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Charge controller - power diode question

Discussion in 'Electronic Design' started by [email protected], Dec 26, 2012.

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  1. Guest

    Could some power electronics guru please
    help ?
    I am trying to put together a charge
    controller for a battery pack/solar
    panel array. The maximum solar panel
    output current is around 20.0A, and
    the maximum battery voltage is 12V.
    So, in the charge controller, I was
    planning to use the power diode 10A02,
    whose IFmax is 10.0A and the BV is
    100.0V. To deal with the 20.0A maximum
    solar panel output current, I was planning
    to have 3 diodes in parallel to tackle
    the current, and the diode's 100.0V
    BV would block reverse battery discharge.
    How does this look ? Also, if I have a
    simple shunt type charge controller,
    what would be a good power transistor
    to use at these current/voltages ?
    Thanks in advance for all your hints
    and suggestions.
  2. mike

    mike Guest

    On 12/26/2012 12:20 AM, wrote:

    There have been ample discussions about the folly
    of solar power when you have any alternatives.
    I'll not repeat that here. I like to help
    people with their hobbies.
    I've helped engineer more than one solar system
    for mountain-top use in the boonies.

    Your statements are worrisome on many levels...
    Depending on what batteries you're using
    and the tradeoffs you've made for life vs
    maximum stored energy,
    your battery voltage ranges from 10V to
    nearly 15V or so.
    Sounds logical on the surface, but there's
    a troll under that bridge.
    Most diodes have a negative temperature coefficient.
    If there's any imbalance, one diode takes more
    current and warms up which makes it hog more current
    which makes it warm up. Pretty soon, you have one diode
    in the circuit and the other two just sitting there.
    From then on, it's a matter of time until the silicon melts
    and shorts or the plastic melts and the device comes apart open.
    According to the spec, that diode at 10 amps is gonna
    be 150 C above ambient. That's too hot, even if you
    could make three share equally. Get something with a bolt hole
    and put it on a heat sink.
    You're better off with one 40A diode.
    And a 20V diode will likely be less lossy and cheaper
    than a 100V one. And I don't mean a lower voltage one
    selected out of the same bucket. I mean a diode designed
    for minimum forward voltage often has a lower reverse voltage.
    It doesn't have to be a fast diode.

    But there's a more basic question. Many solar panels have built-in
    diodes. Are you sure yours don't?

    That diode has a forward voltage spec of 1V.
    You've got 15V at the battery and you're wasting another volt
    or 20 watts.

    And it's more complex than that.
    Look at the family of curves for the panel.
    They're not a straight line. And the 20A is at noon in the desert.
    At 3PM when the panel isn't pointed directly at the sun and there's
    more atmosphere in the way and there's some haze and some bird poop
    on the panel, you're gonna be on another curve of that graph.
    That diode might be the difference between your charge current going to
    zero at 3PM instead of 3:30 PM. Those lost amp-hours add up.
    Also, if I have a
    Shunt controller sounds simple, but again, the devil is in the
    We built one system that used 4 150W resistors and 4 audio power
    transistors. A comparator switched the transistors on at 13.7V and off
    at 13.6V.
    That worked because
    The panel was 20A and the batteries were 800Amp-Hour. They didn't have
    any trouble with that current and voltage. And the thing rarely
    turned on except on the longest days of the year.

    If you had a single car battery and a 20A panel, that wouldn't be a good

    A later version used an oscillator and a crude 300Hz PWM to switch the
    For a less robust battery system, you'd want a fast switcher and much
    better charge management. And today, you'd be able to afford fast
    switching transistors at that current.

    For starters, take any 300W plus buck converter design that can run on
    10-15V. Put 600W of resistors with values that can draw at least
    300W from the output of the converter.
    Control the converter output voltage to give you the shunt load current
    you require. Since you're likely to use multiple resistors in parallel.
    It might be easier to use more buck converters of lower current each.
    Depends on what you can find in the cheapo bin of the electronic surplus

    We built a MPPT controller and a sun tracker. Worked neat and gained
    some additional usable power. Problem was that it was deemed unworkable.
    It's hard to track the sun when the panel is covered with ice
    and the roads to the site are closed eight months out of the year.

