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Thread: Shaded panel wiring.

  1. #1
    Join Date
    Jul 2007
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    Mt. Shasta, Ca.
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    Default Shaded panel wiring.

    I'm modifying my PV system.

    I'm in an area of heavy timber and can't avoid occasional shading of some of my panels. The bottoms of one or more panels will be shaded for some time as the sun moves.

    To increase efficiency do I need to install blocking diodes on the outputs of each panel? Or will the factory installed bypass diodes take care of this?

    I have KC 135's and a couple of similar Sharp's.

    I am also planning to replace my present mppt controller (it's too small for my additional panels) with a pwm controller because of this shading problem. Is it true that mppt controllers have a problem with intermittent shading?

    I am also planning on installing an ammeter that can be switched off. Is it better to open the leads from the shunt or short them with the switch? If shorting is better must the switch and conductors be rated for the ampacity of the circuit?

  2. #2

    Default Re: Shaded panel wiring.

    My opinion, which no doubt someone will disagree with in whole or in part:

    No need for blocking diodes on shaded panels unless you have very high array Voltage (the other panels will not produce enough Voltage to reverse current flow through the shaded ones).

    MPPT controllers don't have a problem with shaded panels any more than PWM controllers do.

    Why would you want to switch the Ammeter off? It won't hurt anything to leave it connected providing it is wired right. Where were you planning on installing this? A good charge controller will measure the current going in and out of it anyway, so an additional meter would be redundant.

    If you do this, bypassing the shunt through shorting is unnecessary; simply open the lead to the meter so that it doesn't read. If you were to short the shunt the switch would indeed have to be able to take full current and the maximum DC (I assume) Voltage. I just don't see any reason to do this.
    1220 Watts of PV, OB MX60, 232 Amp hrs, OB 3524, Honda eu2000.

    Ohm's Law: Amps = Volts / Ohms
    Power Formula: Watts = Volts * Amps

  3. #3
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    Mt. Shasta, Ca.
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    Default Re: Shaded panel wiring.

    Thanks for the reply, Caribo. The reason for switching the meter is so I can use one meter for measuring more than one circuit and so the digital meter is out of the circuit during periods when I'm not on site.

    If, as you say, MPPT controllers work as well as PWM controllers in shaded conditions I will re-think that part of my project. It used to be said that changing from PWM to MPPT is like adding another panel but I didn't notice much difference when I changed controllers from PWM to MPPT on my smaller system. And today I can buy quite a bit of PV power with the difference between buying a C-60 and an Outback 80.

  4. #4

    Default Re: Shaded panel wiring.

    Quote Originally Posted by Lefty Wright View Post
    Thanks for the reply, Caribo. The reason for switching the meter is so I can use one meter for measuring more than one circuit and so the digital meter is out of the circuit during periods when I'm not on site.

    If, as you say, MPPT controllers work as well as PWM controllers in shaded conditions I will re-think that part of my project. It used to be said that changing from PWM to MPPT is like adding another panel but I didn't notice much difference when I changed controllers from PWM to MPPT on my smaller system. And today I can buy quite a bit of PV power with the difference between buying a C-60 and an Outback 80.
    The actual increase in power of an MPPT vs. PWM is minimal under normal conditions. The big advantage they offer is ease of array design (able to take in a wide variety of Voltages and get the right charging out). Otherwise you need either a large array or cold temps to see any significant power advantage. It's a question of scraping up the unused power from the Voltage difference available between array Vmp and system (battery) Voltage. Higher system Voltages see more improvement for the same reason: less V-drop in the wiring due to the higher Voltage means more of it is available to convert to charging current. Note that this only matters with the Bulk charging stage where current is what you're after.

    I personally like MPPT controllers because they make system design easier.
    I also wouldn't buy a Xantrex charge controller, or anything else they make. Too many problems with the equipment (not the C60 specifically) and lousy customer service.
    1220 Watts of PV, OB MX60, 232 Amp hrs, OB 3524, Honda eu2000.

    Ohm's Law: Amps = Volts / Ohms
    Power Formula: Watts = Volts * Amps

  5. #5
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    Default Re: Shaded panel wiring.

    I didn't know that about Xantrex. Reliability matters more to me than saving a few bucks.

    My system requires 12V so design isn't a problem. But things can change so I'll keep flexability in mind.

    Cost wise I think PWM rather than MPPT and put the savings into higher grade batteries. Or beer.

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