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BQ78PL114, cell differential for balancing too tight for LIFEPO4 ?

Mathieu Lafrance
Intellectual 365 points
Other Parts Discussed in Thread: BQ78PL114

Hi,

 

 

I have a pack with consists of 6x LIFEPO4 cells in parralel, 4 in series.

 

I have some problems with some packs regarding their Cell over-voltage and Cell under-voltage caracteristics. Some packs exhibit the same behavior during charge or discharge; they have a lot of trouble getting to the fully charged state (or the fully discharged state) because there is often a safety condition that temporarely disables the pack.

 

I charge the cells at 3.65V, they have a nominal voltage of 3.3V and end of discharge voltage (per cell) is 2.9V. Under voltage is at 2.7V and over voltage is at 3.9V

Configuration file : 6406.Z120.zip

Here is a discharge profile of the cells we're using (We cutoff at 2.9V, not at 1.8V as per the graph, because the chip will shutdown at that voltage)


You will notice the extremely flat discharge voltage between 10 and 90%. We have set the default cell differential for balacing to 10mV, but I believe this is too tight.

 

I have noticed that some BQ78PL114 chips have some offset on their ADC for voltage measurements. I have seen chips having offsets as high as 8mV per cell VS reality.

For example, I have a pack here with a measured error for the voltages:

cell1 => +7mV

cell2 => +8mV

cell3 => 0mV

cell4 => -1mV

 

At approx 50% SOC, I believe the chip is wrongly balacing the cells because it sees a differential higher than 10mV, effectively debalancing the cells for quite some capacity since the voltage doesn't change much for a given change in capacity.

 

Is the 10mV default too tight for the precision of the BQ78PL114 with LIFEPO4 cells ? At what differential should it be set for correct operation ?

  • 0
    TI__Intellectual 1505 points

    Hi Mathieu,

    The 10mV differential is a TI recommended and default value for the cell balance to be active.

    Note: If you are using the PL114, the Voltage can be calibrated to attain a more accurate measurement.

    You have the option to change the Cell Balanceing threshold if that will improve your system perfromance.  There is also the option to turn-off balance during discharge.

    The PL114/ PL116 provides the flexibility for the user to make adjustments that best optimizes their system.

    More infor mation on Balancing can be found here: http://focus.ti.com/general/docs/lit/getliterature.tsp?literatureNumber=slua524a&fileType=pdf

    Regards,

    Ben 

  • 0 in reply to Benjamin Sarpong
    Intellectual 365 points

    I already attempted to calibrate the voltage myself, and I was never able to get a better voltage calibration pack after pack compared to the default calibration. My concern is that I observe offsets that are outside PL114 specifications.

     

    I tried changing the cell balancing threshold to 25mV and to my comprehension this will not adversely affect performance much. My question is the following: have you ever had to increase the cell balancing threshold in order for LiFEPO4 cell to function correctly without being always under-voltage or over-voltage at end of charge/discharge?

     

    LIFEPO4 cell have a -very very- flat OCV curve. I believe that the cells are wrongly balanced when they are at the flattest point, caused by the little off-spec voltage offset.

  • 0 in reply to Mathieu Lafrance
    Intellectual 515 points

    Benjamin,

    Any update on Mathieu update?

    Thanks,

    Alex

  • 0 in reply to Alex Marcoux
    TI__Intellectual 1505 points

    Hi Mathieu

    To your Question:

    My question is the following: have you ever had to increase the cell balancing threshold in order for LiFEPO4 cell to function correctly without being always under-voltage or over-voltage at end of charge/discharge?

    The answer is no -- You have to ensure you have the correct ChemID  so that the PL116/PL114 can balance successfully based on the OCV. Check : http://www.ti.com/lit/an/slua524a/slua524a.pdf (Page 6) and TRM (http://www.ti.com/lit/ug/sluu481/sluu481.pdf) page 60

    An understanding of the the characteristics of your pack including the appropriate taper currents to ensure that all the cells are not Overvoltage at the end of charge. The cells manufacturer normally has recommendations on the ideal taper currents for the cells. However, for your pack you will have to study the characteristic and optimize the PL116 parameters. The same applies to the discharge as well. This is nornally controlled with the settiongs of your CUV.

    Regards,

    Ben