    For remote stuff that just has to work, it's often better to add another
    panel than to try to eek out a few more percent on the one you have.
    Reliability trumps efficiency every time.
    The hardest part of any project is writing the spec.
    Decide exactly what you expect to happen under any and all
    conditions of insolation and load current and battery charge level.
    Map that all out and decide what to implement. Then figger out HOW
    to implement...repeat the cycle until it looks like what you want.
    Then start ordering parts.

    If you're on the grid and expect to generate power, you're likely to
    give up a that point and go play a round of golf.

    If you're indulging an expensive hobby, you can have great fun
    with solar power. It's a lot cheaper than golf...and less risky
    than a mistress.
  3. Guest

    Thank you very much for your detailed and insightful explanation.
    I really loved your last paragraph.
  4. Guest

    The usual method to avoid this is to put individual series resistors
    in front of each diode, thus, sharing the current more evenly.

    Of course, this dissipates some power.

    However, since there are usually quite a long distance between the
    panels and the batteries (and diodes) at such low voltage as 12 V
    (instead of 24 V or 48 V), there are still going to be some voltage
    drop in the cable with cross section A, before to current goes into
    three 10 A diodes.

    Why not use three separate insulated conductors with A/3 cross section
    from the panels, each connected separately to each diode ? The total
    power loss in the wiring will be the same, but the separate wiring
    will now act as small series resistors to distribute the current more
  5. John S

    John S Guest

    Acutually, the solar panels I have in front of me have the diodes in
    _parallel_ with the panel output.

    The purpose, AIUI, is to provide a path for the current of shaded panels
    when wired in series.

    In that case, the provided diodes will not prevent current back into the
  6. SoothSayer

    SoothSayer Guest

    They prevent current from an illuminated panel from being loaded by a
    non-illuminated panel. IOW, electrons only get to spit in one direction
    from all elements. That being INTO the charging circuit input.
  7. mike

    mike Guest

    I didn't want to complicate the discussion.
    What you suggest probably works fine. In our case, there was also a wind
    generator that had some control mechanisms in the head. It didn't like
    to be disconnected when the wind was blowing hard. And the solar
    had a crude controller. The two controllers wanted to fight.
    And the pieces were a hundred miles away and a mile straight up.
    Was easier to just shunt the thing into a resistor to regulate it.

    It can be a minor consideration, but series control is loss when
    you need the juice most. Shunt control is loss when you've got excess
    you can't use.
  8. Hmm, well first I know squat about solar panels. But that seems
    Doesn't it cost a bit more than one solar panel 'photovoltage' to
    overcome the
    diode drop of the panel in the shade?
    Or is there a series stack of 'PV's with one diode across the whole

    George H.
  9. Guest

    Ummm- that observation only applies to the small signal stuff you work with. The power Schottky's stay negative TC all the way: see Fig. 3

    Yeah- if the battery was the only energy load on the system, but there might be other things like an MPP inverter which will get all confused with that scheme.
  10. Guest

    That suggestion is insane for a professional project.
  11. It sounds like a 24vdc PV panel. usually they are wired up in series for
    600vdc to run a Grid-tie inverter.
    The shading Diode sounds plausable and may be in there for other
    issues;) It's your standard ~0.5v drop power diode.
    If you partially shade an array the GTI's tend to shutdown altogether,
    so I'm not sure there is any benefit.

  12. Guest

    As a rule, if there is a single component rated to handle the job, it is always more economical than using multiple copies of a component that can't handle the job. There may be exceptions.
    I know for a fact the MPP in-/con-verter will not like you periodically clamping its input to Vbatt, so you will have to think of something else.
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