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BQ76930: BQ76930 general question and curious behavior for cell voltage

Part Number: BQ76930

Hello TI experts,


I use a 17S BMS board with two BQ76930, the 1st for 1 to 8 (from gnd), and the 2nd for cells 9 to 17. The 2nd is supplied with an isolated DC/DC and have an I2C isolator to communicate with the MCU which is referenced on pack GND.

Everything works well, but I have some questions and a curious behavior.


Question 1 :

For the temperature reading we must firstly calculate the voltage at TSx pin and then calculate the resitor value of the sensor.
The datasheet gives this formula page 25 :

VTSx = (ADC in Decimal) x 382 μV/LSB
RTS = (10,000 × VTSX) ÷ (3.3 – VTSX)

But from the internal chip diagram, the ADC for temperature is the same for cell voltage measurement.
Thus I think we should also use the Gain and Offset of the chip to calculate VTSx, as for cell voltage ?

Question 2 :

What is the precision of the 3.3V regulator applied to the internal 10K and external thermistor for the temperature measurement ?

Question 3 :

In my software in the main loop I pull the CC_READY bit in the SYS_STAT register for the 2 chips, and when the bit is 1, I read all data register to get cell voltage and temperature. For debug I flash a different led for each chip when data are ready, and I can clearly see the "phase" changing on led flashes. It clearly indicates that the 250ms sample interval are not exactly the same between the 2 chips.

What is the precision or tolerance for this 250ms timing ? The datasheet doesn't talk about this.
It can be a problem because my MCU calculate the ampere-hour outgoing of the pack based on this timing. The Ah calculation will be wrong if the 250ms is not precise.

Curious behavior :

I use a serial port of my MCU to output debug message, and when I display the cells voltage measurements (each time CC_READY is 1 thus at 4Hz rate), I can see sometime a deviation of the value on some cell. More precisely on the upper cell of the internal five cell group (VC5 and VC10). There is nothing connected on the battery pack (no charge, no discharge, thus zero current). It's the same behavior for the 2 bq76930 of the board, for example, for the low side bq76930 (cell 1 to 8), incorrect values are in bold :

