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Original question:

bq24123 charge status outputs

BQ24123: Stops charging after 20 minutes

David Baltz
Prodigy 80 points
Part Number: BQ24123

We have a new revision of a board with a BQ24123 that has worked fine in previous revisions.  In the latest revision, the GND trace to VSS was inadvertently left off so we have added a solder bridge to the contact to make it work.  What we are seeing is that the charger works fine for between 10 to 30 minutes, then just shuts off as if in a fault condition.  It does not start charging again if the battery voltage drops below the Vrch threshold.  The 12V input must be removed, then restored, to clear the fault.  I suspect the solder bridge to be the culprit but we have had at least 6 boards that have charged w/o problems from the same revision with a solder bridge. About a third of the boards in this batch have this same charging issue--shutting off after thirty minutes or less.

We are charging 2 cells (8.4V) at 1.5A, have a 4-hour charge timer, and are not doing battery temperature sensing.  The IC does not appear to be overheating (<110 °F). What else might be causing this.

  • 0
    TI__Guru**** 166416 points

    Hi David,

    I was unable to read your schematic.  Can you reload with higher resolution?

    What is the TTC pin voltage?  Is the precharge timer expiring?  If you short TTC to ground and/or toggle /CE does the board start working again?

    Regards,

    Jeff

  • 0 in reply to Jeff F
    Prodigy 80 points

    The TTC voltage is zero.  Pins 2-3 are jumpered on JP1.  I don't think it is timing out because STAT2 never goes ON.  /CE is tied to ground so I can't toggle it.

  • 0 in reply to David Baltz
    TI__Guru**** 166416 points

    David,

    Does your solder bridge ensure that the power pad is at the same GND potential as the PGND pins?  If your board layout allows ground return using both paths, I could see an issue where charge is slowly building up, creating a potential at a critical node.

    Regards,

    Jeff

  • 0 in reply to Jeff F
    Prodigy 80 points

    They both use the same GND polygon so I don't think that is the problem.

  • 0 in reply to David Baltz
    TI__Guru**** 166416 points

    David,

    Since the charge is timing out at different times, I suspect a floating node somewhere.  What is the state of STAT1 and STAT2 when this occurs?  It the IC is reporting charge complete but the battery is too low then the BAT pin has an issue.  If charge suspend, it could be TS pin.

    Regards,

    Jeff

  • 0 in reply to Jeff F
    Prodigy 80 points

    We have discovered the issue.  An incorrect value timer capacitor Cttc was installed on the boards causing a very short (24 minute) charge timeout.  Since the battery voltage at timeout was below the threshold voltage, a fault condition must have occurred, preventing the charger from restarting.  As long as a battery was nearly fully charged, the short timeout was sufficient to keep the battery within the Voreg and Vterm limits and not enter the fault condition.  Thanks for your help.  I better understand the TTC function now.