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bq78PL114 battery lockout

Steve14663
Prodigy 170 points
Other Parts Discussed in Thread: BQ78PL114, BQ78PL116

We have discovered an interesting problem after manufacturing a number of battery packs using the bq78PL114 management controller. The problem is that if a battery pack has been fully charged, then left on the shelf for a couple of months, the cells discharge significantly due to self discharge and small standby currents and hence require charging prior to use. But as the battery pack has not yet been connected to any equipment, the "Fully Charged" bit in the battery status register is still set and the charge and pre-charge FET's are still open because no discharge current has been sensed. The only way we have found to get around this problem is to connect some form of equipment to the battery packs, to draw current from the pack and clear the "Fully Charged" bit prior to connecting a charger. This is fairly inconvenient for the end user, and as such any suggestions would be appreciated.

  • 0
    TI__Intellectual 1940 points

    Steve -

    Your description does accurately describe how the charge control rules and gas gauge operates for the typical configuration.  The Charge Control Rule that is at play here is Charge Completion.  It is described at the bottom of this entry.

    A few suggestions/options:

    1.    Do not change your configuration and continue to insert battery into load to cause a discharge current to reset CC flag.

    2.    Change the FET Activation Time to zero.  This will prevent the charge and pre-charge mosfets from opening when the CC point is reached.  Then, either the charger will stop charge or a safety rule will stop charge, ie COV or POV.  Or, the cells will just gracefully reach the charger voltage.

    3.    Do not charge your pack to full prior to placing on the shelf - maybe charge to 80%.  The CC flag will not set and CHG and PRE FETs will remain on.  The downside here is that their FET drives will draw more quiescent current.  We have a feature called System Present FET control that will turn off the CHG & PRE FETs when the SMBus connection is removed.  This would save current.  HOWEVER, we have found that this function as described in the documentation does not work.  Sorry about that.  It is being fixed in the next product release, called bq78PL116.

    Thanks for bringing your concern to our attention.  This undoubtedly will help other users.

    ***********************

    Charge Completion

    Associated parameters: Charge Completion Taper Current, Charge Completion Time and Charge Completion FET Activation Time.

    Activation criteria/behavior: If the pack voltage is at or above Charge Completion Pack Voltage Qualifier, and the charge current is below Charge Completion Taper Current but above Charge Completion Taper Current value divided by 2 for a time duration of Charge Completion Time, then the status flag for this

    condition is set, and the following actions are taken:

    (a) If FC Set SOC Threshold is –1, then SBS:BatteryStatus(0x16):[FC] FullyCharged status flag is set.

    (b) If TCA Set SOC Threshold is –1, then SBS:BatteryStatus(0x16):[TCA] Terminate Charge Alarm bit is set.

    (c) SBS:ChargingVoltage(0x15) is set to 0 after the FETs are opened.

    (d) SBS:ChargingCurrent(0x14) is set to 0 after the FETs are opened.

    (e) SBS:FullChargeCapacity(0x10) may be updated.

    (f) Additional internal capacity updates may be calculated.

    (g) SBS:RelativeSOC(0x0d) is set to 100%.

    (h) SBS:MaxError(0x0c) may be reset or reduced.

    (i) SBS:RemainingCapacity(0x0f) is updated.

    (j) SBS:AverageTimeToFull(0x13) is set to 0.

    (k) SBS:AbsoluteSOCl(0x0e) is updated.

    (l) Timer for Charge Completion FET Activation Time (if non-zero) is started.

    If the charge current does not drop below the Transition to Idle Current value for a non-zero time duration of Charge Completion FET Activation Time after the end of Charge Completion Time, then the following action is taken:

    (a) Charge and precharge MOSFETs are opened.

    Deactivation criteria/behavior: If a discharge current is observed, OR if the charge current drops below the Transition to Idle Current value before the Charge Completion FET Activation Time elapses, then the following actions are taken:

    (a) Status condition for this event is cleared.

    (b) Charge Completion FET Activation timer is reset.

    (c) Charge and precharge MOSFETs are closed.