BQ 0 : 3749mV 3749mV 3750mV 0000mV 3740mV 3745mV 3748mV 3746mV 0000mV 3742mV
BQ 0 : 3749mV 3749mV 3750mV 0000mV 3748mV 3745mV 3748mV 3746mV 0000mV 3742mV
BQ 0 : 3749mV 3750mV 3750mV 0000mV 3748mV 3745mV 3748mV 3746mV 0000mV 3742mV
BQ 0 : 3749mV 3750mV 3750mV 0000mV 3748mV 3745mV 3748mV 3746mV 0000mV 3742mV
BQ 0 : 3749mV 3750mV 3750mV 0000mV 3748mV 3745mV 3748mV 3746mV 0000mV 3742mV
BQ 0 : 3749mV 3750mV 3750mV 0000mV 3748mV 3745mV 3748mV 3747mV 0000mV 3742mV
BQ 0 : 3749mV 3750mV 3750mV 0000mV 3748mV 3745mV 3748mV 3747mV 0000mV 3742mV
BQ 0 : 3749mV 3750mV 3750mV 0000mV 3748mV 3745mV 3748mV 3747mV 0000mV 3742mV
BQ 0 : 3749mV 3750mV 3750mV 0000mV 3743mV 3745mV 3747mV 3747mV 0000mV 3724mV
BQ 0 : 3750mV 3750mV 3751mV 0000mV 3748mV 3745mV 3747mV 3746mV 0000mV 3742mV
BQ 0 : 3749mV 3750mV 3750mV 0000mV 3748mV 3745mV 3748mV 3747mV 0000mV 3742mV
BQ 0 : 3749mV 3750mV 3751mV 0000mV 3748mV 3745mV 3749mV 3747mV 0000mV 3742mV
BQ 0 : 3749mV 3750mV 3750mV 0000mV 3748mV 3745mV 3749mV 3747mV 0000mV 3742mV
BQ 0 : 3749mV 3750mV 3750mV 0000mV 3748mV 3745mV 3748mV 3747mV 0000mV 3742mV
BQ 0 : 3749mV 3750mV 3750mV 0000mV 3748mV 3745mV 3748mV 3747mV 0000mV 3742mV
BQ 0 : 3749mV 3750mV 3750mV 0000mV 3748mV 3745mV 3748mV 3747mV 0000mV 3724mV
BQ 0 : 3749mV 3750mV 3750mV 0000mV 3743mV 3745mV 3748mV 3747mV 0000mV 3742mV
BQ 0 : 3749mV 3750mV 3750mV 0000mV 3748mV 3745mV 3749mV 3747mV 0000mV 3742mV
BQ 0 : 3749mV 3750mV 3750mV 0000mV 3748mV 3745mV 3748mV 3747mV 0000mV 3742mV
BQ 0 : 3749mV 3750mV 3750mV 0000mV 3748mV 3745mV 3748mV 3747mV 0000mV 3742mV
BQ 0 : 3749mV 3750mV 3750mV 0000mV 3748mV 3745mV 3748mV 3747mV 0000mV 3742mV
BQ 0 : 3749mV 3750mV 3750mV 0000mV 3748mV 3745mV 3748mV 3747mV 0000mV 3742mV
BQ 0 : 3749mV 3750mV 3750mV 0000mV 3748mV 3745mV 3748mV 3747mV 0000mV 3742mV
BQ 0 : 3749mV 3750mV 3750mV 0000mV 3748mV 3745mV 3748mV 3747mV 0000mV 3724mV
BQ 0 : 3749mV 3750mV 3750mV 0000mV 3742mV 3745mV 3748mV 3746mV 0000mV 3742mV
BQ 0 : 3749mV 3750mV 3750mV 0000mV 3748mV 3745mV 3748mV 3746mV 0000mV 3742mV
BQ 0 : 3749mV 3750mV 3750mV 0000mV 3748mV 3745mV 3748mV 3746mV 0000mV 3742mV
BQ 0 : 3749mV 3750mV 3750mV 0000mV 3748mV 3745mV 3748mV 3746mV 0000mV 3742mV
BQ 0 : 3749mV 3750mV 3750mV 0000mV 3748mV 3745mV 3748mV 3747mV 0000mV 3742mV
BQ 0 : 3749mV 3750mV 3750mV 0000mV 3748mV 3745mV 3748mV 3746mV 0000mV 3742mV
BQ 0 : 3749mV 3750mV 3750mV 0000mV 3748mV 3745mV 3748mV 3746mV 0000mV 3742mV
