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Slighlty Different Voltages

Discussion in 'Home Power and Microgeneration' started by Danno, Aug 6, 2004.

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  1. Danno

    Danno Guest

    I know I saw a post similar to this recently, but I can't dredge it up...
    I currently have an ICP 75W panel, it apparently pushes at 15V. I would like to
    wire a Sharp panel in parallel with it, but the Sharp panels push at 17.2V.
    Will it be OK to hook these up like this? I use a standard (non MPPT) charge
    controller, but would eventually like to get into an MPPT. Would the differing
    voltages affect the efficiency?
  2. Vaughn Simon

    Vaughn Simon Guest

    I have dissimilar panels wired together and find it to be a non-issue.
    A simple test is to connect them both together with an ammeter in the
    circuit. If you can remove the wire from either panel and see the charge
    current decrease (and I think you will) , you are home free.
    Doubtless, it would be better if they were the same, but you are
    probably better off with the two dissimilar panels wired together than with
    either one of the two by itself.

  3. Danno

    Danno Guest

    Thanks for the responses. My concern was that, in my zest to increase my
    charging capacity, I would ruin my current (limited) ability. I am running
    my current system through a "standard" charge controller (ICP CC20), with a
    float of 14.4V. It is capable of handling the increased amperage of the new
    panel. I guess I'll have to put the MPPT idea on the back shelf for a while...
    Am I correct in assuming that the maximum voltage of the Sharp
    will not be reached; that it will be held at 15V by the first panel. Or does
    the charge controller limit both to 14.4V? I would like to take a look at the
    output curve of the panel to see just how much power I'd actually be making.
  4. Winston

    Winston Guest

    Are the panels rated for the same amount of current?
    If so, what would happen if you put them in series and drove the controller at
    32.2 V? Ferinstance, these MPPTs have a maximum array voltage of 50 V:

    Wouldn't that just about double your available power?

    --Winston, the Solar Noob :)
  5. Danno

    Danno Guest

    I had been thinking along the same lines, but that is too far off in the
    future for me to worry about right now. They are not rated for the same current
    anyway, so I doubt it would work here. Had considered maybe using a DC-DC
    transformer to lower the voltage of the 17.2V to 15V, but then the power output
    is not that different between 15V & 17.2V on the panel anyway.
  6. Danno

    Danno Guest

    Good stuff, I think this is enough info for me to safely justify purchasing
    the new panel. Thanks a lot!
  7. Starbase

    Starbase Guest

    I asked the same question a while back and got a few conflicting answers so
    I just wired them all up and it seems to work OK to me, the main thing seems
    to be that the battery bank dominates what the voltage is and if the battery
    is not at ~13.8 volts (14.2v) then the charge controller will just whack the
    power through, the panel voltage can't go above battery voltage if the
    battery will accept a charge and all my panels are quite capable of
    supplying the required equalization voltage. I have diodes on the panels
    (just one on each) as all the panels are not pointing south or at the same
    angle. As I get into it more I will learn more and make improvements but I
    have not bust anything yet and always treat my batteries with the utmost

    I can see that if the battery was fully charged and the charge controller
    shorting the PV array then the higher voltage panels might reverse bias the
    lower voltage panels but no more than a battery would on a night without a
    blocking diode, would this harm the panel?

    One other thing which confuses me a little and I have more to learn about is
    the maximum amperage which a panel can handle (how would that happen in
    parallel connections, if a panel is shaded does it's resistance not go up?,
    is a panel not a diode itself?), a blocking diode should prevent current
    from one panel flowing through another? Using a charge controller means you
    do not normally have to use a blocking diode (which wastes about 0.6 volts x
    X amps = X watts) but as I mix my panels I used them just to be safe.

    These are just my thoughts on the subject and the way I am doing things at
    present, things might (will) change as I learn more. I am grid connected and
    treat it purely as a hobby.


  8. Danno

    Danno Guest

    I have also tried to be gentle with my setup; it would seem there are no
    inexpensive pieces to "take chances" with <grin>. ICP's setup is pretty slick,
    I'm quite certain that they have the protective diodes built in. They market
    their panels (or "systems", actually) with the "Plug'n'Play" branding, which
    allows a total noob like me to just plug an additional panel into a specifically
    shaped outlet on the bottom of the first panel. I'v got three 15-watt PV-film
    panels spliced into an extra PnP wire I had, and it's working well; will do
    the same with the Sharp 123 when I get it.

    <grin>. I started that way last August... I think I am addicted... The
    question for me, now, is: Can I make this work year-round in my 3rd fl, west-
    facing, Canadian apartment? Summertime sun is awesome, it practically sets in
    north, lots of afternoon and evening sun, but wintertime is, IIRC, 6 hour days
    hanging very low in the southern sky. Now, the winter wind, OTOH...
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