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BQ77915: CHG drive output being disabled erroneously?

Phil Wise
Prodigy 190 points
Part Number: BQ77915
Other Parts Discussed in Thread: BQ24610, , BQ76925

I'm actually using the BQ7791501 version of this part in a 5S configuration.

This particular chip offers and OV setting of 4.25V.  For my 5S configuration, I am charging them up to 4.15V (20.75V for the whole pack) using a BQ24610 based circuit.  My problem is that once the pack gets fully charged, I can remove it from the charger and find that the CHG pin has gone low (by the action of removing it from the charger).  Connecting a 1A resistive load to the battery restores the CHG pin high but upon removing the load, the CHG pin goes low again.  If I allow the battery to drain to approx. 20.5 or lower, this problem is not present.  The main problem this presents is that if I remove then reinsert the battery into the charger, the charger faults outs thinking that no battery is connected.

For now, I'm assuming it is an OV fault since I have no way to absolutely verify none of the other CHG disable qualifiers are taking place (OT, UT, etc.).  My OC set points are quite high (Rsense is 1.5milliohm) so I can't see it being that.  I've tried capturing an overvoltage event with an oscope connected to the battery but I'm not seeing anything come that close to 4.25V (21.25/pack).  The cells are balanced to within 10 mV.  I have numerous 5S packs using several 18650 cell mfr's (sony, samsung, LG, Molicel) and they are all doing the same thing.  It's as if there is some "glitch" taking place when I remove the load that I haven't been able to capture yet.

Thoughts?

1804-310-401_Sht1.pdf

  • 0
    TI__Guru 54715 points
    Hi Phil,
    It looks like the battery could support the hibernate mode of the bq77915. If the part hibernates when removed from the charger, it will por on re-insertion and as shown in section 9.3.2.1 "The device assumes an OV fault after reset, and clears automatically after OV delay if all cell voltages are below the OV threshold minus hysteresis." So although the cells never reached OV, it will behave as if it had.
  • 0 in reply to WM5295
    Prodigy 190 points

    WM5295,

    Thanks for the response.

    My setup uses the hibernate feature but in my problematic scenario, I don't believe that is causing the problem.  I narrowed being able to reproduce the problem to when I have already removed the battery from the charger. 

    1.  Battery charges up to 20.7V (4.15/cell) and my charger's status indicates a full charge and the BQ77915's CHG drive is high.  All is good.

    2.  I remove the battery and the chip goes into hibernate, CHG drive goes low.  This is expected.

    3.  Attach a test connector to battery pack which has a jumper on it that disables hibernate mode and also contains leads to connect battery pack up to my load.  CHG drive is still low.  This is expected.

    4.  Pull load on battery (~ 1A).  Battery discharges fine.  CHG drive goes high (any fault it thought it had was reset by pulling load?).  This is expected.

    5.  Turn off load but do not allow battery pack/chip to re-enter hibernate mode (left hibernate disable jumper intact), CHG drive goes low.  This is NOT expected.

    6.  Repeat 4 & 5 above.

    I put a scope on the PRES (hibernate) pin during steps 4 & 5 above and saw no low going glitches.  I have it pulled up to pack voltage through a 10K resistor.  I even added a 10uF cap to the pin to help hold it.  No help.

    The crazy thing is that if I let the pack decay to, say, 20.5V, this problem never happens.  

    I'm still puzzled.  Help?

  • 0 in reply to Phil Wise
    TI__Guru 54715 points
    Hi Phil,
    When it comes out of hibernate in step 3, it assumes OV mode since it does not know where the battery has been (9.3.2.1).
    In step 4, the battery is not recovering from the OV, it is just turning on the CHG for body diode protection. See the bq76925 data sheet table 5 for a fault and recovery summary. OV recovery for bq7791501 occurs at 4.25 V - 200 mV or 4.05 V nominal. For 5 cells 20.25 V.
  • 0 in reply to WM5295
    Prodigy 190 points
    WM5295,

    Hmmm, you are right. I see that what I was trying to do to shortcut the reproduction of the problem was actually adding to my problem.

    The key points that your response(s) have brought home is:

    1. Toggling hibernate mode will reset the chip.
    2. If the pack voltage is not < Vov - Vhyst (4.05V/cell or 20.25V/pack in my case), the chip will be stuck in OV mode after exiting hibernate or reset mode.

    In our bench setup, we kept getting hung up because we would charge the pack, remove it, and later reinstall it expecting the charger to kick on again, if for no other reason that to give us a fully charged status indication. In reality, our product is configured to expect the battery to be plugged up to the charger board all the time (it's a UPS type system), so the hibernate signal usually never gets asserted in the field, so this should never be a problem. We were own worst enemy with this bench testing.

    In applications where the battery is taken on and off a charger, and the hibernate feature is implemented (for safety reasons), I could see this as being a big problem unless the design purposely chooses to max charge the battery at least 200mV below the OV threshold.

    Thank you for your help.

    Phil