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bq27541 V1.12 Very low discharge current appears to mess up the Ra Table and cause Qmax estimate errors

David Gunderson
Prodigy 180 points
Other Parts Discussed in Thread: BQ27541-V200

I have a design with a single Moli cell (chem ID 0110) that is being discharged at about C/27 (70mA). In this situation, the Ra Table appears to be updated to very strange values and Qmax is high (2100mAh for a 1900mAh cell). Here's the golden good IT section and Ra Table programmed at battery production.

[IT Cfg(Gas Gauging)]
Load Select = 1
Load Mode = 1
Terminate Voltage = 3000
User Rate-mW = 0
Reserve Cap-mAh = 0
Reserve Cap-mWh = 0
[Current Thresholds(Gas Gauging)]
Dsg Current Threshold = 30
Chg Current Threshold = 75
Quit Current = 25
Dsg Relax Time = 1800
Chg Relax Time = 60
Quit Relax Time = 1
[State(Gas Gauging)]
Qmax Cell 0 = 2020
Cycle Count = 0
Update Status = 06
Avg I Last Run = -462
Avg P Last Run = -1723

[OCVa Table(OCV Table)]
Chem ID = 0110

Cell0 R_a flag = 0055
Cell0 R_a 0 = 70
Cell0 R_a 1 = 138
Cell0 R_a 2 = 109
Cell0 R_a 3 = 131
Cell0 R_a 4 = 136
Cell0 R_a 5 = 152
Cell0 R_a 6 = 177
Cell0 R_a 7 = 179
Cell0 R_a 8 = 175
Cell0 R_a 9 = 182
Cell0 R_a 10 = 181
Cell0 R_a 11 = 120
Cell0 R_a 12 = 134
Cell0 R_a 13 = 139
Cell0 R_a 14 = 258

And here's that info after full charge, rest, low current discharge, rest.

[IT Cfg(Gas Gauging)]
Load Select = 1
Load Mode = 1
Terminate Voltage = 3000
User Rate-mW = 0
Reserve Cap-mAh = 0
Reserve Cap-mWh = 0
[Current Thresholds(Gas Gauging)]
Dsg Current Threshold = 30
Chg Current Threshold = 75
Quit Current = 25
Dsg Relax Time = 1800
Chg Relax Time = 60
Quit Relax Time = 1
[State(Gas Gauging)]
Qmax Cell 0 = 2101
Cycle Count = 2
Update Status = 06
Avg I Last Run = -75
Avg P Last Run = -296

[OCVa Table(OCV Table)]
Chem ID = 0110

Cell0 R_a flag = 0055
Cell0 R_a 0 = 198
Cell0 R_a 1 = 389
Cell0 R_a 2 = 81
Cell0 R_a 3 = 214
Cell0 R_a 4 = 311
Cell0 R_a 5 = 256
Cell0 R_a 6 = 175
Cell0 R_a 7 = 381
Cell0 R_a 8 = 500
Cell0 R_a 9 = 436
Cell0 R_a 10 = 1002
Cell0 R_a 11 = 623
Cell0 R_a 12 = 3734
Cell0 R_a 13 = 12732
Cell0 R_a 14 = 23709

It appears that the low current is causing IT to make errors in calculating the Ra table entries.

 Is there a way to disable Ra Table update on a low current discharge that may not give IT enough OCV to discharge voltage baseline for the Ra table entry calculation?

Is this due to my use of the V1.12 gas gauge firmware? I had to remain at that release due to configuration management requirements on this product.

Thanks much

  • 0
    TI__Genius 14985 points

    Hi David,

    I'm surprised this happened since such low currents should have cause the updates to have been disqualified.  This might be due to the old version (v1.12), or it might also affect the newer bq27541-V200.

    You could change Ra Filter from 500 to 1000.  This will prevent all updates to the Ra table. 

    Ra Filter = 500 means write new Ra grid point by combining 50% of the old value with 50% of the new calculated value. 

    Ra Filter = 1000 means combine 100% of the old value and 0% of the new value.

    To prevent Qmax from changing so much (~4% in your case) you can limit it with

    Qmax Max Delta % = 5 -> 1

    You might be able to use 0, but setting some parameters to 0 means that feature is disabled.  I'm not sure about this one, so you could try 0 and see if it blocks Qmax from updating totally or not.

  • 0 in reply to dMax
    Prodigy 180 points

    Note that both Ra filtering and Qmax Max Delta are not supported in V1.12.

    So, it appears that I'm a bit stuck. Can't update to V2.00 because of configuration control issues, can't make IT work very well, especially at low current discharge with V1.12.

    Bit of a bummer - any advice would be appreciated.