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Balancing?

  • firethorn Sunday, January 12, 2014 10:16 PM Reply
    I think you have misunderstood the point. It's not that the inaccuracy of the zeners will produce unbalanced cells. It's that it will produce improperly charged cells (i.e. shorter cell life, explosions, fire, armageddon).

    Matching the zeners (I'm not even sure how this is done), will only ensure that all batteries "explode" at the same time. Li Ion batteries require precise voltages/temperatures/current for charging: whereby precise I mean that it has an actual limit.

    You cannot hope to control a zener (of any size) with thermal regulation. You cannot hope to get the precise voltage that has been stated on the zener. You will need proper regulation such as a linear LM317 or other switching regulator. You have been warned. Your life is in your hands!

    Try the copter-x charger on here. It balances my cells very nicely but is unfortunately, very slow.
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  • Jerommel Sunday, January 12, 2014 10:51 PM Reply
    Matching them would (in my case) mean measuring current around the area of U = 4 Volts.
    An identically responding trio of zeners would / might (in my mind) balance the lot sufficiently.
    Armageddon is not yet due.. ;)
    But they will at least explode together.. :P

    I'll have to properly experiment and measure to say anything useful about it further.
    But it'll probably not work satisfactory..

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    post edited by Jerommel on 1/12/2014 at 10:53 PM
  • desolder Top 10 Forum Poster Monday, January 13, 2014 4:13 AM Reply
    A "4.2V" zener would probably have no current at 4.0V. The rolloff is pretty steep. It may start conducting at 4.1V, then blow up at 4.3V. A more reliable measurement method would be to force a constant current through the zener, or connect it to a significantly higher voltage source in series with a large resistor, and measure the voltage across the zener. The resistor should be sized such that the current [(Vs - Vz)/R] will result in acceptable power dissipation in the zener [P = (Vs-Vr)^2 / R]

    Matching could be tedious. You'll probably have to buy 10x more 5~10% zeners in order to get a set matching to within 1%. But as firethorn stated, matching to other zeners is not the real issue, it's accuracy compared to an absolute standard (4.2V +-50mV) (OK OK.. being a bit pendantic... but worth pointing out the difference neverthless)

    Sourcing a "4.2V" zener could also be a problem. That's an unusual voltage for a zener, and might require a special order, which means expensive. "4.3V" zeners seem to be a bit more common, and may actually work for your intended purpose, since they may start conducting at 4.2V.

    As for any explosion... hopefully that won't happen since the protection circuit should cut off the charging current if any zener fails to start allowing bypass current at the expected voltage, e.g. if there's a break or cold solder joint or the zener burns up or just unexpectedly fails.

    Personally I would like to see you give it a shot and tells us how it went. Your idea has some merit. You might need to get "4.3V" zeners, since "4.2V" zeners don't seem to be readily available.
    What's up with the scrubbed LED driver IC markings?
    What are they trying to hide?

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  • firethorn Monday, January 13, 2014 11:35 AM Reply
    I was being a bit alarmist perhaps (:P) but the points still stand. That being said, I too would like to see how it turns out.

    With monitoring, it should be safe enough for a test. Don't forget to terminate charge though! Li-Ions can't take float charging (some say they do but most say no). Some say LiFePOs will take trickle charging though.
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  • Jerommel Tuesday, January 14, 2014 6:36 AM Reply
    I think you're right about warning me (and others) of the dangers of overcharged or undercharged LiPo cells and about messing around with the balancing (or lack of it..).
    Many times it's better to think inside the box, when you're not an expert..

    About the 4.2 Volt zeners, I was planning to combine values in series.
    This probably will not improve it's accuracy though..
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