BQ 0 : 3749mV 3750mV 3750mV 0000mV 3748mV 3745mV 3748mV 3747mV 0000mV 3721mV
BQ 0 : 3749mV 3750mV 3750mV 0000mV 3742mV 3745mV 3748mV 3747mV 0000mV 3742mV
BQ 0 : 3749mV 3750mV 3750mV 0000mV 3748mV 3745mV 3748mV 3747mV 0000mV 3742mV
BQ 0 : 3749mV 3750mV 3750mV 0000mV 3748mV 3745mV 3748mV 3747mV 0000mV 3742mV
BQ 0 : 3749mV 3750mV 3750mV 0000mV 3748mV 3745mV 3748mV 3746mV 0000mV 3742mV
BQ 0 : 3749mV 3750mV 3750mV 0000mV 3748mV 3745mV 3748mV 3746mV 0000mV 3742mV
BQ 0 : 3749mV 3750mV 3750mV 0000mV 3748mV 3745mV 3748mV 3746mV 0000mV 3742mV
BQ 0 : 3749mV 3750mV 3750mV 0000mV 3748mV 3744mV 3748mV 3747mV 0000mV 3742mV
BQ 0 : 3749mV 3750mV 3750mV 0000mV 3748mV 3745mV 3748mV 3747mV 0000mV 3724mV
BQ 0 : 3749mV 3750mV 3751mV 0000mV 3742mV 3745mV 3748mV 3747mV 0000mV 3742mV
BQ 0 : 3749mV 3750mV 3750mV 0000mV 3748mV 3741mV 3748mV 3747mV 0000mV 3742mV
BQ 0 : 3749mV 3750mV 3751mV 0000mV 3748mV 3745mV 3748mV 3747mV 0000mV 3742mV
BQ 0 : 3750mV 3750mV 3751mV 0000mV 3748mV 3745mV 3748mV 3747mV 0000mV 3742mV
BQ 0 : 3750mV 3750mV 3751mV 0000mV 3748mV 3745mV 3748mV 3747mV 0000mV 3742mV
BQ 0 : 3750mV 3750mV 3751mV 0000mV 3748mV 3745mV 3748mV 3747mV 0000mV 3742mV
BQ 0 : 3750mV 3750mV 3751mV 0000mV 3748mV 3745mV 3748mV 3747mV 0000mV 3742mV
BQ 0 : 3750mV 3750mV 3751mV 0000mV 3748mV 3745mV 3749mV 3747mV 0000mV 3723mV
BQ 0 : 3750mV 3750mV 3751mV 0000mV 3743mV 3745mV 3749mV 3746mV 0000mV 3742mV
BQ 0 : 3750mV 3750mV 3751mV 0000mV 3748mV 3748mV 3746mV 3746mV 0000mV 3742mV
BQ 0 : 3750mV 3750mV 3751mV 0000mV 3748mV 3745mV 3748mV 3747mV 0000mV 3742mV
BQ 0 : 3750mV 3750mV 3751mV 0000mV 3748mV 3745mV 3748mV 3747mV 0000mV 3742mV
BQ 0 : 3750mV 3750mV 3751mV 0000mV 3748mV 3745mV 3748mV 3747mV 0000mV 3742mV
BQ 0 : 3750mV 3750mV 3751mV 0000mV 3748mV 3745mV 3748mV 3746mV 0000mV 3742mV
BQ 0 : 3750mV 3750mV 3751mV 0000mV 3748mV 3745mV 3748mV 3746mV 0000mV 3724mV
BQ 0 : 3750mV 3750mV 3751mV 0000mV 3748mV 3745mV 3748mV 3747mV 0000mV 3742mV
BQ 0 : 3749mV 3750mV 3750mV 0000mV 3743mV 3745mV 3748mV 3747mV 0000mV 3742mV
BQ 0 : 3749mV 3750mV 3750mV 0000mV 3748mV 3745mV 3748mV 3747mV 0000mV 3742mV
BQ 0 : 3749mV 3750mV 3750mV 0000mV 3748mV 3745mV 3748mV 3747mV 0000mV 3742mV
BQ 0 : 3749mV 3750mV 3750mV 0000mV 3748mV 3745mV 3748mV 3747mV 0000mV 3742mV
BQ 0 : 3750mV 3750mV 3751mV 0000mV 3748mV 3745mV 3748mV 3747mV 0000mV 3742mV
