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BQ40Z80: BMS CELL BALANCING

Shubham Yadav
Intellectual 275 points
Part Number: BQ40Z80

Respected sir,

We have designed a bms with 6s configuration with MAX Bat voltage up to 25.2 V with important protection parameters configured are listed below:
COV : 4.2 V/cell
CUV : 1.8  V/cell
OCD : 2500 mA
OCC : 2000 mA

There is one problem I am accounting and I am confused and don't know how it works. Actually sir, I discharged my completely charged battery pack (all cells were balanced) while discharging there is cell 2 whose voltage come around 2.4 V while others cells were 3 to 3.2 V. I don't know how these happen if it discharge it should discharge in a balanced manner ( If I am correct ). Even CB was also not working to balance these cells, so I tried different ways so  to see in which condition CB flags gets set. while doing these process my cell 2 voltage becomes < charging voltage and PV flag sets i.e my bms goes in precharge condition. After these when I started charging my battery pack PCHG flag got set and it was charging with 72 mA till it overcome voltage threshold. What I noticed that while charging my battery pack CB flag got set and it remains set till my battery pack got completely charged and they all were balanced. 

Sir, i am attaching my schematic design, srec file as well as gg.csv file for your reference please review it and let me know if any changes required in that. Also, sir please guide me how I can learn these cell balancing concept in BMS as I can't understand it through technical reference manual and also when its parameter gets activated i.e during charging only or discharging or in both conditions.
Also, please specify is there any document which can explain about cell balancing and how its parameters behave in different conditions.

2437.BMS.pdfNEW_2.gg.csvGolden_Image_PDSG_set_PRECHG-3-26-2024.zip

Regards,
Shubham

  • 0
    TI__Mastermind 28450 points

    Hi Shubham,

    Shubham Yadav said:
    Actually sir, I discharged my completely charged battery pack (all cells were balanced) while discharging there is cell 2 whose voltage come around 2.4 V while others cells were 3 to 3.2 V. I don't know how these happen if it discharge it should discharge in a balanced manner ( If I am correct ).

    Actually, the cells typically will not discharge in a balanced manner since they can have different impedances that are learned over time. This is typically why the gauge will do calculations during relax to determine how to go about the balancing in either relax or charge mode.

    Shubham Yadav said:
    Even CB was also not working to balance these cells, so I tried different ways so  to see in which condition CB flags gets set. while doing these process my cell 2 voltage becomes < charging voltage and PV flag sets i.e my bms goes in precharge condition. After these when I started charging my battery pack PCHG flag got set and it was charging with 72 mA till it overcome voltage threshold. What I noticed that while charging my battery pack CB flag got set and it remains set till my battery pack got completely charged and they all were balanced. 

    If possible, can we please receive a log file of the discharge-relax-charge cycle so we can take a deeper look into this?

    Regards,

    Anthony Baldino

  • 0 in reply to Anthony
    Intellectual 275 points

    Dear Sir,

    Correct me I am wrong on below points as I am confused in these. As per my understanding

    1) Wake up mode : This is an active state of BMS in which it monitors battery parameters like voltage, currents and triggers protection as per it. In these mode, we are performing charging and discharging of battery.
    2) Sleep mode : It is the mode where BMS is active but in power saving mode just monitors battery parameters and charging and discharging is not happening in these mode.
    3) Shutdown mode : It is an inactive mode where bms is non-operational and not monitoring batteries. It should be wake up in order to perform protection operation on batteries.

    And sir, i dont know what is relax mode and how does bms goes in relax can you explain it with example.

    Also, as per our previous discussions I had attached below log file based on yesterday issue discussed. Please review it and let me know if some parameter changes are needed. There are separate logs for  battery discharging and battery charging .

    Waiting for your reply,
    BMS_CHARGING_LOG.logBMS_DISCHARGING_LOG.log

    Regards,
    Shubham

  • 0 in reply to Shubham Yadav
    TI__Mastermind 28450 points

    Hi Shubham,

    Shubham Yadav said:
    1) Wake up mode : This is an active state of BMS in which it monitors battery parameters like voltage, currents and triggers protection as per it. In these mode, we are performing charging and discharging of battery.
    2) Sleep mode : It is the mode where BMS is active but in power saving mode just monitors battery parameters and charging and discharging is not happening in these mode.
    3) Shutdown mode : It is an inactive mode where bms is non-operational and not monitoring batteries. It should be wake up in order to perform protection operation on batteries.

    This seems correct, Wake up mode is also known as NORMAL mode in many of our documents.

    Shubham Yadav said:
    And sir, i dont know what is relax mode and how does bms goes in relax can you explain it with example.

    I believe the image below can be helpful in explanation:

     

    Relax mode is a state where the cell is neither being charged or discharged, and is normally where the gauge takes a large sum of its measurements. To enter Relax mode from either Charge (where the current on the sense resistor is greater than the CHG current threshold) or Discharge mode (where the current on the sense resistor is less than the DSG current threshold), the current needs to be less than the Quit Current but greater than the -Quit Current seen in the range below:

    If the gauge is in Charge mode and is attempting to enter Relax, the current needs to stay in that range for at least the time depicted by the parameter CHG Relax Time. 

    If the gauge is in Discharge mode and is attempting to enter Relax, the current needs to stay in that range for at least the time depicted by the parameter DSG Relax Time.

    This depiction below can also help:

     

    Regards,

    Anthony Baldino