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BQ78350-R1A: Charging Stops Before COV Event (in combination with BQ76930)

Jonas Ebert
Prodigy 20 points
Part Number: BQ78350-R1A
Other Parts Discussed in Thread: BQ78350-R1, BQSTUDIO

Hi TI-Team,

for a battery pack (7S1P) we have designed a BMS using the BQ78350DBT-R1A in combination with the BQ7693000DBT.

In general the BMS still works. 

Except that the pack cannot be fully charged. That means that the SoC is set to 100% although the voltage of every cell is only about 4050mV but the COV is set to 4250mV and typical charge termination is about 4200mV per cell.

We use the BAK N18650CNP with a capacity of 2500mAh and have programmed the specific Chemistry with ID 2447.

While charging with voltage of 29,4V the process stops at about 4050mV per cell (pack voltage 28,3V). SoC is at 100% and the Remaining Capacity is equal to the Full Charge Capacity.

But it is striking that the FCC is only about 1700mAh. I would have expected this to be about 2500mAh.

During my research I noticed that the FCC will be adapted during discharge. So if the cells are only loaded until 4050mV it is clear that the discharged capacity is less than 2500mAh.

I programmed the Chemistry 2447 again but the FCC was not set to 2500mAh. (I don't know if this work regular or not).

 

Can you tell me why the charging stops already at 4050mV per cell and does not charge until 4,2V per cell corresponding to the charging voltage of 29,4V.

Is it because of the SoC is still set to 100%, because the FCC is reached or are there any other reasons?

Or do I have to change someting in the "Data Memory"?

Maybe is there any fault with the Chemistry (maybe I have forgotten some settings)?

It's also striking that if discharge started for a short time after charging stops at 4050mV per cell, the cells could be charged up to 4200mV and only stop at COV of 4250mV.

But during discharge of the cell once more the FCC was not set to about 2400mAh, which I have measured with an Electronic Load.

So what can I do / have to change, that the battery will be charged until the Charge Voltage of 4200mV per cell or until Cell Over Voltage Event of 4250mV is set?

If you need some settings files, please let me know.

Thanks for your support

Jonas

  • 0
    TI__Guru* 81034 points

    Hi Jonas,

    Programming the chemistry does not set the FCC. You should set both the Design Capacity and Learned Full Charge Capacity settings to 2500 for your cells. Hopefully you have also followed the steps in the Using the BQ78350-R1 guide to find and program your CEDV (compensated end-of-discharge) parameters - this is very important for accurate gauging.

    As for why the charging stops early, the best way to debug this is to collect a log file during charging using BQStudio (click the Start Log button on the main screen). The log file will show the state of all of the device registers, so it should reveal why charging was stopped. Also save your register settings to a .gg.csv file to share so that we can see if there is a problem with any of your settings.

    Best regards,

    Matt