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BQ28Z610: Cells Imbalance

Alexey Bulatov
Prodigy 185 points
Part Number: BQ28Z610

Hello.

I've a problem with Cell Balancing during charging. I made 2 learning cycles and my UpdateStatus is 0x0E, so it should work. I can see OperationStatus()[CB] sometimes, when any Cell X Bal Time > 0, but these timers recalculate their values only after sending RESET command. So the main issue that cell disbalance > 100-400 mV at some time and charging is switching off (CUV = 1).

I've setup Bal time like this:

Bal time/mAh cell1 = 3600 / (3.2 / (100 + 150)) * (0.66 / 1000) = 186

Bal time/mAh cell2 = 3600 / (3.2 / (5 + 150)) * (0.66 / 1000) = 115

There's a graphic of charging:

And Registers view:

And params:

balancing_params.gg.csv.zip

What can be wrong? Is there a balancing restriction? Should I send RESET command for Bal Time calculation everytime?

Thanks a lot.

  • 0
    TI__Guru* 76230 points
    Hi Alexey
    Our gauge performs cell balancing based on capacity difference between the two cells and not based on voltage. In order for cell balancing to work properly, you need to have learned successfully, which it appears you have so the qmax for both cells is accurate. Now afterwards, The device needs to take proper DOD0 measurement so as to calculate the delta q properly. You do not have to send the reset button as that forces a DOD measurement which could be wrong depending on oif the cells are relexed. You also need to enable the CB and CBR flags. See the TRM for additional details on cell balancing.

    Did you change the cell balancing resistors. If no, why are you modifying the balancing time. Try using the default values.

    thanks
    Onyx
  • 0 in reply to Onyx Ahiakwo
    Prodigy 185 points
    Hello Onyx
    Thanks for the reply.

    So in my case balancing may not work correctly because of the wrong learning cycle? How can I estimate the values of QMAX and DOD0?
    I've already enabled CB and CBR flags since they are required for cell balancing, so the test was performed with these parameters.

    In our current design, we are using 5 and 100 ohms which we saw in the datasheet. So according to datasheet we have to recalculate standard BalTime values for each cell, because we have different voltage than at the datasheet and they are different from default. What effect does that make?

    If we have 5 ohm resistor, balancing should be with current Vbat/(Rdson+Rext), then 3.2/(150+5) = ~20 mA. Is that current enough for actually balancing if charging current 1.4A? It seems like a low current. Can you please explain what bq28z610 is actually doing at balancing with charging current, which more bigger than balancing current?
  • 0 in reply to Alexey Bulatov
    Prodigy 185 points
    Should I decrease charging current during cell balancing?
  • 0 in reply to Onyx Ahiakwo
    Prodigy 185 points
    I've noticed with oscilloscope that duty cycle of balancing FET is 20% and seems like this value is not enough. Can I increase duty cycle?
  • 0 in reply to Alexey Bulatov
    TI__Mastermind 42535 points
    No, you don't need to. In this case you are using cell balancing internally. The gauge will take care of that.
  • 0 in reply to Alexey Bulatov
    TI__Mastermind 42535 points
    The cell balancing duty cycle is controlled internally by the gauge. You cannot change that. Also we are sorry, for a 2s config the duty cycle should be 80.8%. So please use 0.808 for it instead of 0.66. The TRM has to be changed to fix it.
  • 0 in reply to Batt
    Prodigy 185 points
    Hello Batt
    Thanks for the replies.

    So I should recalculate bal timers for my 6.4V 2S2P battery as:
    Bal time/mAh cell1 = 3600 / (3.2 / (100 + 150)) * (0.808 / 1000) = 227 s/mAh

    Bal time/mAh cell2 = 3600 / (3.2 / (100 + 5 + 150)) * (0.808 / 1000) = 232 s/mAh

    I'm right? Does wrong bal timers have a strong effect on balancing process?
  • 0 in reply to Alexey Bulatov
    TI__Mastermind 42535 points
    Yes and yes. The balancing timer gets the time in s/mAh. If you set that to a wrong value you can overshoot or undershoot the charge that needs to be balanced out and end up with a pack that is not balanced.