BQ 0 : 3750mV 3750mV 3751mV 0000mV 3748mV 3745mV 3748mV 3747mV 0000mV 3742mV
BQ 0 : 3750mV 3750mV 3751mV 0000mV 3748mV 3745mV 3748mV 3747mV 0000mV 3724mV
BQ 0 : 3750mV 3750mV 3751mV 0000mV 3748mV 3745mV 3748mV 3747mV 0000mV 3742mV
BQ 0 : 3750mV 3750mV 3751mV 0000mV 3742mV 3745mV 3748mV 3747mV 0000mV 3742mV
BQ 0 : 3750mV 3750mV 3751mV 0000mV 3748mV 3745mV 3748mV 3747mV 0000mV 3742mV
BQ 0 : 3749mV 3750mV 3750mV 0000mV 3748mV 3745mV 3748mV 3747mV 0000mV 3742mV
BQ 0 : 3750mV 3750mV 3751mV 0000mV 3748mV 3745mV 3748mV 3747mV 0000mV 3742mV
BQ 0 : 3749mV 3750mV 3750mV 0000mV 3748mV 3745mV 3748mV 3747mV 0000mV 3742mV
BQ 0 : 3749mV 3750mV 3750mV 0000mV 3748mV 3745mV 3748mV 3747mV 0000mV 3742mV
BQ 0 : 3749mV 3750mV 3750mV 0000mV 3748mV 3745mV 3748mV 3747mV 0000mV 3724mV
BQ 0 : 3750mV 3750mV 3751mV 0000mV 3748mV 3745mV 3748mV 3747mV 0000mV 3742mV
BQ 0 : 3749mV 3750mV 3750mV 0000mV 3743mV 3745mV 3748mV 3746mV 0000mV 3742mV
BQ 0 : 3749mV 3750mV 3750mV 0000mV 3748mV 3745mV 3748mV 3747mV 0000mV 3742mV
BQ 0 : 3749mV 3750mV 3750mV 0000mV 3748mV 3745mV 3748mV 3747mV 0000mV 3742mV
BQ 0 : 3749mV 3750mV 3750mV 0000mV 3748mV 3745mV 3748mV 3747mV 0000mV 3742mV
BQ 0 : 3749mV 3750mV 3750mV 0000mV 3748mV 3745mV 3748mV 3747mV 0000mV 3742mV
BQ 0 : 3749mV 3750mV 3750mV 0000mV 3748mV 3745mV 3748mV 3746mV 0000mV 3742mV
BQ 0 : 3749mV 3750mV 3750mV 0000mV 3748mV 3745mV 3748mV 3746mV 0000mV 3724mV
BQ 0 : 3749mV 3750mV 3750mV 0000mV 3748mV 3745mV 3748mV 3747mV 0000mV 3742mV
BQ 0 : 3749mV 3750mV 3750mV 0000mV 3743mV 3745mV 3748mV 3747mV 0000mV 3742mV
BQ 0 : 3749mV 3750mV 3751mV 0000mV 3748mV 3745mV 3747mV 3747mV 0000mV 3742mV
BQ 0 : 3749mV 3750mV 3751mV 0000mV 3748mV 3745mV 3748mV 3747mV 0000mV 3742mV
BQ 0 : 3749mV 3750mV 3751mV 0000mV 3748mV 3745mV 3748mV 3747mV 0000mV 3742mV
BQ 0 : 3750mV 3750mV 3751mV 0000mV 3748mV 3745mV 3749mV 3747mV 0000mV 3742mV
BQ 0 : 3750mV 3750mV 3751mV 0000mV 3748mV 3745mV 3748mV 3746mV 0000mV 3742mV
BQ 0 : 3750mV 3750mV 3751mV 0000mV 3748mV 3745mV 3748mV 3746mV 0000mV 3721mV
BQ 0 : 3750mV 3750mV 3751mV 0000mV 3748mV 3745mV 3748mV 3747mV 0000mV 3742mV
BQ 0 : 3750mV 3750mV 3751mV 0000mV 3743mV 3745mV 3748mV 3746mV 0000mV 3742mV
BQ 0 : 3749mV 3750mV 3750mV 0000mV 3748mV 3745mV 3749mV 3747mV 0000mV 3742mV
BQ 0 : 3749mV 3750mV 3751mV 0000mV 3748mV 3745mV 3748mV 3746mV 0000mV 3742mV
BQ 0 : 3749mV 3750mV 3750mV 0000mV 3748mV 3745mV 3748mV 3746mV 0000mV 3742mV

