BQ27750: Gas Gauging

Did you run a learning cycle where the gauge measured the (scaled) impedance? How does Ra change over the 4 cycles? What is your ChemID?

Dear Dominik, I have ChemID = 0x1636. 

I have not information at datalog about Ra.

I didn't run the "special" learning cycle, just made a few cycles of full charge/discharge without interrupt.

Does it enough for learning system or necesasry to go at special learning  mode by I2C command or something like this?

Best regards, Danil

The gauge will update Ra automatically in regular use. A learning cycle is just a well controlled charge/relax/discharge/relax cycle. If the gauge hasn't learned Ra yet, then you don't have to do anything via I2C.

http://www.ti.com/lit/an/slua777/slua777.pdf

Please read the Ra table and attach it to this thread. I'd like to compare this to the default from ChemID 1636

Ok, please give me time to get information about Ra.

But please answer for a two important questions:

1) Is it possible if I calibrate current with factor ( 1:2 ), at this case corelation part of IT algorithm ( depend of Ra = deltaU/I )  make a wrong calculation?

2) If gauge has already updated the information about Ra, how to start the update again?

Best regards, Danil

#1: It will calculate scaled impedance, which will result in the same IR drop with your scaled current so this is usually not an issue.

#2: The gauge will continuously update Ra but with different filter settings between first and subsequent updates. You can manually change Update Status to 0x04 to reset this.

#1: You know BQ27750 there is not "Scale Factor" register ( as opposed to BQ78350)? How algorithm to know about I_real = I_measuring *2  and as result Ra_measuring = Ra_real/2 ?

#2: Thank you!

There's no scale factor here.

Dear all, I made "clean" test, - used original firmware file "bq27750_v0_05_build_13.srec", not changed any parameters and made learning cycle (charge/relax/discharge/relax):

1) with 1:1 current calibration

2) with 1:2 current calibration

Result:

Impedance Track™ algorithm make correct calculation in both cases!

Resume:

I was wrong! Really Impedance Track™ independent of current calibration!

Now my opinion: According to "The Impedance Track Theory" document the 
OCV correction is based on the impedance from the table Ra and calculated by formula: OCV` = OCV-I * R.
And Ra table is updated based on the current and the difference between the actual voltage and the voltage of the corresponding DOD (from table):

R(DOD) = ( V – OCV(DOD,T) ) / I.

If a current with the same calibration factor is used in these calculations, 
as a result calibration factor does not matter.

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#1: But why Ra table on batteries with the different calibration factors is the same???

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1:1 current calibration Ra Table after learning cycle 

 1:2 current calibration Ra Table after learning cycle 

Dear Dominik, above I show Ra table after 4 cycles ( two types calibration ) Please your comment! I do not understand why the Ra tables has the same values? Maybe that not update. Chemisty of my last test CH_ID = 0x3142.

Thank you!

Did the gauge set the RDIS bit at any time during your discharge test? What was your load (in C rate) during the learning cycle?

RE: BQ27750: Gas Gauging

I found RDIS at IT Status Register.
So, if  Ra Table is not update by reason of  negative sign impedance calculation, - for my opiniom to better disable update Ra, becouse calculation is perfect without  this correlation part of the algorithm.

#:Q_2: How to use only integration part?

Dear Dominik Hartl11,  RDIS bit was reset at any time of discharge test (Bit = false, green color).

Load was 0.08C.

How to guaranteedly disable the update Ra table?