Qmax update in impedance track fuel gas gauges bq20z75v181

Jason,

The published 8500mAh Qmax is a design target for the pack. The device measures the actual Qmax during its' learning cycle. Qmax will be recalculated during future learning cycles and the value will drop as the cell ages.

Regards

Tom

Hi, Tom

I don't think it's a reasonable explanation.   The errors is about  7%. (8500/9100=93%) I want to verified in theory and calculation.  From that, we can guarantee the product possibly.

Below is process of Qmax learing which is described above. 

8764.X300.rar

SBS logging data is from the step 2(send the command "0021").

Best regards

Jason

Jason,

Thanks for sending the data files. I reviewed them and have some concerns with the results. The .gg files show that the learned UpdateStatus = 0D, which is not a valid value. It also looks like the Ra-Table was not updated, so I think that something sent wrong with your learning cycle. I checked  the .log file and did not see any discharge current applied  to the pack during the learning cycle. I recommend that you re-initialize the device by reloading the .senc file  and rerun the learning cycle.  This  is the best precedure that  I have used.

When no close Qmax value is known:
1. Even though accurate capacity is unknown, set Qmax and design capacity to the cell capacity shown in the data sheet. Do not send IT enable command 0021.
2. Send SMB command 46, word 0006 to turn on the FETs.
3. Discharge the pack down to termination voltage at a C/5 rate.
4. Let it relax for 5 hours or more.
5. Send the IT enable command (0021). Bits QEN and VOK in Operation Status are set.
6. Charge pack to full capacity until the taper current is reached and the FC bit is set.
7. Let it relax for 2 hours. Qmax is learned at this point; this can be verified by MaxError changing to 3% and Update Status to 5%.
8. Discharge the pack down to terminate voltage at the typical low rate of your application. For laptops, it is usually C/5. The MaxError changes to 1%, and the Update Status changes to 6%.
9. Repeat steps 4 and 6 to 8 one more time to verify the gas-gauging accuracy.

I hope that this helps.

Best regards,

Tom

Hi, Tom

The process in fact is below:

    1) Discharge to terminal voltage(6V), wait 5 hours

    2) Send the command "0021" 

    3)Charge the battery pack with 8.4V/3A .   3.5 hours

    4)wait for 5 hours......................This step is let the IC-20z75 to update the Qmax..

    5) Charge the battery pack to full status.

The data is logged from step-2 to step-5..Within the step-4, I think the Qmax is updated as the MaxError is changed to 3%.

 1)There is a question about that  the update status is changed to 0D not 05. I changed the critical data font color in red.7266.X300_NO_Close_Qmax_golden pack_2_Jan_2012.xls

Another about the Ra-table. The Ra-table is updated during the discharge mode and in several points about the RSOC (every 11.1%).  The Ra variables is updated based on the Qmax at typical point(eg. 77.7%). I think the Ra variables fill the gap with model and real voltage.I have one question here:

1)R_an and R_anx

what's the real difference between them? Can you give me a full explanation about them. I can't find that in the application notes. How to check the values of Ra-table?

Best regards

Jason

Jason, I discussed the UpdateStatus issue with one our firmware engineers and he is going to have to try to find out how the update to 0D can occur. It looks like two QMAX updates may have occurred simulanteously, so something went wrong with your learning cycle. You should probably try re-initializing the device and trying again. The .log file shows that you did not discharge the pack after the QMAX update, so this may have prevented the Ra-table from updating. The device has two Ra-tables. (R_an and R_anx) One of them is redundant and it is used while the other one updates. The device then switches over to the updated one after the Ra-table update has completed. Best regards Tom

Hi, Tom

    Thanks for you to deal with the problem..Wait for good news..

    As you mentioned about the discharge cyles.  The update status is changed to 0E during the relax mode after discharged. But the Qmax is quit bigger than the real cells' capacity. Why? I want to verify the result. Please tell me the details. If you need the logged data, just tell me. I will  re-do it for you...If that, please tell me the test process.

Best regards

Jason

Jason, If the Ra-table was updated and the UpdateStatus changed to 0E, then the pack has completed learning successfully and is ready to use. I believe that you mentioned that the device calculated approximately the same QMAX using two different learning techniques, so I would believe it to be accurate. We have seen customer's cell capacity to be measured higher than what was published by the manufacturer. This could be be due to improper cell characterization or improper chemistry. I recommend that you run the pack through a few charge and discharge cycles and see if the the gas gauge reports properly. QMAX may drop some after a few cycles of usage as well. Best regards, Tom

I would also like to add that the Design Capacity (printed on the battery or in the cell datasheet) is usually a MINIMUM spec based on a typical current (C/5, for example).

The Qmax used by Impedance Track is the theoretical chemical capacity of a cell/battery at NO LOAD.  Therefore it is quite normal for Qmax to be larger than Design Capacity.