Battery Charger Voltage?

Discussion in 'General Electronics Chat' started by spinnaker, Nov 12, 2011.

  1. spinnaker

    Thread Starter AAC Fanatic!

    Oct 29, 2009
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    I bought one of these chargers

    http://www.solarpanelstore.com/solar-power.small-charge-controllers.sunguard.html

    as part of a larger PIC controlled solar lighting project.

    The PIC monitors both the solar panel and battery voltages.

    What I found strange is that when I have everything connected, the input to the charger attains the same voltage as the charging battery.

    The charger is supposed to have a blocking diode and appears to be working and the panel voltage towards zero as the sun goes down while the battery maintains it's charged voltage.

    Why would the input on the charging seek the same level as the battery? I would expect to see the actual voltage.

    How can I read the actual panel voltage? Can I place another blocking diode between the panel and the charger then read the voltage from the panel?
     
  2. iONic

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    Nov 16, 2007
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    Actually the charging battery is seeking the charging voltage which is higher. As the battery charges the voltage at the terminals will rise.
     
  3. spinnaker

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    But why is the panel voltage lower when the charger is connected?
     
  4. SgtWookie

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    They might be using a P-ch power MOSFET as an "ideal diode"; the voltage drop across the MOSFET would be very low.

    The solar charger may just monitor that voltage drop; if it gets to zero, then it turns the MOSFET off until the solar panel side again is higher than the battery voltage.

    As far as your PIC measuring the panel voltage as being lower than the battery; it may be the tolerance of the resistors you used in the voltage divider.
     
  5. spinnaker

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    Thanks Wookie.

    I think you misunderstood when I used "lower" or maybe I did not phrase it correctly.

    When the charger is connected my PIC reads somewhere around 22 volts or so depending on the amount of sunshine.

    With the charger connected that same measurement drops to 12.5 and less depending on the charge on the battery.

    The PIC itself is reading the voltage fairly accurately on both battery and panel.
     
  6. iONic

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    Sgt. Wookie may be able to explain the WHY(physics) behind this, but I'll stand by my first comment and add that this is how it should be reacting. You would not want to see 22V with the two devices connected as the battery would be severely overcharged and not charging. You'll know it's charging when you see 12.6V, 12.7V....
     
  7. tom66

    Senior Member

    May 9, 2009
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    Solar panels drop voltage when loaded.

    There are two values: V(oc) and I(sc).

    V(oc) is the open circuit voltage; it is often 1.5x - 2x the loaded value. No power is available at V(oc) because no current is drawn.
    I(sc) is the short circuit current; it's often very large, but again little or no power is available when I(sc) is drawn.

    It's expected that a panel will drop voltage when loaded. Otherwise they would be overunity. Which would be nice, but somewhat impossible.
     
  8. SgtWookie

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    If your solar charger happens to be MPPT (maximum power point tracking), the voltage on the input could be just about anything; as a MPPT charger will adjust the current draw from the panel to maximize the power transfer to the battery until it's charged.
     
  9. spinnaker

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    No I completely understand that the battery cannot be overcharged. That is the whole purpose of the charger. So I expected to see the output of the charger at most the charge voltage.

    What I did not expect to see is the input to the charger be the same as the output.
     
  10. spinnaker

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    So if the current from the panel is adjusted, the load is the same, so therefore the voltage from the panel will drop?

    Why do they adjust the current draw from the panel as opposed to adjusting the current to the battery?

    But I am not sure if that is what is going on here as it is an old and cheap panel. I doubt it is supplying much current at all though apparently enough to charge the battery in a day.

    Here is a hint of what might be going on inside the charger. It was at the end of the day my panel voltage was equal to the battery voltage. I brought the whole works inside and covered the panel. The panel voltage was still equal to the battery voltage. After about 10 seconds the panel voltage dropped to zero. I suspect that after a delay, the charger shut down the connection to the panel.

    After thinking about it, I think your comment on the MOSFET is right on the mark.

    Not that is really important but is there anyway to measure the true voltage of the panel other than using something like a power MOSFET to disconnect the input to the charger then read the panel voltage?
     
  11. spinnaker

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    Yeah I understand voltage will be dropped under load but it is over 10V. That is a lot and it would be a strange coincidence that the resulting voltage of the load is exactly equal to my panel voltage. :)
     
  12. Adjuster

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    Dec 26, 2010
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    Unless the charger can be seen to be definitely overcharging, with an excessive charge current or the battery voltage climbing to an unacceptable value, this may be normal. The maximum output will be obtained from a simple PWM charger circuit without voltage conversion with its pass element fully turned on at 100% duty. As the Sgt. says, a modern machine may use a low-resistance PMOS FET which may not drop very much at all.

    You would only expect the series element to begin closing down (narrowing its duty cycle) if the charge current approached its limit value, or for more sophisticated chargers when the battery state of charge dictated a lower current.

    Of course it is possible that your charger has a fault, but it is also possible that the charge current / battery charge state have not reached values which would cause the charger to throttle down.
     
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