BQ27426: Questions about Qmax and RA update rules

A1: The basis is in the method that the gauge uses to model OCV as a function of temperature.

A2: 0deg.C and only limited by max signed int /10.

A3: Qmax relies on DOD and DOD is a function of OCV measurement and temperature measurement. The gauge models OCV over temperature. This mathematical model isn't accurate enough for temperatures below 0deg.C and above 40deg.C to accurately determine DOD, hence the gauge excludes OCV measurements outside this temperature range.

A4: The DOD error is the difference between true DOD and measured DOD.

A5: For practical reasons, the discharge current should exceed C/10 during a discharge for the gauge to measure Ra.

A6: No. Ra will not update if the current is less than C/10. This is a hard requirement.

A7: The Ra table will not update if the current is less than C/10.

A8: It can be modified to C/25. But the gauge's analog front end and integer math doesn't support Ra updates for C/25 so this is not supported by TI.

A9: Reserve capacity is subtracted from capacity predictions. It is not subtracted from Design Capacity. Design Capacity is a reference capacity. The gauging results do not use Design Capacity in a simple formula but instead the gauge models a discharge scenario. It will then subtract Reserve Capacity from the results.

A10: The basis is empirical data by the team that developed the algorithm. 37% DOD change was deemed the minimum acceptable limit for Qmax updates.

Hello dominik，

1、The minimum value of Design Capacity in TRM is 0. If the Design Capacity of the battery is 500mAh, then C/10=50mA, can it make Ra update?
What is the difference from (Design Capacity=1230mAh,C/25=49mA)?

2、What is the minimum discharge current supported by the TI algorithm to bring the RA updated?

Dominik Hartl11 said:

A8: It can be modified to C/25. But the gauge's analog front end and integer math doesn't support Ra updates for C/25 so this is not supported by TI.

3、Can it be amended to C/13 and what are the implications?

4、Can it be amended to C/15 and what are the implications?

5、Don't quite understand this statement, can you elaborate more?

Dominik Hartl11 said:

A9: Reserve capacity is subtracted from capacity predictions. It is not subtracted from Design Capacity. Design Capacity is a reference capacity. The gauging results do not use Design Capacity in a simple formula but instead the gauge models a discharge scenario. It will then subtract Reserve Capacity from the results.

6、Can you provide information or screenshots of the conclusions of the 37% test data? Purpose: We may modify the parameter "Min % Passed Chg for Qm", so we would like to know the steps and criteria to determine the accuracy of Qmax.

Dominik Hartl11 said:

A10: The basis is empirical data by the team that developed the algorithm. 37% DOD change was deemed the minimum acceptable limit for Qmax updates.

#1: Yes, if the current is above C/10, then Ra can update. It's not just about the absolute current 50mA vs. 49mA in your example. Besides having a reasonable current (the gauge uses a 1mA resolution and integer math so this limits accuracy if absolute currents are small), there also has to be a significant voltage drop OCV vs. loaded voltage and that is what makes a difference between your 500mAh and 1230mAh example. 50mA for the 500mAh cell will produce a larger voltage drop than 50mA for the 1230mAh cell (the latter will not produce enough voltage drop, hence the gauge can't measure Ra).

#2: It's C/10. There is no absolute limit. The Ra calculation will become less accurate for lower absolute currents. You can calculate an error for a specific case using integer math (e.g. if your true current is 1.5mA but the gauge measures 2mA (because of the resolution) and your true voltage drop is 10mV but the gauge measures 9mV, then you can calculate the error as Rmeasured = 9mV / 2mA = 4500mOhm with Rtrue = 10mV / 1.5mA = 6667mOhm).

#3 and 4: Yes, it can. See answer #2 how this will impact the Ra calculations.

#5: Remaining Capacity = Simulation Result - Reserve Capacity.

#6: No.

hello Dominik

The discharge current of our application does not satisfy the C/10 rule. So we need to modify the rule of discharge current C/10.

1. What does 'C' in C/10 stand for? What is the value corresponding to 'C' in our configuration?

2、Modify C/10 to C/13, how much error will RA have?

3、Modify C/10 to C/13, how much error in RA will lead to error in FCC?

4、Modify C/10 to C/15, how much error will RA have?

5、Modify C/10 to C/15, how much error in RA will lead to error in FCC?

6. To modify C/10, should modify the FS file? Can you help to provide the two FS files for C/13 and C/15?

#1: C stands for C rate in [h]. It's a way to specify load as a function of Capacity. C/10 equals a current of DesignCapacity / 10[h] (a current that would accumulate to Design Capacity in 10 hours).

#3-5: This depends on the chemistry and can't be determined with a simple formula. It would have to be analyzed separately for a specific cell and chemistry.

You are trying to "fix" the algorithm's configuration in a way that will break it, not make it work. These parameters are private for this reason. The gauge won't be able to measure Ra reliably for smaller currents than C/10.

Hello Dominik，

Since our equipment has a minimum discharge current of 100mA, please help us with the corresponding FS file for C/13. Thanks!

We need to do the verification to confirm if we can modify the parameter.