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bq20z655-R1: Pack not exiting sleep mode when providing current

George Mortimer
Expert 1205 points
Part Number: BQ20Z655-R1

Hey TI experts,

I have a situation in which the battery pack remains in SLEEP mode even when providing discharge current to the load.  When the pack is idle, the CHG FET is OFF as indicated by the output voltage.  When a nominal load is applied, the CHG FET turns ON and the pack operates normally. However, if the pack is turned on with a light load, the CHG FET stays OFF.  Note that this only occurs when the SMBus connection to the bqEv is disconnected.  In the description of SLEEP mode in the bq20z655 TRM, it states that the pack will emerge from SLEEP mode if either a transition is seen on the SMBC or SMBD lines, so this I can understand.  However, it also states that SLEEP mode is exited if the absolute value of current is greater than the Sleep Current value.  In my case the Sleep Current is only 10mA, and the applied current under light load is between 500mA and 400mA.  Interestingly enough, if the pack is loaded to above around 500mA, the pack will wake up (CHG FET will turn ON) and will remain ON even as the load current is reduced.

This may be more of a nuisance than a major problem in that in the application the pack is connected to the utilizing equipment via the SMBus, so that connection is always there.  However this appears to be an anomaly either in my application or in the description of the function in the TRM.  Note that in previous designs with your gas gauges I have always had NRCHG set (which would eliminate the problem), but for this design I chose to clear that bit in order to minimize standby power.

  • 0
    TI__Guru* 76230 points

    Hi George,

    We would test this behavior your described and get back to you if we figure out a  work around fix.

    thanks

    Onyx

  • 0 in reply to Onyx Ahiakwo
    TI__Guru 73830 points

    I tested the Sleep exit features and verified that the gauge does exit Sleep mode, if the current is slightly above the Sleep Current setting. Remember that current is only measured every 20s when in Sleep mode, so wake may be delayed by that much time. If you cannot get it to work, then send a senc file from your pack and I can test your setup.

  • 0 in reply to ThomasCosby
    Expert 1205 points
    Thanks, Tom! I forgot that the GG would have to wait one sleep current timeout cycle before it would recognize that the current had changed. And to make matters "worse", I had reset the current measurement time in sleep mode to 240 seconds (4 minutes) in order to reduce the current consumption (hence the self-discharge) when the pack was in storage on the shelf. I checked the pack and it took just under 4 minutes to recover.