Is this MPPT OR PWM

Discussion in 'General Electronics Chat' started by mrel, May 29, 2016.

  1. mrel

    Thread Starter Active Member

    Jan 20, 2009
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    Hello
    Recently purchase from ebay this item
    LCD 10/20/30/40A 12V/24V MPPT Solar Panel Regulator Charge Controller 3 Timer KJ.
    Start to wonder if this is true mppt regulator charger,after reading more from they website came across this sentence
    ( MPPT+PWM charging mode(Controller Provided MPPT Maximum power point tracking Function,charging mode is PWM ).
    Is there such a thing MPPT + PWM in Solar Panel Regulator Charger,can both MppT ,PWM work together.
    I thought mppt does charging of the battery.
    What is purpose having pwm when mppt is the one that doing the work.
    mrel
     
  2. AlbertHall

    Well-Known Member

    Jun 4, 2014
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    The MPPT part refers to adjusting the effective load on the solar panel to get the maximum power from it as the insolation changes. The PWM part is how that loading is implemented. So the MPPT controls the PWM to set the loading and control the battery voltage and current.
     
  3. Lestraveled

    Well-Known Member

    May 19, 2014
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    @mrel
    I don't have permission to access your link, I am not you.

    PWM is an inexpensive yet effective way to control the charging of the battery from a solar panel. It is a control method not a true power conversion method so it will always leave about 30% of the power generated unused, (13V/18V = 72%). It can not output more current than what the panel can supply.

    As Albert said, MPPT adjusts the battery charging parameters to optimize the power transfer. MPPT is used to "harvest" a few more percent of power as the sun is going down and coming up. MPPT, on high end systems, is used with a switch mode power converter that is between 85% and 95% efficient.

    Because a PWM controller is only 70% efficient to begin with, adding MPPT will have little real improvement and is more of a marketing ploy.
     
  4. crutschow

    Expert

    Mar 14, 2008
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    Les, you seem to be confusing linear regulation with PWM.
    A linear regulator's efficiency is equal to the output voltage divided by the input voltage, but PWM is indeed a switch-mode type of conversion whose efficiency can exceed 90% with proper design.
     
  5. Lestraveled

    Well-Known Member

    May 19, 2014
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    They are 100% efficient.......in wasting 30% of the panels power. PWM controllers have no inductors. They utilize the constant current nature of the solar panel to limit current. They use PWM to control the panel voltage by shunting the battery across it. In other words, a PWM controller forces the panel to operate at the desired battery voltage, not at the voltage that produces the most power.

    A PWM charge controller is not a switch mode power converter.
     
    Last edited: May 29, 2016
  6. AlbertHall

    Well-Known Member

    Jun 4, 2014
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    Switch mode power converters use PWM! That's what controls the switch.
     
  7. Lestraveled

    Well-Known Member

    May 19, 2014
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    But a switch mode power converter uses inductance to store energy and change the power form. A SMPS can output more current than what is put into it, a PWM can not.
     
  8. crutschow

    Expert

    Mar 14, 2008
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    Okay, I understand where you are coming from.
    I just assumed that any PWM circuit used to charge a battery would include an inductor.
    Otherwise it makes little sense to use PWM.
     
  9. Lestraveled

    Well-Known Member

    May 19, 2014
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    They are cheap, simple, requires little or no heat sink, small, rugged and has a cool name. In the lower power range, where buying a bigger panel is cheap, they are the perfect solution.
     
  10. crutschow

    Expert

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    You are sure they have no inductor?
    It seems odd that they would leave out an inductor for a regulator where efficiency is important.
    Can you post a link to such a device.
     
  11. Lestraveled

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    Tonyr1084 likes this.
  12. AlbertHall

    Well-Known Member

    Jun 4, 2014
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    Indeed for MPPT to work you need to draw a reasonably steady current. If you just had a switch (no inductor) between the solar panel and the battery that would not be the case. While the switch is closed the solar panel voltage would be dragged down to the battery voltage. You can do that with just a diode.
     
