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BQ77PL900: Cell Balancing Not Being Performed

John Rossi
Prodigy 30 points
Part Number: BQ77PL900

Hello,

I am having trouble balancing a 6S LiPo battery using the bq77pl900 EVM. I was able to complete several charge cycles, but wanted to see evidence of the cell balancing algorithm taking over once the AFE gives an OV warning. So far I am not able to see the cell balance FETs turning on/off at all. The CBEN bit in the OCD_CONFIG register is in fact set to 1. The IC is being used in stand-alone mode for charging. I have the OV threshold set to 4.2V with OVHys set to 100mV.

For my latest test, I discharged the battery to about 3.83V per cell, and discharged one cell (Cell 6) even further to 3.7V, in an attempt to verify the cell balancing functionality. I have the bq77pl900 EVM plugged into the EV2300 module and am viewing the register output on a PC. During the charge cycle, the AFE triggered OV towards the end of the charge but it was not evident that any cell balancing was occurring. After leaving the battery plugged into the EVM for a few hours, the total battery voltage as well as the individual cell voltages still remained the same as they were when the OV warning was shown. Also, during all of my testing I have not seen the cell voltages actually reach 4.2V before showing OV. I figured this could just be a voltage measurement calibration issue. 

My ending cell voltages are as follows:

- Cell 1: 4.12 V

- Cell 2: 4.11 V

- Cell 3: 4.11 V

- Cell 4: 4.10 V

- Cell 5: 4.12 V

- Cell 6: 3.83 V

As per the datasheet, the bq77pl900 should enable the respective discharge FET(s) once any of the cells reach the OV threshold and another cell is at a greater difference than 100mV.

Is there any reason that am I not seeing this action being performed? 

Thanks

Hello,

I am having trouble balancing a 6S LiPo battery using the bq77pl900 EVM. I was able to complete several charge cycles, but wanted to see evidence of the cell balancing algorithm taking over once the AFE gives an OV warning. So far I am not able to see the cell balance FETs turning on/off at all. The CBEN bit in the OCD_CONFIG register is in fact set to 1. The IC is being used in stand-alone mode for charging. I have the OV threshold set to 4.2V with OVHys set to 100mV.

For my latest test, I discharged the battery to about 3.83V per cell, and discharged one cell (Cell 6) even further to 3.7V, in an attempt to verify the cell balancing functionality. I have the bq77pl900 EVM plugged into the EV2300 module and am viewing the register output on a PC. During the charge cycle, the AFE triggered OV towards the end of the charge but it was not evident that any cell balancing was occurring. After leaving the battery plugged into the EVM for a few hours, the total battery voltage as well as the individual cell voltages still remained the same as they were when the OV warning was shown.

My ending cell voltages are as follows:

- Cell 1: 

- Cell 2: 

- Cell 3: 

- Cell 4: 

- Cell 5: 

- Cell 6: 

As per the datasheet, the bq77pl900 should enable the respective discharge FET(s) once any of the cells reach the OV threshold and another cell is at a greater difference than 100mV.

Is there any reason that am I not seeing this action being performed? 

 

  • 0
    TI__Guru 54715 points
    Hi John,
    You are correct, if it is in standalone, CB_EN is set and it has gone to OV it should cell balance until the cell voltages have recovered through the OV_hysteresis level and the OV status is cleared. The bq77PL900 does not need to have a difference between the cells, it will balance all cells in the OV hysteresis region.
    The cell voltages shown look like some should still be in the OV hysteresis region, so they should balance, but as you indicate there can be calibration or offsets for an exact value.
    Confirm the EEPROM settings as you indicate.
    Be sure the part is in OV mode. You can read the status with the register.
    Be sure host mode is not being set.
    Observe the cell voltages at the charge to OV. If the cells drop through the OV hysteresis threshold when the charge current stops there will not be any significant balancing. In addition to the register bits you can watch the charge FET gate with a scope as well as a cell voltage and a VCn pin to see if balancing occurs. If multiple cells are to be balanced you may need to find the one being balanced since the part will not balance adjacent or every other cells.