BQ40Z80: Cell 7 Measurement Safety issue

Klaus

Part Number: BQ40Z80
Other Parts Discussed in Thread: BQSTUDIO, BQ34Z100

Hi Experts,

we are using the bq40z80 in a 7S1P (Samsung SDI INR18650-25R) Applicaiton since about one year.
We have delieverd about 3500 BMS since there.

From this thread

I know that there are some inaccuracies with the measurment of the cell 7 for the Lifetime-Data-Collection.

From this thread

I know that the measurement of the 7th could cause some gauging errors.

My question now is, can the inaccurate measurement of the 7th cell lead to safety problems, like a unintended triggering of Cell over voltage (COV), or something else?

Thanks for helping us.

Klaus

over 4 years ago

0 Kang Kang over 4 years ago

TI__Mastermind 24870 points

Hello Klaus,

We have removed the 7th cell from the bq40z80.

0 Kang Kang over 4 years ago

TI__Mastermind 24870 points

Hello Klaus,

We have actually removed the 7th cell from the bq40z80.

0 Klaus over 4 years ago in reply to Kang Kang

Expert 1431 points

Hi Kang,

thanks for your two answers.

My question in other words:

Is it possible that the less accurate measurement of the 7th cell could unintended trigger the COV event?

Thanks for helping us.

Klaus

0 Kang Kang over 4 years ago in reply to Klaus

TI__Mastermind 24870 points

Hello Klaus,

It may happen depending on what you set the COV to.

We wouldn't know unless you capture it in a bqStudio log event.

Thanks!

0 Klaus over 4 years ago in reply to Kang Kang

Expert 1431 points

Hi Kang,

thanks for your answer.

Here are my settings for COV:

The Charging Voltage from the Charge is 29,4V or 4,2V per Cell.

Untill now I have not capture it in the bqStudio. I think it is not so easy.

Is this a good setting or not?

Thanks.

Klaus

0 Klaus over 4 years ago in reply to Kang Kang

Expert 1431 points

Hi Kang,

in the past I have made a measurement with a fully relaxed battery.

I have made following thread:

In the following chart you can see the Voltage of Cell 7 and Cell 4 :

You can see that cell 7's measurement is noisier than cell 4's.

Also you can see the measurement has spikes with a voltage change of about 35mV.

Between 2 measurements is a time gap of 4 seconds.

Does this mean if the one cell is charged to about 4220mV and we get a spike of about 35mV for longer than
3 seconds, we get a COV event?

The complete log you can download from the thread mentioned in the beginning of the post.

Thanks for helping us.

Klaus

0 Kang Kang over 4 years ago in reply to Klaus

TI__Mastermind 24870 points

Hello Klaus,

Yes, this is part of the reason we removed 7S support on bq40z80.

If you need to do a 7S design, the only option we have is bq34z100.

0 Klaus over 4 years ago in reply to Kang Kang

Expert 1431 points

Hello Kang,

ok. For new designs we should do that, but what should we do with the designs that have been in the field for about a year?

Thanks.

Klaus

0 Rushi_Dalal over 4 years ago in reply to Klaus

TI__Intellectual 2025 points

Hi Klaus,

If you already went to production in 7S design we will continue to support, but otherwise for new designs we recommend considering an alternative TI solution for 7S battery gauging or monitoring like mentioned above.

Thanks,

Rushi

0 Klaus over 4 years ago in reply to Rushi_Dalal

Expert 1431 points

Hi Rushi,

thanks for your answer.

At this point of the project I think we can not change the BMS. As I mentioned above we have delivered about 3500 BMS.
They have all certification they need.

Klaus

0 Kang Kang over 4 years ago in reply to Klaus

TI__Mastermind 24870 points

Hello Klaus,

Unfortunately, there's no good work-around for the 7th cell measurement accuracy.

Power management

Power management forum

BQ40Z80: Cell 7 Measurement Safety issue