  13. Lestraveled

    Well-Known Member

    May 19, 2014
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    @AlbertHall
    But, one thing that a PWN does well is control the charging profile; bulk, absorption and float modes, all require direct control of the voltage and current. You can't do that with just a diode.
     
  14. tom_s

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    Jun 27, 2014
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    and here's me thinking i was the pedantic one around here :)
     
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  15. Roderick Young

    Member

    Feb 22, 2015
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    To add to this, there actually is a storage element, a capacitor across the solar panel input. Let's say that for a specific set of conditions, the solar panel is most efficient at an output voltage of 24.0 volts. Then the switch is engaged at a duty cycle that makes the capacitor voltage hover around 24.0 volts. When the switch is on, the circuit draws power from both the panel and capacitor, and the capacitor slowly discharges, let's say to 23.9 volts. When the switch is off, the panel puts all of its current into charging the capacitor, and the voltage rises to perhaps 24.1 volts. This switching goes on at a high frequency, of course, so the panel actually is putting out a more or less steady current.

    But I'm with Crutschow in thinking that there must be an inductor somewhere thereafter. Unless it's one of those cheap controllers that just says MPPT as a marketing ploy, and they actually just throw away excess energy as heat, defeating the purpose of MPPT.
     
    Last edited: May 29, 2016
  16. Lestraveled

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    I thought that also. I was looking at picture of the main board for a Xantrex C35 PWM charger controller. There was no inductor. It is a 35 amp controller, if it had a inductor it would be clearly visible. No inductor. The spec sheet said, "Highly Efficient", no numbers, just "Highly Efficient". I looked at other PWM charge controllers, same thing, no inductor and no real efficiency numbers. The big ($) MPPT models were big on boasting their efficiency numbers, not the PWM models. Then reading a few white papers of PWM charger controller confirmed it, they are not power converters, they are shunt regulators.
     
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  17. ronv

    AAC Fanatic!

    Nov 12, 2008
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    I think you mean series regulators?
    See if I understand it right.

    Since the solar cell is kind of a CC device you can just turn on the switch and hook it to the battery until the voltage gets to high. Then start pwm to control the voltage? What keeps the voltage from instantly being to high?
     
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  18. Roderick Young

    Member

    Feb 22, 2015
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    Xantrex is a legitimate brand. That makes me rethink how these things work.

    As many of us know there is a certain output voltage at which one can extract maximum power from a solar panel. But perhaps that voltage does not need to be constant. If the panel is 30 volts open circuit, and the battery being charged is 15 volts, maybe it's still valid to have the voltage at 30 volts 70% of the time, and at 15 volts 30% of the time, if the frequency is high enough.

    My description of using a capacitor at the input was based on my own design, which functions that way. But it needs to have an inductor for a number of reasons - one of which is that the panel voltage could be higher or lower than the battery.

    I also read reviews of the cheap "Harbor Freight" panel kits, where the reviewer said their battery charged better with a direct connection to the panel, compared with the so-called MPPT charger.
     
  19. Lestraveled

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    Because the panels are constant current devices and there is a good amount of capacitance at the input of the PWM switch.

    No, what a PWM is actually doing is controlling the panels output voltage by shunting it with the battery. In other words, the PWM is using the battery to control the panel output voltage. You can call it what you like. A PWM charge controller does not allow the panel to operate at its maximum power output voltage. It forces the panel to operate at the voltage needed to charge the battery.
     
  20. ronv

    AAC Fanatic!

    Nov 12, 2008
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    @Lestraveled

    Yes, I dig it. :D In reality just a switch and since the panel is a current source it's voltage is clamped slightly above the battery voltage.
    What I don't understand is this:
    It's a bright sunny day in Southern Arizona and the panel is humming.
    Oh wait I think I just answered my own question.:eek:
    I've set the switch to turn off at 14.6 volts and on at 14.4 volts.
    It gets to 14.6 and turns off, but is slow to drop to 14.4 because of chemistry. :rolleyes:
    It drops to 14.4 and turns on. The voltage quickly rises to 14.6 then repeats.
    So it's really the battery that keeps it from oscillating around the set point faster than a speeding bullet.
     
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