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Secondary discharge overcurrent protection if gas gauge controlled FET is shorted?

Joe LeKostaj
Prodigy 140 points

I have a 7.2V 6.6ah 2S2P battery pack design using a bq20z75 gas gauge IC. To meet a provision of the UL 2054 battery safety standard, the discharge overcurrent trip threshold must be no greater than 8.0 amps. The difficulty is that the battery must do this even in a faulted condition, such as when the discharge FET is intentionally shorted. Other than adding another gas gauge IC (and discharge FET) solely for secondary current monitoring, is there any other T.I. monitor/protection IC that can provide the discharge overcurrent detect function with the same degree of accuracy as the gas gauge? Note, the host product's current drain can go up to 8A, so there is no margin for the overcurrent detection threshold to be below 8.0A.

  • 0
    TI__Guru 54715 points
    Since the gauge is calibrated at board or pack production, it will or can be more accurate than a protector which is calibrated at IC production and then subject to tolerances at the board as well as its own. Depending on your gauge it may have a capability for a secondary protection to activate a fuse if it determines that the FET has failed. Check the features of your gauge and setup parameters. I will direct the post to the gauge forum where there are experts on that part should you have additional questions.
  • 0 in reply to WM5295
    Prodigy 140 points

    That confirms my belief that none of the protector ICs has as accurate an overcurrent threshold as does the gas gauge IC that is calibrated at board production.


    I don't wish to use the gas gauge's fuse output because:

    1) my intention is not to blow a fuse, just to control a redundant discharge FET that can turn back on after the overcurrent condition is gone.

    2) my application needs a secondary overcurrent protection that doesn't rely on the gauge's current sensing resistor (as the fuse output does) in case the fault condition* is a short across that resistor.

    * UL 2054 Limited Power Source test is performed with a single simulated fault in the active protection circuitry; this may take the form of a short across the sensing resistor.

    Presently I can find no monitor/protector IC that meets the precision of the gas gauge's current monitoring. Adding a second gauge IC solely for overcurrent monitoring is not feasible due to PCB space constraints and the cost.