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BQ77PL900 cell balancing function

daewon Lee
Prodigy 30 points
Other Parts Discussed in Thread: BQ77PL900

Hi

I tested cell balancing function of BQ77PL900.

test condition:

8S1P 2200mAh , imbalance state, charge  to 4.110V at 7th-cell, other cells voltage : 4.010V. imbalance voltage :0.1V ,

stand-alone mode, OVP set 4.25V, hysteresis 100mV, CBEN set 1, after charge to 33.6V(CC/CV mode), state is OV alram. CHG FET off,  charging stop.

series input resistor - 510 ohm

after 1hour, measure 7th cell voltage- 4.2534V, other cell voltage-4.1421V (charger connect, OV state)

after 12 hours, measure 7th cell voltage drop -0.013V(some -0.001V/hour), other cell voltage drop 0.0008V. (charger connect, OV state)

Thus if 7th-cell vlotage drop down to 4.15V, 1th function time(tb-ta) is some 100 hours for charging again. this result show that balancing function is very slow.

Is that functioning right? or test is fail?

slus844b -33page - 'When a certain cell (cell A) voltage reaches cell overvoltage, the battery charging stops and then cell balance starts working at ta. The cell-A voltage
decreases by the bypass current until the voltage reaches (VOV – ΔVOVH).'

 

 Thank you for read.

  • 0
    Prodigy 30 points

    Hello~~ Is anyone there?

  • 0
    TI__Guru 54715 points

    It certainly seems the balancing is not functioning.  The cell voltage drops seem small even for relaxation.

    If you are monitoring the state of the the status register, be sure the software has not inadvertently put the device in host mode.  In host mode balancing is controlled by the host control of the register bits and the CBEN state is not applicable. Read the registers to be sure the device is in standalone mode and that CBEN was actually written to the EEPROM.

    You should be able to see the cell voltage vary with balancing at the device pins for the balancing cell.  For the balancing cell the pin voltage will likely vary about 1.4V with the setup described.  This is a low current balancing, ~3mA with a 80% duty cycle or ~2.4mA.  Cell voltage drop over time with the balancing will depend on the cell characteristic.

    You might also compare the performance of balancing on other cells, and of course check that the series input resistors are the values expected.

     

  • 0 in reply to WM5295
    Prodigy 30 points

    Thank you for your answer. 

    At test beginning, I thought that cell voltage drops seem small even for relaxation. me, too.

    Well, I am check CBEN bit while testing. and voltage drop of 7th cell is diffrent  other cells. Voltage drop of 7th cell is much large.

    Anyway, testing continued.

    * Test start + 4days measure:  7th cell voltage -> 4.1529V, other cell voltage -> 4.0883V - OV release state(first release), (charger connect) after 1 hour  OV state.

    * Test start + 5days measure:  7th cell voltage -> 4.1475V, other cell voltage -> 4.0938V - OV release state(second release), (charger connect) after 1 hour  OV state.

    * Test start + 6days measure:  7th cell voltage -> 4.1480V, other cell voltage -> 4.1430V - OV release state((third release), (charger connect)

    after 3 hour  OV release state, cell imbalance release , Full charging state(33.6V), 7th cell voltage -> 4.2456V, other cell voltage -> 4.2372V


    I will try test again.

    Thank you.