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Li-ion under-voltage protection circuit

  1. sweben

    sweben

    18
    15
    Jul 3, 2015
    sweben submitted a new Showcase Item:

    Li-ion under-voltage protection circuit

    Read more about this showcase item here...
     
  2. HellasTechn

    HellasTechn

    1,363
    190
    Apr 14, 2013
    I would recommend to set the cut out voltage to something about 3.6 to 3.7 volts.

    3.7 volts is about the voltage indication that a Li-ion battery is almost depleted.
     
    sweben likes this.
  3. Harald Kapp

    Harald Kapp Moderator Moderator

    8,797
    1,723
    Nov 17, 2011
    The resistance from Vbat to GND by the resistive divider is a bit on the low side. It will drain the battery even with the olad turned off. You should increase this divider's resistance considerably.
     
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  4. sweben

    sweben

    18
    15
    Jul 3, 2015
    I'll raise the cutoff on the modules, but 3.7V is still quite high isn't it?

    @Harald Kapp: What do you think, changing by a factor of 10? 18K/2K?
     
  5. sweben

    sweben

    18
    15
    Jul 3, 2015
    @Harald Kapp: So, using 18M/2M would work the same? the current will truely be minimal there.

    Also, for the next generation, I'm considering getting proper comparators instead of amp ops. I'm looking at some LM2903D, can I just use it the same way as the lm358, or do I need additional components?
     
    Last edited: Jul 23, 2015
  6. Harald Kapp

    Harald Kapp Moderator Moderator

    8,797
    1,723
    Nov 17, 2011
    The answer will have to wait until I can see your diagrams, they seem to be blocked by the firewall here.
     
  7. Harald Kapp

    Harald Kapp Moderator Moderator

    8,797
    1,723
    Nov 17, 2011
    These values my be too high a you'l have to account for the input current of teh comparator.
    I suggest not more than 1.8M/200k, probably 900k/100k.

    Also note that ypur circuit is prone to oscillations when the battery voltage comes in the treshold range:
    When the power is turned off due to low-batt indication, current flow from the battery stops and the battery will recover a few mV, thus turning power on again. Add some hysteresis to the comparator to form a Schmitt Trigger circuit and prevent oscillations.
     
  8. Old Steve

    Old Steve

    734
    169
    Jul 23, 2015
    Is that a 2K pot? That will also drain the batteries even when the load is switched off. You need to consider a much higher resistance pot, as suggested by Harald for the voltage divider.
     
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