Battery monitor/ comparator circuit

Discussion in 'The Projects Forum' started by jimmychacha, Dec 4, 2010.

  1. jimmychacha

    Thread Starter New Member

    Dec 1, 2010
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    Good morning, I am an electronics student and sustainable energy enthusiast. I have a small PV array w/battery backup installed in my home. To fully utilize the battery power I want to connect a small heating element, approx 1.5 amp load, that will run when the battery voltage is around 12.3 volts or greater, and turn off when the voltage drops to 11.5. I'm thinking of using an op amp as a comparator, Schmitt trigger to establish the UTP and LTP, and either a zener diode or voltage reg to establish the reference 12v. I'm finding my textbook a bit confusing because it only gives examples of such circuits with AC signals. Can someone help with this design- setting ref, UTP & LTP, choosing an op amp? Thanks
     
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  2. wayneh

    Expert

    Sep 9, 2010
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    I think your heater could easily be a light bulb unless you already have something in mind. A power resistor would also work, but it would need to be rated for >20 watts. Not a problem, but they're not just laying around.

    As for the rest, I think you could get by with nothing more than a comparator driving a MOSFET to turn the load on or off. Use a zener at some reference voltage well below the supply, for instance 5v. Use a pair of resistors to divide the supply voltage so that at "hi", it's above the reference and at "low", it's below. Put the reference and the divided supply voltage to the two inputs of the comparator, with the reference at the inverting input. Pull up the output of the comparator with a ~3K resistor and connect that output to the gate of the MOSFET. Connect the other MOSFET pins Source to ground, and Drain to one terminal of the load. The other load terminal connects to battery positive.

    You may want to add hyteresis to the comparator to avoid chatter as it switches. That's hard to predict and I'd simply experiment a bit. Hmmm...The more I think about it, this is a problem. Anytime you disconnect the load, the voltage will rise substantially and vice versa. You'll definitely need a wide band for hysteresis so that the turn-off-under-load voltage is well below the turn-on-without-load voltage.
     
  3. jimmychacha

    Thread Starter New Member

    Dec 1, 2010
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    Thanks for the quick reply, one thing I'm not understading is the voltage divider. Why is it necessary to divide Vcc and not just monitor around its nominal 12v?
     
  4. wayneh

    Expert

    Sep 9, 2010
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    Because the comparator may have difficulty making comparisons at its supply, or rail voltage. And, it's hard to establish a reference too near the supply voltage. Cutting it all in half makes it a cinch.
     
  5. jimmychacha

    Thread Starter New Member

    Dec 1, 2010
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    Is an LM741 suitable?
     
  6. iONic

    AAC Fanatic!

    Nov 16, 2007
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    Yes, the 741 would work, but I believe that comparator IC's are a bit more efficient in general, so I've heard.


    iONic
     
  7. wayneh

    Expert

    Sep 9, 2010
    12,103
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    Yeah, I can't really comment on the relative merits. A comparator is a just a specialized op-amp, designed to minimize the "in between" region where it's not in one state or the other. That's a good thing and might be harder to achieve with a general purpose op-amp, although there are certainly circuit examples of how to use an op-amp as a comparator. I can tell you from experience that even the comparator is not perfect in regard to switching; you'll definitely see the output voltage swing before it triggers a change.
     
  8. SgtWookie

    Expert

    Jul 17, 2007
    22,182
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    If you drain your batteries down to 11.5v, you'll kill them very quickly.

    Have a look at this post in the Tips 'n Tricks thread: http://forum.allaboutcircuits.com/showpost.php?p=262143&postcount=38

    Battery plate sulfation begins when a 12v lead-acid battery falls below 12.5v @ 25°C core temperature.

    Discharging a deep-cycle lead-acid battery to 50% repeatedly will cut its' service life by 2/3. If you want to get a good service life, size your battery bank so that it won't get discharged below 70%.

    With this new information at hand, where do you want your thresholds to be?
     
  9. jimmychacha

    Thread Starter New Member

    Dec 1, 2010
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    My 3 batteries are all deep cycle, sealed cells. I didn't think 11.5 would be harmful. On consecutive cloudy days I may never reach the 13 volt level, so I would be in the same boat i am in now, rarely using the stored energy at all (the battery supplies undercabinet lighting in my kitchen and a 12 volt pump that fills a 1st floor toilet). What thresholds would you recommend, and how close do you think they could be?
     
  10. jimmychacha

    Thread Starter New Member

    Dec 1, 2010
    10
    1
    Sgt., just looked at your spreadsheet. I didn't realize a voltage around 11.5 indicated such a low state of charge. The bank is stored in my basement which hovers around 60 F this time of year. So I'm going to say by the chart that a good low threshold for my circuit would be 12.5. Where would you recommend the high?
    Thanks for your help
     
  11. Kermit2

    AAC Fanatic!

    Feb 5, 2010
    3,783
    945
    Just to throw more complications into this.

    The battery will show a lower voltage when under load. The larger the load(more amps) the lower the voltage.

    The battery may read 12.1 volts while the circuit is operating, but may go considerably higher when the circuits are off.

    You should base the cutoff on the battery voltage when not loaded, but also know the battery voltage under load when the unloaded voltage is as low as you want to go.

    So it may turn out that your cutoff point would be 11.8 volts under load, and the battery itself off circuit would read 12.1 or 12.2(a good 40-60% discharge)

    The off state(no load) voltage will indicate the state of charge. Full charge on an idle battery being between 12.7 and 12.8 volts.
     
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