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Original question:

BQ40Z50: Balancing Multiple Cells

BQ40Z50: Internal Cell Balancing

Michael Parry
Prodigy 40 points
Part Number: BQ40Z50

Tool/software:

Hello,

I’m working with a 4-cell LiFePO4 battery pack (12.8 V nominal, 3.6 Ah). I’ve noticed about 10% capacity loss within 4 weeks on some packs. After a quick check, I found that the logs show Cell 2 and Cell 4 reaching 3.8 V, while the others remain lower.

The packs are charged at 14.4 V, 750 mA. I ran a full charge cycle and logged the cell voltages on the graph, Cells 2 and 4 consistently rise higher than Cells 1 and 3, especially toward the end of the charge.

When the batteries are left to rest, the voltages eventually balance out, but the imbalance is clear during charging.
Does anyone know why such a large difference appears between the cells during charge and is there anything I can do to help?

Kind Regards,

Mike

  • 0
    TI__Mastermind 25855 points

    Hello Michael, 

    This question has been assigned and will be reviewed in the meantime you can attached the associated .gg/.log files for this project.

    Thank you,
    Alan

  • 0
    Expert 1091 points

    Hi Mike,

    I've experienced this with LiFePO4 cells as well.  They have a large voltage spike (just like in your graph) at the end of charge that causes one or more cells in series to hit an Over voltage condition, which ends up preventing full charge.  Even with cell balancing, there is still a chance that a cell can ramp up faster at the end of charge.  This then results with a loss of some capacity.

    What I've done to help this is actually lower the charge voltage.  Instead of charging at 3.6V per cell, I've used 3.5V or 3.55V.

    This has helped me to prevent the cells from hitting over voltage and get to the constant voltage stage, so that I can get to the taper current portion of charging.  It might be worth testing out on your end if possible.

    I'm sure TI will have more insight for you as well.

    Regards,
    Kevin

  • 0 in reply to Kevin Knicker
    Prodigy 40 points

    Hi Kevin, 

    Thank you for your insight! I tried reducing my charge voltage to 13.7V (3.425V per cell), but found the charge termination to be unpredictable—sometimes it stopped at 50%, other times at 80%. It might be that I set the voltage too low, or perhaps my termination current (set to 300mA) needs to be adjusted lower.

    Regards,

    Michael

  • 0 in reply to Michael Parry
    Expert 1091 points

    Hi Mike,

    That does sound like a result of a little too low of a voltage and too high of a taper current, so you are hitting the End of Charge conditions.

    Certainly try 3.5V per cell to see if that works.
    I'd also lower that taper current threshold.  The taper current portion of charging these cells is very small, so reducing it will help out.

    Good luck!

    Kevin

  • 0 in reply to Kevin Knicker
    Prodigy 40 points

    Hey,

    Yeah, I was hitting the 300mA ILIM threshold causing it to indicated it was charged. I'll set the charge volage to 14V and lower the ilim threshold. Hopfully it wont hit the Overvoltage condition

    Thanks again,

    Mike

  • 0 in reply to Michael Parry
    Prodigy 40 points

    Hi,

    I did another log last night at 14V, 150mA Termination current and I sees similar results with Cell 2 and 4 reading higher than 1 and 3. But with them topping out at a safer voltage, around 3533mV. 

    Here are my main Questions

    - is this perfectly OK?

    - How did a cell get to 3.8V? 

    - Internal balancing is at 10mA this sounds really low, should I be using external?

    Thanks,

    Mike

    PSA10.3_14.0V Battery Charge test180825.log

  • 0 in reply to Michael Parry
    Expert 1091 points

    Hi Mike,

    You could set the over voltage threshold higher to see if you can get past the spike from cell 2 and cell 4.

    I have set the over voltage threshold as high as 3.8V per cell.  That has allowed me to get the charge voltage to a constant voltage before hitting an and allow for taper current to top off the cells.

    External balancing will allow for a higher balancing current versus internal.  TI has some documentation on how to implement external balancing using some additional components.

    Best Regards,

    Kevin