PWM Charge Controller - Pulse Width?

Discussion in 'The Projects Forum' started by Buchy, Jul 15, 2011.

  1. Buchy

    Thread Starter New Member

    Jun 27, 2011
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    I have written code for a fairly simple PWM charge controller, in C for PIC16F886. I have ADC inputs to measure both the battery and panel voltage with LCD display. It also incorporates 4 set points, voltage reg, array reconnect point, low voltage disconnect and load reconnect. I was originally making this a simple On-Off type controller but since moving to PWM I believe I can remove the array reconnect point as the PWM pulse width should take care of this (array can always be connected). Maybe I can add a safety point so the PWM turns off completely if the battery reaches some voltage.

    I understand the basic principle, such that the PWM is essentially always on when the battery voltage is low and is only on slightly as a trickle charge when the battery is at the voltage regulation set point.

    Where I am getting stopped is what is done inbetween? I cannot find any information on how quickly the pulse width should decrease or increase. I take it this is probably the proprietary information within commercial PWM charge controllers. On the battery charging, at what point do I begin to reduce the PWM pulse from full on? What PWM duty cycle is used for a trickle charge ususally? I suppose you would want the PWM on for the longest time possible to do as much charging as possible.

    I am using a home made solar panel, Voc ~19V and Isc ~3A. I've got an old car battery, sealed lead-acid, for my storage.

    Eventually I'd like to progress this into a MPPT type charge controller, but I figured it would be more beneficial to start with a PWM type.

    All thoughts, comments appreciated!
     
  2. iONic

    AAC Fanatic!

    Nov 16, 2007
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    Opinion: PWM is a good start, but MPPT would not be second on the agenda as far as I'm concerned. Solar tracking will get you a greater increase in available Power, possible up to 20 - 60% where MPPT will vary by the season between 10% - 15% in Summer and 20% - 45% in winter if this is practical.

    Solar tracking, however, is limited by available space and desired location of the PV.

    I understand the desire to build such devices oneself as the purchase cost can certainly be prohibitive. It will set back the return on investment a good while....years even.
     
    Last edited: Jul 17, 2011
  3. Roto

    New Member

    Jul 13, 2011
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    What about a simple temperature sensor from the PIC to the battery?
    Charge it at 12f above ambient temperature or what ever temperature diff that give a good charging rate.
     
  4. iONic

    AAC Fanatic!

    Nov 16, 2007
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    Generally speaking you want a bulk charge at the batteries rated high current until you are about 80% charged, then the current is reduced to continue charging to a voltage of about 14.4V. At this point you revert to a trickle charge with a voltage of about 13.6V to 13.8V at an even lower current...just enough to compensate for the batteries self-discharge characteristics. Temperature compensation in this schema is an added bonus for accuracy and battery longevity.
     
  5. Buchy

    Thread Starter New Member

    Jun 27, 2011
    2
    0
    Thank-you for your responses ionic. This gives me a reasonable starting point.

    Cheers
     
  6. iONic

    AAC Fanatic!

    Nov 16, 2007
    1,420
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    This may also be of some help...

    Battery University/
     
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