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Photovoltaic charge regulator

Discussion in 'Electronic Design' started by RST Engineering \(jw\), Apr 13, 2006.

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  1. As best I can tell, a photovoltaic "12 volt" "15 watt" panel is about 19
    volts open circuit in full sun and 13.4 volts at rated current of an amp or
    so. If this is so, it can be described as a "hard" voltage source of 13.4
    volts in series with about 5 ohms.

    To minimize I^2*R loss, it seems that a switching regulator designed for an
    input voltage of 16-19 volts and an output voltage of 13.4 volts at 1.3 amps
    would maximize the efficiency of the system.

    Of course, theory bows to practice every time. Comments appreciated.

  2. Ecnerwal

    Ecnerwal Guest

    Congratulations, you've just invented the MPPT charge controller (well,
    you missed a detail or two, but nevermind that). Unfortunately for your
    bank account, others beat you to it. Fortunately for if you don't feel
    like building one, others beat you to it, and you can rush out and buy
    one. Or you can build one if you prefer to.

    MPPT = maximum power point tracking.

  3. Tim Wescott

    Tim Wescott Guest

    I've reinvented so many things, I assume that if I can't find a
    reference to something new I've done it's 'cause I'm not looking back
    into history far enough.

    There may be new things under the sun, but they don't seem to be coming
    out of _my_ brain.


    Tim Wescott
    Wescott Design Services

    Posting from Google? See
  4. Guest

    Hi Tim,

    Not necessarily....the mppt products tend to be expensive, so there is
    always room for another product made better and cheaper!

    design note 1012 by john shannon

  6. jo

    jo Guest

    I heard about many differents technologies for Mppt. But some of them
    are certainly less expensive.
    I think that some of them are scanning with a processor the
    caracteristic I=f(U) of the PV cells and fixes the not operation.
    But heard that a new way is to find it with an analogic scan, finding
    differents slopes of the caracteristic I=f(U) .
  7. Ban

    Ban Guest

    No, your calculation is not right, but apart from that it is a *current
    source* with the short circuit current and 30 or 31 silicon diodes in
    parallel (count them). Well, that is pretty much what it is. And the current
    is proportional to light intensity in W/m^2 and usually specified for
    1000W/m^2 and 25C. Both together is hard to obtain.
    There might be also a small R in series with the output, but much smaller
    than your 5 ohms.
    Also wrong, that would be 17.4W for a 100% efficient converter and be sure
    the manufacturer would claim that.
    You should brush up your theory as well.
  8. Hi Ban;

    Actually most silicon PV panels have 36 cells.
    Actually they are usually in series.
    Did that, and all of mine have 36 cells.
    Ok, I have one "6 volt" panel that has 18.

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  9. Windsun

    Windsun Guest

    Well, since nearly all "12 volt" panels have 36 cells, not 30 or 31, perhaps
    you should take your own advice.
  10. Guest

    Lead acid charging voltage is the maximum power point for solar panels,
    its why theyre made with the number of cells they are. Maximum power
    point _tracking_ means the mppt will be tracked as it varies with
    varying light levels.

  11. Ban

    Ban Guest

    I looked at my Siemens M55W and it has indeed 36 cells. The equivalent spice
    circuit would then be:
    .--------+-|___|-o +
    | |
    /|\ Is V
    | -
    | |
    / \ V
    ( I ) -
    \_/ |
    | V
    | -
    | |
    | V
    | -
    | |
    | 36 diodes
    | |
    | V
    | -
    | |
    +--------+-------o -
    (created by AACircuit v1.28 beta 10/06/04
  12. There was no calculation done, merely a postulation. If your knowledge is
    superior to mine (which I'm starting to doubt) then give what you think to
    be the correct topology.

    but apart from that it is a *current
    YOU count them. 36 cells in a 12 volt panel.

    If your postulation of a current source is correct, then it wouldn't matter
    if it were a short circuit or high impedance load, the current would be
    constant. At that, it would put out an amp into a short circuit or a 19 ohm
    load, giving 19 watts max. That means the switcher would be 92% efficient,
    which my freshman engineering students can design in a heartbeat.

    My sense is that you are wrong on the current source topology.

    You should take it and stuff it in your ear.

  13. Ban

    Ban Guest

    Thank you , I also `*appreciate* comments on my power supply theory, come
    on. I wouldn't mind, if you give it a try. Don't get angry, else your
    "students" laugh at you.
  14. Utter nonsense.

    PV panels are made with 36 cells because when they get hot their voltage
    drops. If a nominal 12v panel has 36 cells it can drop 5v and still
    produce some charging current. If it only had 33 cells (a once common
    alternative) then when it got hot the battery wouldn't charge.

    PV voltage and max power point varies depending on temperature. I live
    in a mild 4-season climate, and nominally 12v panel's peak voltage goes
    well over 20v in the winter and down to barely over 15v in summer heat.

    Battery voltage varies depending on temperature and state of charge.

    To get maximum power from the PV panel, you need to operate the PV at
    its max power point voltage regardless of the battery voltage.

  15. Ban

    Ban Guest

    If you crosspost to S.E.D. and write something which is wrong and even
    invite commentaries, you should also accept when someone points out the
    mistake. It doesn't have to do with superiority.

    What you thought to have measured would be an ideal voltage source with 19V
    in series with a resistor of 5.6 ohms. Anyway that would be as well
    equivalent to an ideal current source of 3.39A with a resistor of 5.6 ohms
    in parallel. I do not need to tell you that.
    I admit there are mostly 36 diodes in a panel, and this string lies in
    parallel with the current source. I didn't explicitly state the string, but
    it should have been obvious.
    The diodes are slowly shorting the current source, so the output voltage can
    not rise any higher. And the available output current goes to zero.
    Now unfortunately diodes are not linear and thus it will be impossible to
    express them in an equivalent circuit with a voltage source, at least not
    with known components.

    BTW tell us more about you, your students seem to be rather smart.
    You see, that is not a theory, it is only an idea, which has some truth in
    it. The step to a theory would be to study the behaviour, get an equivalent
    circuit that meets this behaviour and then have it peer reviewed. Like
    Shockley with his diode theory, which certainly will be familiar to you.
    Well should I praise you for what you said? I leave that to the google
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