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BQ78PL114 cell balancing

rohit devmore
Intellectual 635 points
Other Parts Discussed in Thread: BQ78PL114

i want to knw how actually cell balancing works in BQ78PL114, i read that document on power pump, but still i have doubts

1)suppose lower cell has voltage of 3.4v and upper has 3v.

then is lower cell going to charge upper cell and both will balance at 3.2v     OR     lower one stops  charging and and wait til upper comes to 3.4v

whats the operation, how cell balances, plese tell me numericallt with example

 

thanks and regards,

rohit.

  • 0
    TI__Guru 54715 points

    The power pump balancing can operate in charge, discharge, or idle. If it is operating during charge, charge current is not interrupted, but the pumping circuit is activated to transfer some energy to the lower voltage or state of charge cell.

    Capacity or state of charge is not linear with cell voltage.  If a cell is at 3.4V and another is at 3.0V, with terminal voltage balancing they will try to equalize the voltage, but it may be at 3.3V or some other voltage due to the differences in capacity - the cell at 3.4V may not loose much voltage with a little energy transfer out while the voltage on the 3.0V cell may rise dramatically with little energy transfer in.  This will depend on the cell chemistry and the particular cells.

  • 0 in reply to WM5295
    Intellectual 635 points

    3.4V cell looses the voltage,   this losing is nothing but charging of inductor.  m i right?

    and rise in 3V cell dramatically,  dramatically means, 3V charge is charging from the charge stored in the inductor , isn`t  it?

    pls reply

     

    thanks and regards,

    rohit

  • 0 in reply to rohit devmore
    TI__Intellectual 1430 points

    To understand power pump, it is better to think in terms of energy transfer.  

     

    If cell 1 has more energy (higher voltage, higher state of charge, etc) bq78PL114 will close one of the FETs.  This will start a current loop and store energy in the inductor. 

     

    Then by opening the FET the current loop will continue through the second FET body diode. 

     

    This will move the energy from the inductor to cell 2

     

    Be sure to read the application note on PowerPump Balancing

    http://focus.ti.com/lit/an/slua524/slua524.pdf