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BQ78350: Coulomb Counter Deadband

Phil Reid
Prodigy 210 points
Part Number: BQ78350
Other Parts Discussed in Thread: BQ76930

We have just tried to get some batteries thru UN38.3 testing.

Unfortunately 6/8 have failed with either a CFET or DFET failure.

The batteries use the PRES pin and this was open during the test, no conenction to the battery terminals.

The black box has recorded a Permanent Failure with either a +/-5 mA current.

The design uses a 15mOhm sense resistor, with the default PF of 5mA on the CFET / DFET.

Deadbands are at defaults, with 3mA and the Coulomb Counter Deadband at 38 (x 264nV units).

The 5mA threshold seems to be well withing the +/-3LSB offset error for the BQ76930 with 15mOhm.

I don't fully understand the Coulomb Counter Deadband setting and how this might factors into things.

Is there any more details on this?

I've just cycled a battery packs from -20 to 70 and the current has remained stable at 0mA for the entire cycle.

Anyone able to offer some advice?

Here's a dump from one of the failed units.

220509.gg.csv

  • 0
    TI__Guru* 81034 points

    Hi Phil,

    The PF Status shows that the Current was measuring 5mA at the time of the fault (see PF Status: Current in the .gg.csv file). The optimum setting for the Coulomb Counter Deadband is 9 (determined from characterization), but I do not think this is causing your issue. I think the Current Deadband would impact this fault, but not the Coulomb Counter Deadband. Your sense resistor is 15mOhms which is larger than many applications, so for a smaller amount of noise, current will be measured. You could increase the CFETF and DFETF current thresholds if you think these are falsely triggered. 

    Regards,

    Matt