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BQ29200: BQ29200 balancer question

DAN ROSS1
Intellectual 670 points
Part Number: BQ29200
Other Parts Discussed in Thread: BQ24103

Hello, we're looking at using the BQ29200 for a new project here a work. Also, The batteries we are planning on using are 3.7V Li-Ion 800 mAh batteries. The datasheet in Section 8.3.6 mentions T.I. does not recommend turning on the balancer if the battery pack voltage is less than 6V. What happens if is lower than 6V?

I tried 2 battery packs, with one battery in each of the packs being less than 2.0V and the other being ~3.25V. When the charge was done, on both battery packs, one battery measured 4.40V & the other battery measured 3.92V. The 3.40V exceeds the max voltage of the charger. Is this because both packs used in this test were below 6V?

Also, I repeated the same test with both packs when both batteries were slightly above 3.0V The results were on both battery packs the voltage difference between the batteries in each pack was 2mv.

Thanks

Dan Ross - AST INC.

  • 0
    TI__Guru 54715 points

    Hi Dan,
    The bq29200 data sheet does not include characteristic curves for the VMM_DET_ON and VMM_DET_OFF over voltage. Since the calibration point is at 7.6V one might expect the accuracy is not as good moving away from the calibration point. The section 8.3.6 in the data sheet seems to indicate good performance from 6 to 8.4V, but does not indicate what that tolerance is.
    In your example where one pack was 2V and 3.25V, the difference was 1.25V and the part should have easily identified this difference and balanced. If the charger completed with cells at 4.4 and 3.92V, the charge voltage must be 8.32V or slightly above if the cells relax some.  The difference is 480 mV and the device should still be balancing the high cell.  If CB_EN is low the part would continue to balance the high cell down, after sufficient time the cells might be approximately 3.92 +/- 9 mV if the device tolerance is still close to VMM_DET_OFF. 

    It will take some time for the part to balance the high cell down.  A charger will normally provide an average voltage and can easily push a cell voltage above its OV limit.  The OUT pin voltage should be a signal to the charger to stop charging.

    If I'm not understanding the voltage conditions of your cells please let me know.

  • 0 in reply to WM5295
    Intellectual 670 points
    Thanks much.
  • 0 in reply to DAN ROSS1
    Intellectual 670 points

    Hello, I was a bit premature in closing the case - I apologize. One last question - Were using (for this prototype the BQ24103 charger. I'm trying to find out why the charger allowed the battery to charge to that voltage level (4.4V). Is that tolerance? Just concerned about the battery taking that high of a charge voltage.

    Thanks

    Dan Ross - AST INC

  • 0 in reply to DAN ROSS1
    TI__Guru 54715 points
    Hi Dan,
    I'm not a charger expert, but the bq24103 shows it is configurable for 1 or 2 cells. At 1 cell it provides 4.2V, at 2 cells 8.4V (figures 1 & 2). The device does not have a way to monitor the voltage of the individual cells, that is left to other circuits. The bq29200 monitors individual cell voltages and provides an overvoltage detection signal on OUT to stop a charger or provide some other overvoltage protection switching.