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Original question:

BQ40Z60: charging at full current until over COV fault, and not transitioning to taper current charge

WEBENCH® Tools/BQ40Z60: Overcharging until COV fault is logged

tybo1mos
Prodigy 175 points
Part Number: BQ40Z60

Tool/software: WEBENCH® Design Tools

I'm having troubles with the chip charging the battery back in a 4S configuration at full current until the COV flag is set.  Was reading through the application report slua750.pdf, and it seems I should be setting some register values to compensate for this.  I currently values of R1=332k and R2=26.1k, but when I use these values as done in the application report, I end up with a step value of more than 256.

Looking for some help to get the register values I have (attached) set up correctly so that the pack reaches Constant Voltage vs overcharging the cells.

4-21-18.gg.zip

  • 0
    TI__Prodigy 680 points
    A BMS expert has been assigned to your question.
    Regards,
    Bethlehem
  • 0
    TI__Mastermind 42535 points
    What is your max charge voltage per cell? Please use this to perform calculations.
  • 0 in reply to Batt
    Prodigy 175 points

    I have used 4.2v in my calculations, but the numbers still don't come out correct, and i'm having difficulty applying the example in the reference.

    Here is what I have so far with R1=332K, R2=26.1K:  

    Maximum output voltage = 1220 mV × (1 + R1 / R2)   = 1220 x (1 + 332/26.1) = 16738mV

    Cell configuration: 4S

    Desired Charging Voltage: 16800 mV (4200 mV per cell)

    Minimum Output Voltage = 610 mV × (1 + R1 / R2) =  610 x (1 + 332/26.1) = 8369mV

    Voltage Resolution = (610 mV × (1 + R1 / R2)) / 256. = (610 x (1 + 332/26.1)) / 256 = 32.69mV,   Assume a Voltage resolution of 32 for the register entry to be conservative.

    Charging Voltage = Minimum Output Voltage + Voltage Resolution × Voltage Register 

    16800  = 8369 + 32 x Voltage Register

    Voltage register = (16800 - 8369) / 32  

    Voltage register = 263

    Per sluua04C, section 4.15.3, the max value for this register is 255.

    From this point, i't not clear to me what values I should be changing or reducing so the charger obtains the charging voltage i'm looking for, since the calculation above obtained a register value (step value) higher than 255.

  • 0 in reply to tybo1mos
    TI__Mastermind 42535 points

    I'm working on the calculations, I'll get back to you on Monday.

  • 0 in reply to tybo1mos
    TI__Mastermind 42535 points

    Please use the attached file. Your parameters are in yellow.40z60 v reg calculations.xlsx