BQ40Z50-R2: QMAX update not occurs with LFP cells during the learning cycle, update status stays 04.

Hello Kovacs,

The question has been assigned and will be answered when possible.

Thank you,
Alan

Thank you Alan,

So far nothing is changed, QMAX is still not updated.

One thing I noticed is the [FC] turned off sometime in the previous 3 days, the cells voltage went below 3400 mV. (there is absolutely no current drawn from the pack except for the ~300uA for the fuel gauge)

This is an LFP pack... the default 3500mV value of FC Clear Voltage Threshold is good for this chemistry?

Hello Kovacs, 

Can you send me the GPCCHEM report you would've gotten to select your ChemID, also make sure to run a calibration on your system before doing a learning cycle.

GPCCHEM:
https://www.ti.com/tool/GPCCHEM

Thank you,
Alan 

Hello Alan,

GPCCHEM was unable to find suitable chemistry for these cells. It was talked about in a previous thread where Adrian suggested to us send cells to TI for analysis. https://e2e.ti.com/support/power-management-group/power-management/f/power-management-forum/1439174/bq40z50-r2-ocvfr-set-high-during-lfp-learning-cycle-when-qmax-update-should-happen-instead

ChemID 4207 was identified for these cells by TI in request from the cells manufacturer. I got some details:

• CC Number : 2501024
• Cell Maker : GoldenCell
• Cell Model : 18FC

Is these info enough?

Both voltage and current calibration was done before the learning cycle and those measurements are quite accurate.

I'm still thinking about the FC flag... LFP cells voltage drops quickly after the end of charge. Some time after the full charge, VOK goes to 0, so the first OCV reading is done and REST bit sets... But what happens after that if the cells voltage drops more and FC turns off?
It's hard to get solid info about this.

I'm thinking about the 4 possibilities that prevents a QMAX update:

• Temperature — If Temperature is outside of the range 10°C to 40°C.

• Delta Capacity — If the capacity change between suitable battery rest periods is less than 90% of the
full scale range of the OCV table during the initial QMax Update and 37% during field updates (2nd
and subsequent updates) of QMax.

• Voltage — If Voltage() is inside the flat voltage region. This flat region is different for each ChemID.
The GaugingStatus[OCVFR] flag indicates if the cell voltage is inside this flat region.

• Offset Error — If offset error accumulated during time passed from previous OCV reading exceeds
1% of Design Capacity, update is disqualified. Offset error current is calculated as Coulomb Counter
Deadband/sense resistor value.

I can rule out the first three. But about the fourth I'm not sure, it's a bit hard to understand.


In the config, the design capacity is 8000mAh, because I'm using 1600mAh cells in a 4S5P arrangement. But according the Qmax passed Q params, the pack actually has 8490mAh capacity. Should I change the design capacity value in the config to match this value?

Someone else had this question too, but the answer there not seems to help. https://e2e.ti.com/support/power-management-group/power-management/f/power-management-forum/866184/bq40z50-how-to-understand-offset-error-parameter

Hello Kovacs,

We are working into giving you a response,

Thank you,
Alan

Hello Alan,

Did you manage to find any solution?

Since production deadlines are pressing, I manually stepped the learning cycle to the next (05) state so that at least the impedance measurement would be available and the balancing would work (at 0E state). - Of course, this is not a very good solution, because remaining capacity and status measurement do not work.

Tamas

Hello Kovacs,

If you are stuck on the learning cycle and can't find a chemID for your cell than you would need to send your cell to create a chemID. Please contact your local FAE for steps on getting a chemID created. 

Thank you,
Alan

Hello Alan,

It is already done, TI created the 4207 ChemID for these cells. Please check my previous posts for details.

Tamas