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BQ24630: Charging Issue for 4-Series LiFePO4 Battery Pack

Dat Pham
Prodigy 35 points
Part Number: BQ24630
Other Parts Discussed in Thread: BQ76905,

Tool/software:

Dear all,

I would like to describe my issue:

I'm using the BQ24630 to charge a 4 - series cell (Lithium Werks 18650 1100mAh 30A LiFePO4), combined with a BQ76905 for cell balancing. I'm facing a strange problem that I cannot explain.

My setting parameters are:

I_Charge: 0.7A

I_Term: 70mA

VBat: 14.6V

I_balance: 15mA

During the constant current charging phase, all 4 cell voltages are equal. However, when switching to the constant voltage stage and the charging current decreases to I_term (70mA), I noticed that one or two cells start to increase in voltage, while the others drop. This keeps the total battery voltage (VBAT) the same, even though one or two cells suddenly rise. 

The constant current charging phase:


The constant voltage charging phase:



The charging termination phase:



Discharge phase:



I have tested this many times, and the same issue keeps recurring. I even turned off cell balancing function and reduced the VBAT threshold to 14.4V but the problem still occurs. The balancing circuit works fine — when one or two cells spike, the IC still activates the MOSFETs to discharge the higher cells. So, I believe the issue might be related to the BQ24630.

After many charge and discharge cycles, the cell with a higher voltage gets even higher with each charge. If I do not fix this, the overall battery capacity of the 4 cells will gradually decrease after every charge cycle.

I hope to receive help soon.

Regards,
Dat

  • 0
    TI__Mastermind 42060 points

    Hello Dat,

    Do you see any voltage drops on the board or connectors during the charging cycle?

    If this only shows up during charging, I am wondering if it may be related to IR drops on the cell connectors. The BQ76905 should only be measuring the cells and would only do balancing if the host allows it.

    Best Regards,

    Luis Hernandez Salomon

  • 0 in reply to Luis H. S.
    Prodigy 35 points

    Hi Luis,

    Thanks for your response.

    This is my design. This issue only occurs during the constant voltage (CV) phase of charging, when the charging current starts to decrease. I haven't measured any voltage drop on the board or connectors. Why do you think IR drop is the cause?

    The issue only really spikes when the charging current drops close to the termination threshold. I tried increasing the termination current above I_charge/10 (I_charge is 0.9A and I_term is 0.3A), and it seems to terminate early and the issue no longer occurs. However, this significantly reduces the battery capacity.

    Regards,
    Dat

  • 0 in reply to Dat Pham
    Prodigy 35 points

    Hi Luis,

    I just measured the voltage drop on the board. The voltage at the PH pins of the BQ24630 matches my configuration, but the battery voltage is 24 mV lower. I also noticed that the VFB voltage doesn't quite reach 1.8 V (it's at 1.797 V). Do you know why the charge current is decreasing even though VFB hasn't reached 1.8 V?

    Regards,
    Dat