Same thing with the high side bq76930 (cell 9 to 17) :

BQ 1 : 3742mV 3743mV 3743mV 3744mV 3743mV 3740mV 3742mV 3743mV 0000mV 3738mV
BQ 1 : 3742mV 3743mV 3743mV 3745mV 3743mV 3740mV 3742mV 3743mV 0000mV 3738mV
BQ 1 : 3742mV 3743mV 3744mV 3745mV 3744mV 3740mV 3742mV 3743mV 0000mV 3738mV
BQ 1 : 3742mV 3743mV 3744mV 3745mV 3743mV 3740mV 3742mV 3743mV 0000mV 3738mV
BQ 1 : 3742mV 3743mV 3744mV 3744mV 3743mV 3739mV 3743mV 3743mV 0000mV 3738mV
BQ 1 : 3742mV 3743mV 3744mV 3745mV 3743mV 3740mV 3743mV 3743mV 0000mV 3738mV
BQ 1 : 3742mV 3743mV 3744mV 3745mV 3743mV 3740mV 3742mV 3743mV 0000mV 3738mV
BQ 1 : 3742mV 3743mV 3744mV 3744mV 3738mV 3740mV 3742mV 3743mV 0000mV 3738mV
BQ 1 : 3742mV 3743mV 3744mV 3745mV 3743mV 3740mV 3742mV 3743mV 0000mV 3720mV
BQ 1 : 3742mV 3743mV 3744mV 3745mV 3743mV 3740mV 3742mV 3743mV 0000mV 3738mV
BQ 1 : 3742mV 3743mV 3744mV 3744mV 3744mV 3740mV 3742mV 3743mV 0000mV 3738mV
BQ 1 : 3742mV 3743mV 3744mV 3745mV 3743mV 3740mV 3742mV 3743mV 0000mV 3738mV
BQ 1 : 3742mV 3743mV 3744mV 3745mV 3743mV 3739mV 3743mV 3743mV 0000mV 3738mV
BQ 1 : 3742mV 3744mV 3744mV 3745mV 3743mV 3739mV 3743mV 3743mV 0000mV 3739mV
BQ 1 : 3742mV 3743mV 3744mV 3744mV 3743mV 3740mV 3742mV 3743mV 0000mV 3738mV
BQ 1 : 3742mV 3743mV 3744mV 3744mV 3744mV 3740mV 3743mV 3743mV 0000mV 3738mV
BQ 1 : 3742mV 3743mV 3744mV 3745mV 3743mV 3740mV 3743mV 3743mV 0000mV 3720mV
BQ 1 : 3742mV 3743mV 3744mV 3745mV 3744mV 3740mV 3743mV 3743mV 0000mV 3738mV
BQ 1 : 3742mV 3743mV 3743mV 3744mV 3744mV 3740mV 3743mV 3743mV 0000mV 3737mV
BQ 1 : 3742mV 3743mV 3743mV 3745mV 3743mV 3740mV 3743mV 3743mV 0000mV 3738mV
BQ 1 : 3742mV 3743mV 3744mV 3745mV 3743mV 3740mV 3743mV 3743mV 0000mV 3738mV
BQ 1 : 3742mV 3743mV 3744mV 3744mV 3744mV 3740mV 3743mV 3743mV 0000mV 3738mV
BQ 1 : 3742mV 3743mV 3744mV 3744mV 3744mV 3740mV 3742mV 3743mV 0000mV 3738mV
BQ 1 : 3742mV 3743mV 3743mV 3744mV 3738mV 3740mV 3742mV 3743mV 0000mV 3738mV
BQ 1 : 3742mV 3743mV 3744mV 3745mV 3744mV 3740mV 3742mV 3743mV 0000mV 3720mV
BQ 1 : 3742mV 3743mV 3744mV 3744mV 3744mV 3740mV 3742mV 3743mV 0000mV 3738mV
BQ 1 : 3742mV 3743mV 3744mV 3745mV 3743mV 3740mV 3742mV 3743mV 0000mV 3739mV
BQ 1 : 3742mV 3743mV 3744mV 3745mV 3743mV 3740mV 3743mV 3743mV 0000mV 3738mV
BQ 1 : 3742mV 3743mV 3744mV 3745mV 3743mV 3740mV 3742mV 3743mV 0000mV 3739mV
BQ 1 : 3742mV 3743mV 3744mV 3744mV 3743mV 3740mV 3743mV 3743mV 0000mV 3739mV
BQ 1 : 3742mV 3743mV 3744mV 3745mV 3743mV 3740mV 3743mV 3743mV 0000mV 3738mV
BQ 1 : 3742mV 3743mV 3744mV 3744mV 3738mV 3740mV 3743mV 3743mV 0000mV 3738mV
BQ 1 : 3742mV 3743mV 3744mV 3744mV 3743mV 3740mV 3743mV 3743mV 0000mV 3719mV
BQ 1 : 3742mV 3743mV 3744mV 3744mV 3743mV 3740mV 3743mV 3743mV 0000mV 3719mV
