BQ77915: Cell current high in lower IC of the stack, OCCDT higher than spec, Cell Balancing does not function when shunt is < 1 milliohm

Hello Anil,

Could you please share the schematic of your system?

1.  Single Cell Consumption current  high  for lowest ic in the stack. (connected with mosfets & current sense shunt resistor). This will cause unbalance in the battery pack. Measured current is 22uA for  cell 1-4 , 11uA for Cell 5-16. (NO cell balancing at this time)

I may need to see the schematic to see if there's anything that could affect it. However, it may be likely that this is due to load by the FETs. The bottom device has a greater load than upper devices due to the drive needed for the FETs. You may want to match the loads of the upper devices to more closely match the FET loads. Albeit for a different part, I believe Section 3 of the bq77905 20S Cell Stacking Configuration application report will be useful for this and understanding more about it, as these two parts are very similar. 

2. Over Charging Current Delay Time measured  is 17 - 19mS vs datasheet number of 8 – 12 msec.

What are your test conditions? I may need to see the schematic to further help. How much above the OCC voltage threshold are you in? If it is just at the threshold, the sense resistor filter may add some delay.

3. When we use shunt below 1milli OHM , Cell balancing does not function.

What is your sense resistor value and charging current? Cell balancing is activated in relation to the voltage measured by the sense resistor; smaller sense resistors means that a higher charge current will be needed to activate cell balancing. Generally you will want to make the charging current as small as possible (above the cell balancing threshold) to balance. Cells have IR drops that can cause them to show higher voltages than reality during charging, which can lead to imbalances of the cell balancing. This is just something to keep in mind.

Best Regards,

Luis Hernandez Salomon

Thanks Luis  for the quick reply.

Would like to send schematic in a private message. Could you please enable  private message ?

Regards,

Anil