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BQ76PL455A-Q1: Automatic load balancing activation and levels

Andre Hattingh
Prodigy 30 points
Part Number: BQ76PL455A-Q1

Hi

I'm having trouble with the bq76pl455 as its load balancing doesn't seem to kick in when I increase the voltage across one cell. Everything else works, I can read the cell and AUX voltage levels correctly, config and use the GPIO pins and there are no errors when reading the fault registers. I made sure all the voltage levels and grounds are stable and correct, at least on the outside there doesn't seem to be any problems.

My setup for the load balancing is:

register 0x13 = 0x28 (1 minute balance, BAL_CONTINUE = 1)

register 0x14 / 15 = 0x07 0xff (11 cells balanced)

register 0x1e = 0x00 (EQ_SQUEEZE_EN = 0)

I made sure the registers were configured successfully afterwards by reading all the registers. Am I missing anything? 

At what voltage level should the balancing kick in? My average cell voltages are 2.2V so when I increase the voltage of one cell to 4V the balancer should have activated a long time ago.

Any ideas or help will be appreciated.

Kind regards.

  • 0
    TI__Intellectual 2520 points

    Hello Andre,

    I didn't quite understand your problem.

    What does "Kick in" mean?

    What will you expect when you increase cell voltage from 2.2V to 4V?

    Thanks!

    Andy

  • 0 in reply to Andy Li55
    Prodigy 30 points

    Hi Andy.

    As I understand it, when one cell's voltage rises above the levels of the other cell's voltages, the Balance FET of that cell needs to switch on to drain the cell through the Balance Resistor. This never happens. In other words, If the voltage on channel 1 (vsense0 and vsense1) rise but the voltages on channel 2 to channel 11 stays the same, then at some stage EQ1 needs to switch on. 
    I only found 7.3.7 Passive Balancing in the datasheet regarding the setup. Is there anything else that can keep the EQx pins from pulling high? I am using a matched resistor voltage divider to simulate the cells and a floating power supply to simulate a cell that is "unbalanced". Is there a problem with this setup? I continuously read the cell voltage values and everything looks stable. All cells read 2.2v while cell 1 slowly ramps up from 2.2v to 4v. Yet EQ1 never switch on. Am I misunderstanding something?
    One more thing, my UV and OV levels are set to 1.5v and 2.8v respectively and successfully trigger fault warnings when 2.8V is exceeded, but the actual balancing of the cell never happens.
  • 0 in reply to Andre Hattingh
    TI__Intellectual 2520 points

    HI Andre,

    Cell balancing is manually controlled, it should start after you set CBENBL register,

    such as 92 00 14 FF FF F8 69.

    Another thing that you should pay attention to is below specs in DS.

    min cell voltage for balancing is 1.8V.

    Andy

  • 0 in reply to Andy Li55
    TI__Genius 10785 points

    HI Andre,

    Check Vgs of balancing FET spec.

    You don't expect to see big voltage drop. It's balancing current X impedance of your cable.

    If yo use resistor divider then you will see noticeable voltage changes.

    Roger  

  • 0 in reply to Roger Hwang
    Prodigy 30 points

    Hi Roger.

    Thanks a lot for the reply, this was indeed the problem. My cells are only 2.2v and the FETs I used had a higher Vgs value. 

    Kind regards,

    Andre

  • 0 in reply to Andre Hattingh
    TI__Genius 10785 points

    HI Andre,

    I am glad you found the issue.

    Thanks

    Roger