BQ 1 : 3742mV 3743mV 3744mV 3744mV 3743mV 3740mV 3743mV 3744mV 0000mV 3738mV
BQ 1 : 3742mV 3743mV 3744mV 3745mV 3743mV 3740mV 3743mV 3743mV 0000mV 3738mV
BQ 1 : 3742mV 3743mV 3744mV 3745mV 3743mV 3740mV 3742mV 3743mV 0000mV 3738mV
BQ 1 : 3742mV 3743mV 3744mV 3745mV 3743mV 3740mV 3742mV 3743mV 0000mV 3738mV
BQ 1 : 3742mV 3743mV 3743mV 3744mV 3743mV 3740mV 3742mV 3743mV 0000mV 3738mV
BQ 1 : 3742mV 3743mV 3743mV 3744mV 3739mV 3740mV 3742mV 3743mV 0000mV 3738mV
BQ 1 : 3742mV 3743mV 3743mV 3744mV 3743mV 3736mV 3743mV 3743mV 0000mV 3738mV
BQ 1 : 3742mV 3743mV 3744mV 3744mV 3743mV 3740mV 3743mV 3743mV 0000mV 3720mV
BQ 1 : 3742mV 3743mV 3744mV 3744mV 3743mV 3740mV 3742mV 3743mV 0000mV 3738mV
BQ 1 : 3742mV 3743mV 3744mV 3745mV 3744mV 3739mV 3743mV 3743mV 0000mV 3738mV
BQ 1 : 3742mV 3743mV 3744mV 3745mV 3743mV 3739mV 3742mV 3743mV 0000mV 3738mV
BQ 1 : 3742mV 3743mV 3744mV 3744mV 3743mV 3740mV 3742mV 3743mV 0000mV 3738mV
BQ 1 : 3742mV 3743mV 3744mV 3745mV 3744mV 3740mV 3742mV 3743mV 0000mV 3738mV
BQ 1 : 3742mV 3743mV 3744mV 3745mV 3738mV 3740mV 3742mV 3743mV 0000mV 3738mV
BQ 1 : 3742mV 3743mV 3744mV 3745mV 3743mV 3740mV 3742mV 3743mV 0000mV 3738mV
BQ 1 : 3741mV 3743mV 3744mV 3745mV 3743mV 3740mV 3742mV 3743mV 0000mV 3718mV
BQ 1 : 3742mV 3743mV 3744mV 3744mV 3743mV 3740mV 3742mV 3743mV 0000mV 3738mV
BQ 1 : 3742mV 3743mV 3744mV 3745mV 3743mV 3740mV 3743mV 3743mV 0000mV 3738mV
BQ 1 : 3742mV 3743mV 3744mV 3745mV 3743mV 3740mV 3742mV 3743mV 0000mV 3738mV
BQ 1 : 3742mV 3743mV 3743mV 3744mV 3743mV 3740mV 3742mV 3743mV 0000mV 3738mV
BQ 1 : 3741mV 3743mV 3743mV 3745mV 3743mV 3740mV 3742mV 3743mV 0000mV 3738mV
BQ 1 : 3742mV 3743mV 3744mV 3745mV 3739mV 3740mV 3742mV 3743mV 0000mV 3738mV
BQ 1 : 3742mV 3743mV 3744mV 3744mV 3743mV 3740mV 3742mV 3743mV 0000mV 3739mV
BQ 1 : 3742mV 3743mV 3744mV 3745mV 3743mV 3740mV 3743mV 3743mV 0000mV 3720mV
BQ 1 : 3742mV 3743mV 3744mV 3744mV 3743mV 3740mV 3743mV 3743mV 0000mV 3738mV
BQ 1 : 3742mV 3743mV 3743mV 3745mV 3743mV 3740mV 3742mV 3743mV 0000mV 3738mV
BQ 1 : 3742mV 3743mV 3743mV 3744mV 3743mV 3740mV 3742mV 3743mV 0000mV 3738mV
BQ 1 : 3742mV 3743mV 3744mV 3745mV 3743mV 3740mV 3742mV 3743mV 0000mV 3738mV
BQ 1 : 3742mV 3743mV 3744mV 3744mV 3743mV 3740mV 3742mV 3744mV 0000mV 3738mV
BQ 1 : 3742mV 3743mV 3744mV 3745mV 3739mV 3740mV 3743mV 3743mV 0000mV 3738mV
BQ 1 : 3742mV 3743mV 3743mV 3744mV 3744mV 3740mV 3741mV 3743mV 0000mV 3738mV
BQ 1 : 3742mV 3743mV 3743mV 3745mV 3743mV 3740mV 3744mV 3743mV 0000mV 3738mV
BQ 1 : 3742mV 3743mV 3744mV 3745mV 3743mV 3740mV 3743mV 3744mV 0000mV 3719mV
BQ 1 : 3742mV 3743mV 3744mV 3745mV 3743mV 3740mV 3742mV 3744mV 0000mV 3738mV
