BQ20Z70: Cell Balancing and Qmax Cell

Sherard Yu

Intellectual 260 points

Part Number: BQ20Z70

Hello,

I'm now trying to generate the golden file for 3S3P battery pack, 7800 mAh, 11.1V typ.

I have two concerns about the work.

When I charge and discharge the battery pack, I can see more than 150mV of the different voltage level gap between maximum and minimum.
- This should be less than 100mV, what parameters I need to check in order to get this target value?
- I set the Min Cell Deviation to 1314, = dQ *R / (V * duty cycle) = 3600*540/(3700*0.4).
The values of Qmax Cell 0/1/2/Pack kept changing to the overcapacity of the battery pack.
- Initially, I set the values of each parameter to 7800 (mAh), and the values were changed to 8445/8456/8450/8445.
- What can cause this change?

Thanks,

Sherard Yu

over 6 years ago

0 Batt over 6 years ago

TI__Mastermind 42535 points

1. When you dsg, at deeper levels of dsg, you can't control the voltage delta between cells as that's dependent on the relative strength of the cell. However, on charge once balancing is turned on, this can be controlled.
2. Qmax is the theoretical capacity of the cell, it's dependent on load, avg I last run etc. If you avg dsg rate is low, then you can see a higher value of Qmax.

0 Sherard Yu over 6 years ago in reply to Batt

Intellectual 260 points

Hello Batt,

Thanks for your response.

1. I've observed the unbalanced charging voltage levels between the cells. What should I take a look at?

2. My understanding is that the value of Qmax would be updated automatically by bq20z70, right? Is it safe to set the nominal capacity of a battery pack for the golden file?

3. What is "Cell x Chg dod at EoC"? I couldn't find it from Configuring the bq20z70_slua405.pdf.

Best Regards,

Sherard Yu

0 Batt over 6 years ago in reply to Sherard Yu

TI__Mastermind 42535 points

1. You should see if CB is set in the registers. If it is, then your cells are being balanced by the gauge when the delta exceeds threshold in df.
2. That is correct. Your design cap is the nominal cap of the battery. Your Qmax though, can be updated by the gauge.
3. DoD at EOC stands for depth of discharge at End of Charge. It's a parameter that we monitor when the gauge reaches charge taper termination to set an appropriate point for starting gauging measurements. It's calculated per cell.

0 Sherard Yu over 6 years ago in reply to Batt

Intellectual 260 points

What is CB? Can you provide the exact name of the register?

Thanks,
Sherard Yu

0 Batt over 6 years ago in reply to Sherard Yu

TI__Mastermind 42535 points

CB stands for cell balancing. It's bit 6 in Charging Status.

0 Sherard Yu over 6 years ago in reply to Batt

Intellectual 260 points

I could see that CB in Charging status was set during charging, but the the voltage difference was +200mV between min and max.
How can I remedy this?

Best Regards,
Sherard Yu

0 Batt over 6 years ago in reply to Sherard Yu

TI__Mastermind 42535 points

At the end of charging or beginning of charging?

0 Sherard Yu over 6 years ago in reply to Batt

Intellectual 260 points

Until one out of 3 cells was reached to ~4.2V, there was 200+ mV of the gap between min and max.
I could see that the gap was disappeared at the end of charging.

Regards,
Sherard Yu

0 Batt over 6 years ago in reply to Sherard Yu

TI__Mastermind 42535 points

If your delta goes away at end of charge that is good. It means balancing is working correctly. The gauge calculates DOD delta and adjusts timers correctly. If you are using the ref design then your balancing parameters are correct. If not, please refer to www.ti.com/.../slua420a.pdf for balancing with ext resistor.

Cells won't be balanced during dsg.

0 Sherard Yu over 6 years ago in reply to Batt

Intellectual 260 points

Thank you for your confirmation of the cell balancing is working correctly.

Best Regards,

Sheard Yu

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BQ20Z70: Cell Balancing and Qmax Cell