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Original question:

BQ76952: Cell Balancing

BQ76952: Passive balancing is not working with high charging current during charging

Shashidhar HN
Prodigy 40 points
Part Number: BQ76952

Hello,

I have BQ76952 AFE with which I've been able to test passive balancing during relaxation with bleed resistor value being 41 ohm(Did add 6 parallel resistors for the cell with max voltage so that dissipation is quick). When I do the balancing during charging, I'm seeing one of the cell quickly hitting over voltage threshold & hence balancing stops. I had the charging current tried with 3A, 0.5A. I had the same problem. Balancing while charging seems to work fine if I have 0.05A for the charging current which I wouldn't prefer doing. 

Battery chemistry: Lithium Ferrous phosphate

Battery capacity: 6Ah, nominal cell voltage 3.3V; My test setup actually has 2 AFE with each connecting to 12 cells. Cell mapping has already been done and verified with TI. 

  • 0
    TI__Mastermind 27321 points

    Hello,

    A cell hitting the over voltage threshold does not have to do with cell balancing. It seems like one of your cells is over charged or its internal resistance is very high. You may want to run a log file and check which cell this is.

    Another possible approach would be to do RELAX cell balancing where the highest cell voltage will balance to the lowest cell voltage. 

    Over voltage protection will keep triggering when charging regardless of when cell balancing or not. The key here is to identify which cell is overcharged, then equal all cell voltages by doing balancing in relax mode.

    Regards,
    Jose Couso

  • 0 in reply to Jose Couso
    Prodigy 40 points

    Hello sir,

    I have 2 questions;

    [1] The cell voltage capacity is 3.7V. If one of the cells always hits over voltage, the battery can never be charged fully. what's the general way to solve this?

    [2] I've run the host controlled passive balancing. Whenever I balance an even cell, the adjacent odd cell is also being balanced(i can see pwm & power dissipation) even if its a cell of lowest voltage(ideally balancing should not drain the cell with low voltages). This is observed even with the autonomous cell balancing as well. Can you explain why does it work this way.

    Thx      

  • 0 in reply to Shashidhar HN
    TI__Mastermind 27321 points

    Hello Shashidhar,

    1- The battery cell should always be fully charged to the specified design voltage in cell manufacturer datasheet. Did you configure the the overvoltage protection accordingly? Please share your settings (export gg file).

    2- Did you configure the Settings: Cell Balancing Config: Cell Balance Max Cells? This register limits the maximum number of cells actively being balanced.  The 0x0083 CB_ACTIVE_CELLS() subcommand can be read to see which cells are actively being balanced.


    Please see the cell balancing app note for reference and debug issues.

    Note: Do not use resistor dividers as cell simulators as this will constantly trigger the COV; hence, cell balancing will be disabled. Instead, please use real cells.

    Regards,
    Jose Couso