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Original question:

BQ27426: Issues when setting trace resistance

BQ27426: Right offset and Value for Trace Resistance

leinho
Expert 1230 points
Part Number: BQ27426
Other Parts Discussed in Thread: BQSTUDIO,

Hello,

Which is the right offset that should be used for Trace Resistance? 

I have tried with the two provided by the datasheet (75 and 92) but the default vaue was wrong. I got 0x023F when using 75 and 0x6300 when using 92. 

Regarding the value, the trace resistance in my device is 827 mOhm. Should I then set the register to 207 as explianed in this post (https://e2e.ti.com/support/power-management/f/196/t/616622

  • 0
    TI__Mastermind 24870 points
    Yes that's correct. Thanks for pointing it out to us.
  • 0 in reply to Kang Kang
    Expert 1230 points
    Thanks for your answer. Please, let me know how I should do as my traces have high resistance and the register looks promising to smooth the SOC jumps when the current changes.
  • 0 in reply to leinho
    Expert 1230 points
    Hello again,

    I have just checked with BQStudio and BQ27426EVM, that the default trace resistance value is 0 Ohm. Is it possible to tell the register offset from bqStudio?
  • 0 in reply to leinho
    TI__Mastermind 24870 points
    This information is on our TRM. Search for trace resistance and you will find a table with the offset.
  • 0 in reply to Kang Kang
    Expert 1230 points

    Do you mean SLUUBB0 (Dec 2015)?

    There is contradictory information there as you acknowledged in your first response.

    Could you please be more specific? 

  • 0 in reply to Kang Kang
    Expert 1230 points
    Ok, we are referring to the same file. Please, look at page 34 and page 44. According to page 34, the offset would be 75. But according to page 44, it would 92. From reset state, I get the following value using each offset: 0x023F when using 75 and 0x6300 when using 92. The reset state for trace resistance should be 0.

    Could you please tell me which offset to use? I believe it should be another one.

    PS: I also checked with BQStudio and the reset value of the register is 0 but I can not tell the offset from bqStudio.
  • 0 in reply to leinho
    TI__Guru 60826 points

    Trace Resistance is in subclass 0x50, block 2, offset 11. Each block is 32-bytes long so this equals to a total offset of 2 * 32 + 11 = 75.

    The read sequence in bqStudio, advanced comm tab, is:

    I2C Address = AA
    Start Register = 3E
    Bytes to Write = 50 02 // subclass 0x50, block 2

    -> click "write"

    Start Register = 4B //B = offset 11
    Number of Bytes to Read = 2

    -> click "read"

    --> Data = 00 00 (because trace resistance is 0)

    Another comment about trace resistance: The bq27426 divides all impedance values by 0.256 * DesignCapacity / DefaultDesignCapacity so you need to scale them accordingly, including trace resistance.

    So if your trace resistance is 1000mOhm, the value that you need to write to Trace Resistance = 1000 * 0.256 * DesignCapacity / DefaultDesignCapacity.

    DefaultDesignCapacity is in subclass 0x6D, block 0, offset 0, 2-byte unsigned integer. It will be different for each selected chemistry so this needs to be read after selecting a chemistry.

  • 0 in reply to Dominik Hartl11
    Expert 1230 points
    Ok, thanks for the long explanation. I was doing the following in my code and that was because I was forgetting that the block size is 32 bytes:

    //Setup for Block RAM Update
    if(batteryI2CWrite(0x61, 0x00)!= HAL_OK){
    break;
    }
    if(batteryI2CWrite(0x3E, 0x50)!= HAL_OK){
    break;
    }
    if(batteryI2CWrite(0x3F, 0x00)!= HAL_OK){
    break;
    }

    I need to write then 0x02 to 0x3F to get the right value. I will check this again with other registers that have offset higher than 32 in the table.

    As I will use always the same battery, I will not have to read the default design capacity in the code. I will read it using either bqStudio or when doing testing. It looks like it should not change.
  • 0 in reply to leinho
    Expert 1230 points
    Related to the Trace Resistance, should it also improve the value of the voltage measured by the gauge or just the calculation of remaining capacity?
    When there is a current of around 150 mA, the gauge reads 4184 mV when the battery is actually 4084 mV.

    Thanks.
  • 0 in reply to leinho
    TI__Guru* 76230 points
    voltage calibration is what will improve the voltage measurement accuracy and not the trace resistance.. that is used for calculation of rem cap .