will pass whatever current is available to it.
    An MPPT type controller can get 'fooled' by panels pointing in different directions, but it isn't likely you'd invest in one of those for 200 Watts of panel.
  • simagicsimagic Solar Expert Posts: 100 ✭✭✭✭
    Re: Do I need a blocking diode??

    Uh oh. I actually have a blue sky 2512 ix MPPT which is where the one panel I have now is going to. Am I not good??
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Do I need a blocking diode??
    simagic wrote: »
    Uh oh. I actually have a blue sky 2512 ix MPPT which is where the one panel I have now is going to. Am I not good??

    I have no experience with that particular controller, but as a rule you'd be better off with an inexpensive PWM like the Morningstar SunSaver.
    However since you already have the Blue Sky there is no harm in trying it. Nothing will go wrong, it just may not pick the perfect power point. Note I said "may not"; not "absolutely will not". (I put that in for the nit-pickers. :p )
  • simagicsimagic Solar Expert Posts: 100 ✭✭✭✭
    Re: Do I need a blocking diode??

    Ok....I will proceed with this plan since I have your guarantee that it will work perfectly ( just kidding "of course") . "The "harm in trying it" is the amount of work needed to make this happen. This is out on my deck and the wires are up, down and all around ( doors/windows/etc ). When it's all finished I'll check in and give you some feedback as to the performance of this concochshin. if i don't double my output, ( at the time they are both facing the sun)........I'm at least hoping for a significant increase throughout the day (9-4). I might wait just a bit before proceeding to hear what your " nit-pickers say "to you" about this. PS. I'm headed over to my other thread about the mistake i made by cleaning my panels with lacquer thinner and acetone. Your answering me there as well/....... Thanks "C"
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