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Balancing?

  • firethorn Monday, December 9, 2013 1:53 AM Reply
    How would one balance a cell when using this as protectection?
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  • desolder Top 10 Forum Poster Tuesday, December 10, 2013 4:08 AM Reply
    Connect your balance cable directly to the cells (or equivalently, connect the balancing cable to the electrically identical spots on the PCB). This will bypass the protection board. There is no need for protection against a balancing charger, since the balancing charger already protects the cell against overcharge.
    What's up with the scrubbed LED driver IC markings?
    What are they trying to hide?

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  • Jerommel Saturday, January 11, 2014 1:56 AM Reply
    @desolder:
    Do you even need a balance charger when you have this?
    Doesn't this take care of the charging business as well?
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  • desolder Top 10 Forum Poster Saturday, January 11, 2014 2:13 AM Reply
    The "need" for a balance charge is a bit of an open question. Personally I prefer to always balance charge. However, with good quality cells and a good protection board, I concede that the rate of imbalancing could be so low that the cells will reach end of life before the amount of imbalance becomes an issue. This has led to a surprising number of consumer devices that do not perform any balance charging (e.g. all the bike light batteries), even though they have poorly matched cells or protection boards that have significant drain on the inter-cell voltage sensing wires (a drain that causes imbalance). So the answer is, no, you do not "need" to balance charge, but it is preferable in order to avoid angry customers who complain about "dead" batteries or short run-times when the only problem is a simple balance issue.

    This board does not perform charge control. You will need a separate CV/CC charger for that.
    What's up with the scrubbed LED driver IC markings?
    What are they trying to hide?

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  • Jerommel Saturday, January 11, 2014 7:35 AM Reply
    I have a Lipo-pack with 3 cells in series, each cell with a tiny protection PCB.
    So every time I charge it, I charge it fully, so each cell's protection circuit stops the charging @ 4.2 Volts.
    It drains to 9 Volts when it shuts itself (or at least one of the cells) off.
    So I guess it's fine.
    I use a 14 Volts SPS and put a 25 Ohm resistor in series.
    Charges to 12.6 Volts, even after forgetting about it for a day and a half..
    For charging I guess a zener diode for each cell would be enough protection and you will end up perfectly balanced too.
    Probably wise to match the zeners, my guess.
    But I guess a lot, sometimes..
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  • desolder Top 10 Forum Poster Saturday, January 11, 2014 9:15 AM Reply
    That should be OK, even though it doesn't follow the recommended Constant Voltage / Constant Current charge algorithm. In fact, a LOT of chargers on DX don't use the CV/CC method. Some only do CC (or semi-CC like your method), then shut off at 4.2V/cell. Since you have protected each cell, there should be no safety issue.

    In theory the zener idea would work - if it was accurate enough. But zener diodes are not especially accurate, typically +-5%, so they won't be able to maintain the tight 4.2V +- 50mV (1.2%) regulation that is required for good charge control. The other issue is power dissipation in the zener. The zener potentially has to dissipate the full power of the charging current that would otherwise go into the cell.
    What's up with the scrubbed LED driver IC markings?
    What are they trying to hide?

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  • firethorn Saturday, January 11, 2014 7:18 PM Reply
    I, too, have considered using zeners many times but as desolder notes, it is impractical due to its inaccuracy. My guess is that you are attempting to apply 12.6V at the top and let the batteries balance themselves via the zeners.

    My current alternate solution to that, is to use simple but accurate step down converters (linear or otherwise). It is not currently implemented because I only trust the lm317s so far and they are horribly inefficient and get hot easily. But you get the idea.

    In all cases, don't forget to isolate them from each other. Otherwise, when you disconnect the charger, your top cell will charge the middle until the top goes flat and the same for the last cell. A schottky diode (personally, I want the voltage drop) or put it on the charging side should do the trick.
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  • Jerommel Sunday, January 12, 2014 4:13 AM Reply
    @desolder, @firethorn :
    Then you would need 3 isolated step down converters, to dedicate one for each cell.. ..if I understand you correctly?
    Bit too complicated electronics for Jerommel, I'm afraid..

    But, what exactly is the problem with zeners?
    Don't they keep their limit too wel (like dependant on temperature), or is it a matter of matching them, to eliminate the difference in voltage between themselves?

    I like the simplicity of the zener idea a lot!
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  • desolder Top 10 Forum Poster Sunday, January 12, 2014 5:19 PM Reply
    Here are the problems:
    1) The reverse voltage tolerance is too loose. Li-ion charging requires a voltage accuracy of 1%. Most zeners are only specified for 5% or even 10% accuracy
    2) The reverse voltage is also significantly affected by temperature and the current flowing through the zener.
    3) The power dissipated by the zener diode can be up to 4.2V times the charging current. If you use a charging current of 1A, then the zener must be big enough to dissipate 4.2W. That's a lot of power, and most zeners are quite small, probably rated for 1/2W or less.
    What's up with the scrubbed LED driver IC markings?
    What are they trying to hide?

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  • Jerommel Sunday, January 12, 2014 9:12 PM Reply
    1) Could be solved by matching zeners with each other?
    2) That's the real problem I suppose.. (Thinking about coupling them thermally in a piece of alu or copper with 3 through holes, 1 hole for each zener, with some cooling paste / stuff, but that's just a thought)
    3) Bigger or more zeners could solve this, I suppose?

    Anyway.. Maybe time to purchase a decent balance charger..
    I got a small one from here, but it doesn't seem to work so well...
    Ordered another type somewhere else, but it never arrived...

    Thanks for all info, guys.
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