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BQ25792: Unable to recover a 3s battery pack in over discharge protection.

David Wickliff
Prodigy 20 points
Part Number: BQ25792

Hello,

We have a BQ25792 design utilizing a 3s battery pack.  The pack has an internal over discharge protection that gets triggered when the pack is discharged down to ~8.1V.   The pack’s protection is released/reset when the applied voltage to the pack reaches ~9.0V. 

With the BQ25792, we have been unable to “recover” a pack that is in over discharge protection.  When a charging source is reapplied to the design, the BQ25792 stays in the Trickle charge state.  The BAT node measures ~7.5V during this Trickle charging state, reg 0x1D reports “Vbat NOT present” and the pack stays in its over discharge protection state.  There is a periodic, every ~1.4 seconds, drop to 0V for ~1.4ms on the battery node.  I assume this is the BQ25792 sampling the actual pack voltage.

How can the BQ25792 be used to recover a battery pack in such an over discharge protection state?

We are also seeing this same inability to recover the battery pack on a BQ25792 EVM we have.

Our BQ25792 design seems to be otherwise performing as expected. 

Thanks,

Dave

  • 0
    TI__Guru**** 155606 points

    Hi Dave,

    Can you provide the top markings of both ICs that cannot recover from trickle charge?  We initially (over 2 years ago) released some ICs that were incorrectly trimmed and could not escape trickle charge.  I thought all of those had been purged from inventory by now but it appears not.  Assuming that is the issue, which I should be able to confirm with the top markings, you can return those ICs for replacement.

    Regards,

    Jeff

  • 0 in reply to Jeff F
    Prodigy 20 points

    Jeff,

    Here are two photos: first our design and second our EVM board.  I had a little more difficulty getting good photo of the EVM board.  Let me know if they are legible enough.

    Thanks,

    Dave

     

  • 0 in reply to David Wickliff
    TI__Guru**** 155606 points

    Thanks. I have submitted to my test manager to confirm whether these ICs were incorrectly trimmed.  Should hear back tomorrow at the latest.

    Regards,

    Jeff

  • 0 in reply to Jeff F
    TI__Guru**** 155606 points

    Hi David,

    288AS2K was screened and should not be failing.  198A068 is from an old lot and is part of the 10% that could fail. Can you confirm the BAT voltage (light blue) and BAT current (dark blue) if possible looks similar to the waveforms in the oscope plot below:

    If so, then we need to start a return so I get those ICs back from you and get you new ICs.  Did you buy from the TI website or a distributor?

    Regards,

    Jeff

  • 0 in reply to Jeff F
    Prodigy 20 points

    Jeff,

    Ok, we'll capture some scope views in this area. 

    I believe the BQ25792 EVM with the oldest date code was purchased via TI website.  I need to check with our contract manufacture where the BQ25792's assembled into our product (the later date code) were purchased.

    Dave

  • 0 in reply to David Wickliff
    Prodigy 20 points

    Jeff,
    We have been working other items.   But recently we have captured some scope plots around our inability to recover/reset a battery pack in over-discharge protection state.
    When we are in the over-discharge protection state, here some scope plots of battery voltage and battery current.     VBAT is generally a constant ~7.75V with a drop to 0V every ~1.4 seconds.  This drop to 0V lasts ~1.4ms.  Note that the battery current (IBAT) is always 0 which is consistent with the battery pack’s protection being active (pack’s protection FETs turned off).
    Although the BQ25792 charge state is reported as Trickle charge state, these plots don’t look similar to the scope plot you shared earlier.  
    I believe that we need apply ~9.0V to the battery pack before the protection logic releases/resets its protection (i.e. the pack’s protection FETs turn back on).  How can we get the BQ25792 to apply >9.0V for this protection reset?
    Thanks,
    Dave

  • 0 in reply to David Wickliff
    TI__Guru**** 155606 points

    Hi David,

    The plots show me that your ICs do not have the trickle charge issue because the battery voltage is higher than 2.5V.  Unfortunately, the behavior above is expected from the BQ25792 when the pack protector opens or there is no battery attached and charge is enabled.  Trickle charge with V(BAT)=~#cells*2.5V only stays on for 1.5s then turns off.  To increase the trickle charge voltage, the host processer can increase the cell count but this is a bit risky since that sets the BATREG higher than your pack needs.  The host can set BATREG to minimum allowed per the higher cell count to minimize the risk.

    Regards,

    Jeff