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BQ40Z50: Clarification on Charge Term Taper Current

Part Number: BQ40Z50
Other Parts Discussed in Thread: BQ25798,

Tool/software:

We have designed a 2S0P battery pack around the BQ40Z50-R5 gauge and a BQ25798 charger on top of Samsung INR-18650-29E cells.

Reading the available technical reference and application notes we've determined the following settings would be required for the learning cycle of the selected cell.  These settings would generate the golden file and then be altered for the specific application.

ChemId: 1737 (INR18650-29E LiMn2O4 (Co, Ni)/carbon, 4.2v) No for Support Turbo Mode.
Cell Count: 2
Design Capacity: 7.3v 2850mAh 2080.5cWh
Charge Term Taper Current: 250mA (Default)
Dsg Current Threshold: 100mA (Default)
Chg Current Threshold: 1375mA
Chg Relax Time: 60s (Default)
Dsg Relax Time: 1s (Default)
Quit Current: 75mA
Term Voltage: 5000mV

However, we have lingering doubts about the appropriate Charge Term Taper Current value.  Please could you offer any assistance in selecting the appropriate value, is it purely driven by the BQ25798 charger?

  • Hey Ben, 

    Yes, the Charge Term Taper Current value is dependent on the taper rate of the charger in constant voltage charge mode. You will want to set the Charge Term Taper Current for the gauge to be 10% higher than the taper rate of the charger to ensure that the gauge detects valid charge termination. 

    Regards, 

    Jonny.

  • Hello Jonny, thanks for the quick reply.

    Perhaps a gap in our knowledge, but we don't seem to be able to determine what this value should be based on the BQ25798 datasheet.

  • Hey Ben, 

    I am looping in the Chargers team to help here. 

    Regards, 

    Jonny. 

  • Hello Ben,

    I'm a confused on the question.

    The BQ25798 does not have a method for choosing the taper current rate, because this is not a selectable value.

    Best Regards,

    Christian

  • Christian,

    It is stated in slua903 section 2.2 Charge Term Taper Current:

    "Most battery chargers have a ±10% error in taper current threshold at which point the charger cuts off charging. It is important to set the taper current programmed in the data flash of the gauge slightly higher than the taper current threshold of the charger. This ensures that the gauge detects the battery is fully charged before the charger cuts off charge. For example, if the charger taper current is 50 mA, it is recommended to set the charge termination taper current in data flash greater than 50 mA."

    This sent us off course attempting to determine how we set the Taper Current in the BQ25798 register map.

    We understand that the Taper Current is set in the BQ40Z50 configuration as the current to be supplied during the Constant Voltage mode of CC-CV charge cycle.

    For our selected cell (Samsung INR18650-29E) it stated a "Standard Charge" means charging the cell with a charge current of 1,375mA and constant voltage 4.2V at 25°C, 0.02C cutoff.

    Therefore, the charge Term Taper Current is 0.02 * 2850 (2s0p) = 57mA.

    This value falls in the range suggested in sula903:

    "A good value to use is 70 mA. Also, it is recommended that the taper current be between C/100 and C/10 to ensure that the battery gets properly fully charged"

    You have stated there is no method of setting this value in the charger; therefore, we can do nothing for the suggested configuration stated in slua903 section 2.2 Charger Term Taper Current.

    Thank you for your time.

  • / was our interpretation correct?  I'm looking to move this ticket to resolved.

  • Hello Ben,

    Based on your description, I think you are refering to termination current. During CV mode when the charge current falls to a certain a current limit the device stops charging.

    If this is the case, This value can be changed in REG0x09[4-0]. 

    Best Regards,

    Christian.