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BQ27426: Validation of some register setting values

xianzhen cao
Prodigy 100 points
Part Number: BQ27426

Dear engineers of TI:

1、Subclass ID：36， Offset：7

Why the default value is  set to 5℃?

What is the amount of change in RM and FCC when the temperature changes by 5°C?

I think that the smaller the setting value, the better the accuracy. What is the reason for not making it smaller?

2、Subclass ID：80，Offset：20

On what basis is Res V Drop=50mV set?

3、Subclass ID：80，Offset：68&70

What is the definition of delta voltage? (Is delta voltage the change in voltage when discharged from a non-discharged state?)

What is delta voltage used as a parameter for? (Is it a calculation parameter for discharge capacity?) 

What is the basis for setting Min Delta Voltage=0mV? 

What is the basis for setting Max Delta Voltage=200mV?

thanks!

  • 0
    TI__Guru 61946 points

    #1: Cell behavior is a function of temperature. The gauge will re-estimate FCC and RM if temperature changes by more than DODatEOC Delta T. 5deg.C is a threshold that was chosen to catch significant changes in FCC and RM when temperature changes. If you set this lower, then the gauge will run discharge simulations more often (than necessary), causing an increase in power consumption. If you set this higher, then the gauge will not adjust FCC and RM enough if temperature changes less than this limit.

    #2: The gauge calculates a voltage drop over the internal cell resistance. This voltage drop must exceed this Res V Drop threshold to qualify for a resistance measurement. This is a check point to ensure the quality of the resistance calculation. The basis for the 50mV is experimental data by TI during algorithm and analog front end development.

    #3: The gauge measures abrupt changes in voltage due to load spikes and records them. It will limit this measurement with these two parameters. Delta Voltage is then added to Terminate Voltage during discharge simulations to adjust capacity predictions for load spikes. 0mV is the default so that Terminate Voltage doesn't get adjusted regardless of load. 200mV was chosen to avoid that glitches cause the gauge to significantly underestimate capacity.

    The reason, why TI has many parameters that limit certain aspects of the algorithm, like min/max Delta Voltage or Res V Drop is that the algorithm must be robust in real life systems. It can't interpret spurious measurement errors or glitches without these limitations as this has the potential to corrupt the gauging algorithm. Most of these parameters are configured by default based on empirical data that was collected by TI during algorithm development and they don't need adjustments in many cases.