BQ 1 : 3742mV 3743mV 3744mV 3744mV 3743mV 3740mV 3742mV 3743mV 0000mV 3738mV
BQ 1 : 3742mV 3743mV 3744mV 3745mV 3743mV 3740mV 3742mV 3743mV 0000mV 3738mV
BQ 1 : 3742mV 3743mV 3744mV 3745mV 3743mV 3740mV 3742mV 3743mV 0000mV 3738mV
BQ 1 : 3742mV 3743mV 3744mV 3745mV 3739mV 3740mV 3743mV 3743mV 0000mV 3739mV
BQ 1 : 3742mV 3743mV 3744mV 3745mV 3744mV 3739mV 3743mV 3744mV 0000mV 3739mV
BQ 1 : 3742mV 3744mV 3744mV 3745mV 3744mV 3739mV 3742mV 3743mV 0000mV 3738mV
BQ 1 : 3742mV 3743mV 3743mV 3745mV 3743mV 3739mV 3743mV 3743mV 0000mV 3720mV
BQ 1 : 3742mV 3743mV 3744mV 3745mV 3743mV 3740mV 3743mV 3743mV 0000mV 3739mV
BQ 1 : 3742mV 3743mV 3744mV 3745mV 3743mV 3740mV 3743mV 3743mV 0000mV 3738mV
BQ 1 : 3742mV 3743mV 3743mV 3745mV 3744mV 3740mV 3743mV 3743mV 0000mV 3738mV
BQ 1 : 3742mV 3743mV 3744mV 3744mV 3744mV 3740mV 3743mV 3743mV 0000mV 3738mV
BQ 1 : 3742mV 3744mV 3744mV 3745mV 3737mV 3740mV 3743mV 3743mV 0000mV 3737mV
BQ 1 : 3742mV 3743mV 3744mV 3744mV 3743mV 3740mV 3743mV 3743mV 0000mV 3738mV
BQ 1 : 3742mV 3743mV 3744mV 3745mV 3743mV 3740mV 3743mV 3743mV 0000mV 3738mV
BQ 1 : 3742mV 3743mV 3744mV 3745mV 3744mV 3740mV 3743mV 3743mV 0000mV 3720mV
BQ 1 : 3742mV 3743mV 3744mV 3745mV 3744mV 3740mV 3742mV 3743mV 0000mV 3738mV
BQ 1 : 3742mV 3743mV 3744mV 3744mV 3743mV 3740mV 3742mV 3743mV 0000mV 3739mV
BQ 1 : 3742mV 3743mV 3743mV 3745mV 3743mV 3740mV 3743mV 3744mV 0000mV 3738mV
BQ 1 : 3742mV 3743mV 3744mV 3745mV 3744mV 3740mV 3743mV 3743mV 0000mV 3738mV
BQ 1 : 3742mV 3743mV 3744mV 3744mV 3738mV 3740mV 3742mV 3743mV 0000mV 3738mV
BQ 1 : 3742mV 3743mV 3744mV 3745mV 3743mV 3740mV 3743mV 3743mV 0000mV 3738mV
BQ 1 : 3742mV 3743mV 3744mV 3744mV 3744mV 3740mV 3743mV 3743mV 0000mV 3738mV
BQ 1 : 3742mV 3743mV 3743mV 3744mV 3743mV 3740mV 3742mV 3743mV 0000mV 3720mV
BQ 1 : 3741mV 3743mV 3744mV 3745mV 3743mV 3740mV 3743mV 3743mV 0000mV 3738mV
BQ 1 : 3742mV 3743mV 3744mV 3744mV 3744mV 3740mV 3743mV 3743mV 0000mV 3738mV
BQ 1 : 3742mV 3743mV 3744mV 3744mV 3744mV 3740mV 3743mV 3744mV 0000mV 3738mV
BQ 1 : 3742mV 3743mV 3744mV 3745mV 3743mV 3740mV 3742mV 3743mV 0000mV 3738mV
BQ 1 : 3742mV 3743mV 3744mV 3745mV 3738mV 3740mV 3743mV 3743mV 0000mV 3738mV
BQ 1 : 3742mV 3743mV 3744mV 3744mV 3743mV 3742mV 3740mV 3743mV 0000mV 3738mV
BQ 1 : 3742mV 3743mV 3744mV 3744mV 3743mV 3740mV 3743mV 3743mV 0000mV 3738mV
BQ 1 : 3742mV 3743mV 3744mV 3744mV 3743mV 3740mV 3742mV 3743mV 0000mV 3738mV
BQ 1 : 3742mV 3743mV 3744mV 3745mV 3744mV 3740mV 3742mV 3743mV 0000mV 3720mV
BQ 1 : 3742mV 3743mV 3744mV 3744mV 3743mV 3740mV 3743mV 3743mV 0000mV 3738mV
BQ 1 : 3742mV 3743mV 3744mV 3745mV 3744mV 3740mV 3742mV 3743mV 0000mV 3738mV
BQ 1 : 3742mV 3743mV 3744mV 3745mV 3743mV 3740mV 3743mV 3743mV 0000mV 3739mV

