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BQ20z95 measures no current if bus lines low

Jan Schreibeis
Prodigy 30 points

Hey,

i have got problems with the BQ20Z95 on our 3s3p battery pack. If i hold the SMBus lines low for 'bus low time' (5 seconds) while charging or discharging (current = 2A) the BQ20Z95 reports current as 23mA.

Our charger communicates with the BQ20z95 using SMBus every 15 seconds. The charger reads several parameters from BQ20Z95 like current, voltage and so on. After all data is acquired the charger holds the bus lines low for 15seconds until it starts communicating to BQ20Z95 again.

So the normal communication cycle is like that:

1. Bus lines low for 15s

2. Bus lines high

3. wait 5ms

4. get all data from BQ20z95

back to 1.

The problem is that the current is reported way lower than it realy is. It's not a calibration problem because if i wait longer, than BQ20z95 reports the right current. Longer wait times won't help me, because of the lowered average current due to false current measurement. Gas Gauging isn't working properly, too.

I found out that if i wait even longer than 15 seconds the BQ20Z95 eventually measures the right value again. Usually 20 seconds after the bus lines got low, BQ20Z95 measures a right current value.

With Bus Low Time i can alter the time which the bus lines need to be low for wrong current measurement to occur.

Any ideas how to get rid of the problem?

Jan

  • 0
    Genius 3970 points

    If the bq20z95's SMBus lines are held low for at least 5 seconds, the gauge automatically enters its coulomb counter auto-calibration routine, which typically lasts 16 seconds. During that time, no gauging or current measurements are performed. See page 62 of SLUU264A for more information.


    Is there a particular reason for your application to pull the SMBus lines low when the bus is idle, and is this for an embedded (non-removable) battery pack?

  • 0 in reply to Jason Gin
    Prodigy 30 points

    Thank you very much Jason. I already thought that this could be the problem, but i thought that auto-calibration is only done when no current is flowing.

    There is no reason to pull the bus lines low but we only did the battery pack. I'll talk to the developer of the charger to change the behaviour of the charger.

    Jan