• State TI Thinks Resolved
  • Locked Locked
  • Replies 3 replies
  • Answers 1 answer
  • Subscribers 63 subscribers
  • Views 824 views
  • Users 0 members are here
Support feedback
Options
Options
Related

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

BQ77PL900: Balancing Issue in Standalone mode

Vaibhav Dhotre
Prodigy 20 points
Part Number: BQ77PL900

We are using BQ77PL900 for 7 cell Li-Ion battery management but facing some issues of cell balancing in standalone mode.

Configuration Settings are as follows:

No of Series cells – 7 Nos

Over voltage Detection – 4.25V (Same as balancing start voltage as per datasheet)

Over voltage Hysteresis – 0.1V

Under Voltage Detection – 2.5V

Under voltage Hysteresis – 0.4mV

External current path (FETs and 56 Ohm resistor) with 10K series resistor used for balancing

 

Observations while Charging the Battery Pack in Standalone mode-

-        When one of the cell reaches to 4.25V (OV threshold), CHG FET is turned OFF.  

-        Balancing gets start as per the internal algorithm.

-        Balancing are observed on all cells (Even on cells which has not reached to OV threshold),

“SET by SET” as per the algorithm described in document SLUA463.

-        Balancing does not end on certain cell even if cell voltage drops to recovery voltage level 4.15V (4.25-0.1, OVth-Hys). It is still going on till last cell voltage in that Balancing SET drops to 4.15V. Result is, further voltage drop on the cell which has already reached to 4.15V.

-        It happens in all Balancing SETs.

-        At the end of balancing, got different voltage readings (Min 4.11, Max 4.15) on all cells in the battery pack. There is 40mV difference observed on different cells.

-        CHG FET is turned ON at the end of balancing process.

 

We need to know:

-        Are above observations matches with the functionality of BQ77PL900?

-        What is tolerance voltage limits of balanced cells?  Which is not specified in datasheet.  (40mV in above observations)

  • 0
    TI__Guru 54715 points
    Hi Vaibhav,
    The expected behavior from table 1 of the application note slua463 is that only the cells above OV would balance, and figure 19 of the data sheet implies the cell which has not reached OV would not balance .
    The part does not have a specific tolerance for balanced cells. Balancing is determined by the OV threshold which has a +/- 50 mV tolerance over temperature. Since balancing will run to the hysteresis range, the adjustments to a cell could be large although some of the voltage change would be due to voltage relaxation when the charge current stopped, and the result will also depend on the entry condition. If a device had a 0 tolerance and a cell was 50 mV high going into OV, if that cell were balanced down to the 100mV hysteresis threshold and none of the voltages changed from the stop of the charge current, the difference between the cells would still be 50 mV after balancing, but in the opposite direction.
    The 10k input resistors are large, and while shown in the application note they will lead to a long settling time dependent on the input capacitors.
  • 0 in reply to WM5295
    Prodigy 20 points
    Hi WM5295,
    Thanks for your attention and valuable reply.
    We carried out one more test attempt as follows
    - Initial battery Open Circuit Voltage at 27.3V.
    (Confirmed approx. 3.9V per cell for 7cell pack with all cells 10mV margin)
    - Charged only Cell 1 to 4.0V (To simulate a known unbalance condition)
    - Installed the BMS circuit (Bq77PL900) on battery having the same configuration settings described previously.
    - Started charging the battery pack at C/20 rate.
    - Cell1 OV (4.25V) detected first as expected, All other cell voltages are below 4.15V (4.25V – 0.1V Hysteresis) CHG FET is OFF, Balancing started only for Cell1. CHG FET turned ON after Cell1 voltage reached 4.132V, all other cell voltages were approx. 4.11V
    - Charging continued further, this time also Cell1 OV detected first as expected, all other cell voltages were approx. 4.17V. CHG FET turned OFF. Balancing started for all cells “SET by SET” as per internal algorithm. (Even if all other cell voltages had not detected OV, but above 4.15V level)
    - After few hours all cell voltages were in the range of 4.128V to 4.146V, CHG FET is tuned ON again.

    Our Conclusion
    - Balancing process started when one of the cell detected for OV
    - Balancing happens on all those cells which are above 4.15V (OV- Hysteresis)

    We need to know
    - Is chip software has been updated and we using outdated part? (“bq 22C7LRK G4” printed on chip, purchased in 2013)
    - Kindly suggest us the settings for our requirement (4.25V OV detection and 4.15V balancing release threshold)
  • 0 in reply to Vaibhav Dhotre
    TI__Guru 54715 points
    Hi Vaibhav,
    Your conclusion looks reasonable although it was not the way I first read the documents. Balancing all cells down to the OV-Hysteresis would provide the best match of cell voltages.
    The chip does not have software and has not been updated. All devices should function the same.
    For your desired settings you would want to set OV for 4.25V and OVH for 0.1V.