Please note that's really a measure problem and not a communication error because I use a bq76930 with CRC option, and the CRC validation is implemented in my firmware. All data registers are read in a single contiguous read with auto increment. And all balancing are OFF.

Any idea for this ~20mV periodic deviation for VC10 and the ~5mV periodic deviation for VC5 ?

Regards
Cyril HAENEL

  • Hi Cyril,
    1. The ADC has stored average gain/offset for the cell voltage range, see the figure 6-2 of the bq76930 data sheet. The temperature measurement is in the low voltage range of the ADC and the cell gain/offset are not needed.
    2. The 3.3V does not have a data sheet value. It is expected to be +/- 100 mV.
    3. Yes there is drift between the cell groups in a part and between devices. I don't immediately find the tolerance. If reading based on an absolute time or with reference to one part the current on the other may eventually read twice or miss a current sample.
    4. I expect you are seeing an effect from the temperature measurement loading the supply. Be sure you have not used schottky diodes for the VC5 to VC5X and VC10 to BAT limits. Also be sure your power filter is connected.
  • Hi WM5295,

    Thank you for answers

    1. Understood, so I will use the fixed 382µV/LSB and no offset for TSx voltage conversion

    2. +/-100mV lower the temperature measurement precision. No way to route this internal 3.3V in the internal ADC to get its real value and calculate temperature with good precision ?

    3. On the board only the low side BQ76930 is used to measure the pack current. On the high side, I just use CC_READY flag to periodically read the ADC data (cell voltage and temperature). Without the 250ms period tolerance, I think I will need to implement the exact interval (for example using a timer and the /alert interrupt) to have a precise Ah counter

    4. You are right this behavior is related to external temperature measurement. I removed the temperature sensor, I have the same behavior, but if I configure the BQ76930 to measure the internal die temperature, all voltage measurement are stable.

    Will check he hardware, but I will not be able to change anything, I work on an existing hardware I have not designed. It's a commercial BMS with a buggy software thus I replace the software by my own....

  • I checked the hardware, it seems diode for VC5 to VC5X and VC10 to BAT are schottky diodes, I measure them with 0.299V threshold.
    Can you explain me why and how have schottky diode here disturb the cell 5 and cell 10 voltage ?
    Regards
  • Hi Cyril,
    For the clock accuracy, see e2e.ti.com/.../576970, expect about 3.5%

    The power will increase to the cell group during the thermistor bias. This will pull more current through the BAT or VC5X pin dropping the voltage. If the TSn pin is shorted to its reference, the maximum current would be about 3.3V/10k = .33 mA. This current through a 1k power filter resistor would drop the supply voltage by 0.33V A standard diode should not conduct. A schottky diode could conduct and start to pull current through the VC5 filter resistor. Certainly with a thermistor rather than a short the impact should be lower. If the power filter resistor is missing or damaged the impact could be greater. If you have a working board/design that should not be an issue.
  • Hi WM5295,

    Thank you for explanation. On cell #10 I have a 20mV error, thus the drop on BAT pin is >= 0.32V during the temperature sensor bias. I have exactly the same 20mV drop on the cell #10 even when I disconnect my 10K sensor. In this configuration there is no current sourced on TS pin, thus I think the voltage drop on BAT pin comes from internal chip circuitry, maybe the 3.3V regulator used to supply the TS pin through the internal 10K resistor ?

    Why these VC10  to BAT and VC5 to VC5x diodes are needed ? VC10 and VC5 must respectively never have voltage 0.6V higher than BAT and VC5x pin to protect internal circuitry ?

    Because I can not change the hardware, I will need to find a software workaround, for example configure the bq76930 to sample internal die temperature when I sample the cell voltage, and for example each minute sample the external sensor but during this time do not take in consideration the cell voltage values

    Regards,

    Cyril

  • Hi Cyril,
    Yes the 3.3V (CAPn) is used to bias the thermistor and its internal circuit. While my explanation was for current from the TS pin there will be some internal bias current also which is apparently what must be creating your error. It is not described in the data sheet as currents are long averages.
    The diodes are needed to prevent large differential voltages between the supply pin and the top input which could damage the part. The data sheet does not have a specification for that limit since the system design is supposed to be done as shown in section 8 and the diode eliminates the condition which would be specified. See figure 20 of http://www.ti.com/lit/slua749 if you want more information on the diode purpose. In your situation it seems you have them.
    You could alternate between internal temperature measurement and external, but there is some time to get an update. Temperature is measured every 2 seconds. Since the scheduler in the bq76930 does not tell you when it is measuring temperature, you may need to wait long enough to be sure you have the new temperature reading, or recognize which temperature you are reading from past performance. You can filter cell voltages for use in your firmware, but the hardware limit will respond to the measured voltage. An intermittent low reading may not cause a false UV, but be sure it does not prevent an OV.
  • Hi WM5295,

    Thank you for the file link, very interesting.
    You are right, I will implement a state machine to remove the bad value during temperature sampling, they can be a problem for OV protection.

    Thank you very much for your help
    Have a good day
    Regards
    Cyril