Confused about MPPT and PWM

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Tricksailing
Tricksailing Registered Users Posts: 22 ✭✭
in Solar Beginners Corner #1
I understood that MPPT and PWM were 2 different types of charging (they are even separated on the NAWS info pages). However, many controller descriptions seem to claim that they are both MPPT and PWM. One example is the BLUE SKY ENERGY SB2000E controller, described in the first 2 paragraphs at http://www.blueskyenergyinc.com/uploads/pdf/SB2000Edatasheet.pdf.

Are all PWM controllers MPPT, all MPPT controllers PWM or is there a mix and, if so, how do we easily identify the MPPT controllers?

Thanks.

John
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,439 admin
    #2
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    Re: Confused about MPPT and PWM

    All MPPT controllers do have PWM internally to operate (pulse width modulation is how they control energy flow through the magnetics). Also, I believe that if your battery is nearly fully charged, the MPPT charge controller goes into a PWM like mode to control current flow to the battery bank.

    MPPT is only used when the battery need lots of current for charging. Maximum Power Point Tracking is the charge controller finding the optimum Voltage and Current from the solar array (Vmp/Imp) at that power level, temperature, amount of sun, etc. to put as much energy as possible into the battery bank.

    There is no reason for MPPT mode once the battery is nearly charged... The battery will accept less than maximum power from the solar charge controller. Therefore the MPPT Charge controller will "fall out" of MPPT mode.

    MPPT charge controller all have a big inductors (and/or transformer) which store energy and "transform" from one voltage/current level to another.
    • If you want to see how a "buck converter" works (most MPPT controllers use a form of buck converters), you can click here. It is actually pretty elegant.
    If you are interested in analogies... Read about Ram Pumps (these are water pumps that use the momentum of water to pump up hill)--They are very similar to Boost Converters (the math between moving water and electrical circuits can actually be the same).

    There are a fair number of charge controllers that claim to be MPPT but are really PWM type... Usually, there are no big inductors in this false MPPT controllers.

    The Rogue 30 amp 12/24 volt MPPT charge controller--Marc has written up a nice manual that describes many of the details and has picture of the toroid (fancy name for a circular inductor or transformer) on page 28 of the manual (pdf download).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • john p
    john p Solar Expert Posts: 814 ✭✭✭
    #3
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    Re: Confused about MPPT and PWM

    I dont know if this will help understanding MPPT and PWM controllers, but have a read anyway You may find a small gem in there somewhere.


    All the gains of MPPT chargers over PWM chargers for small under 500w panel systems are mostly theory .. you never get the claimed gains in most cases. You just pay a lot of extra cash for the charger.
    You will always notice that you never see side by side working examples of MPPT gains over PWM chargers. You are always given the theory only..
    Having tested at least 100 MPPT and PWM chargers (expensive and cheap ones)I have found the gains are about 5 to 15%. Many MPPT chargers produce no more power into your batteries than PWM ones do. This is especially true when the battery/ies are above 90% charged. Under thse conditions the MPPT part usually does nothing.
    Here is one example . but it is only one very well tested example .. of a high priced MPPT charger compared to a Chinese copy of a old well known American PWM charger.. 3x 80 w 12v panels in parallel PWM charger days production 78ahr into the batteries
    3x 80w 12v panels in series MPPT charger days production 81 ahr into the batteries

    different types of battery/ies and depth of discharge would alter these figures. but it does give some idea of what you can possibly expect.

    On systems of panels above 1000w the good MPPT controllers win by a big margin. But you need to use a top of the range controler to achieve that.



    The theory behind MPPT chargers is good.
    In using real ones the results are not so good in most cases..
    Cheap MPPT chargers are usually no better than PWM chargers and in some cases LESS effecient.
    Good ones are far too expensive for the gains on under 1000 w systems Its cheaper to buy a higher output panel/s
    The gains that are claimed are theory. much the same as the rating on panels.
    You are living in a dream if you expect to get 225 w from a roof mounted 225 w rated panel..
    In general even the manufacturers have doubts about the long term reliability of MPPT chargers and most give shorter warranties compared to PWM ones. So what is the consumer to think??
    Its almost impossible to find proof of gains of MPPT chargers ,If you ask the people telling you how good the gains are by comparison tests they have done.the answer is nearly always the same, NO I have never done a comparison test ,and have no documented results to show you BUT I know they are better. hmmmm
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  • RCinFLA
    RCinFLA Solar Expert Posts: 1,484 ✭✭✭✭
    #4
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    Re: Confused about MPPT and PWM

    I never liked the use of the term PWM for on-off switching charge controllers.

    An MPPT DC-DC converter has a power conversion from a high voltage/lower current to a lower voltage/higher current. It uses an inductor to store energy and translate power to the battery side which has lower voltage then the panels (usually).

    To confuse you more, this DC to DC converter pulse width modulates (PWM's) the input to the series connected inductor to get the power conversion.

    Now to confuse you even more, MPPT controllers drop back to a straight on-off chopping when the panel voltage gets too close to the battery voltage for the DC-DC power converter to work properly.

    The MPPT tracking function ceases when battery charge set point voltage is reached or when the controller battery charge current limit setting happens to be lower then the available power from the PV array. No need to get maximum power from panels for these battery charging conditions.

    What we call PWM controllers are just on-off switches that either directly connects the panel output to battery or not. A PV panel is an illumination based current source so its terminal voltage will immediately drop to battery voltage when switch is closed. As the battery voltage reaches its desired charge voltage point the PWM controller starts opening and closing the switch to regulate the battery voltage. This switch opening and closing is called PWM'g as the on to off ratio varies to achieve battery voltage regulation. There is no current gain to battery like there is with the DC-DC converter of a MPPT controller.
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