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Original question:

BQ76942: Potential bug with cell 1 voltage measurement

BQ76952: Cell 1 data issue occasionally while start of charging

Shyama Agrawal
Intellectual 860 points
Part Number: BQ76952
Other Parts Discussed in Thread: TIDA-010247

Hi,

We have been using BQ76952 at production level for our BMS for 1 year. Some field battery packs show sudden drop in Cell 1 voltage just at the start of charging or during balancing of Cell 1. The situation doesn't recreate consistently but we have seen similar behavior at lab level occasionally too. Our CUV is Microcontroller controlled, Shutdown voltage level is BQ76952 controlled. SUV is BQ76952 controlled.

Lab level behavior observed is mentioned below:

In a continuous test of 5 days. Once, when the charger turned ON and current starts to flow from 0A to 8A. Cell 1 voltage dips to 100mV and lower. Cell 1 voltage recovers immediately after charger turns OFF. Balancing is not active as observed in data log. After a while system went to SUV and we had to reset the BMS, since then the issue has not occurred again.

Please suggest the root cause and strategy to handle the same.

  • 0
    Intellectual 860 points

    Above is our schematic for cell 1. D53 is Zener diode of 5.1V. D27 is a Schottky diode.

  • 0 in reply to Shyama Agrawal
    TI__Mastermind 25556 points

    Hello Shyama,

    The schematic looks ok for cell-1. 

    The BQ76952 has a known behavior in Cell-1 Voltage Validation During SLEEP Mode. The explanation is documented in the datasheet in section 10.1.3

    If we go back to section 10.1.3 from datasheet (see below). The idea is to filter the bad Cell1() voltage measurement. This can be done at the MCU side where the host reads the measurements at intervals greater than the defined value in the Power:Sleep:Voltage Time. For instance, say Voltage Time is set to 5 second, this means the voltage measurement will be reported every 5 seconds. What you can do is at the MCU side somewhere in the code constantly monitor Cell 1() voltages and disregard the wrong measurement. 

    Regards,
    Jose Couso

  • 0 in reply to Jose Couso
    Intellectual 860 points

    Hi,

    Can the same occur when the system is not in sleep? We have observed Cell 1 data fluctuation while charging current of 30A is flowing. Please see the log section attached below:

    Cell 1 issue lab test.txt

  • 0 in reply to Shyama Agrawal
    TI__Mastermind 25556 points

    Hello Shyama,

    The Cell1() behavior only occurs during SLEEP mode. Please share the whole schematic.

    Have you referenced to the TIDA-010247 reference schematic? They showcase a Zener and a Schottky diode in Cell0. The Zener protects the pin from high voltages that could exceed abs max and the Schottky diode protects the pin from too low of voltages



    I recommend to look at TIDA-010247 if not already done.

    Regards,
    Jose Couso