BQ29209: <Related to:> BQ29209: Cell balance with 15 mA, when it is enough ?

Hi Stefano,

Your circuit looks a bit different than the datasheet recommended typical schematic. You may want to verify the state of the CB_EN pin during charging to verify cell balancing is enabled. You are also using different resistor values on VC2 and VC1, and VC1_CB. 

As far as balancing current - you can estimate the balancing time based on the capacity of your cells. 15mA is not a large amount of current, so it could be that balancing will take a long time if you have higher capacity cells.

Best regards,

Matt

Hi Matt, thank you for your answer.
I know the difference of resistor but this mean a difference of 0,5% in Icb2 current, I think this is not the issue.
CB_EN is low during the charge. 
Me too, seems little 15mA for a correct balance but in your other answer (see "BQ29209: Cell balance with 15 mA, when it is enough")  you sad :

" Since your charge current is low, 15mA is a big percentage of the charging current, so it should be able to balance cells effectively."

Let me know.

BR

Stefano 

Hi Matt, any feedback for this issue ?

best regards

Stefano 

Hi Stefano,

What is the cell capacity? In the previous thread, my thinking was that if the charge current is only 100mA and the cells are at a similar state of charge, the 15mA balance current is a big enough percentage of the charge current to keep the cells from becoming imbalanced. If however the cells are at a large difference in state of charge, it may take multiple charge cycles (total balancing time) to balance the cells. 

If you have verified that balance current is flowing as expected, you should be able to calculate the expected balancing time based on the capacity difference between the 2 cells.

Regards,

Matt

Hi Matt, as I write in my previous post:

- battery capacity 700mA/h (680-760mA/h)
- charging current: 100mA
- initial charging voltage: 3,7-3,9V

I've check the balance current with 2 batteries (3,5V and 4V) and the balance current is 8mA.

I repeat the discharge and charge sequence but the charge difference is increased.

What do you think ?

BR

Stefano 

Hello Stefano,

Matt is on holiday breaks and will be returning January 3rd. Responses will be delayed until then. I apologize for the wait. I will attempt to support as best as possible. 

Something I notice from your schematic, is that the CB_EN needs to be shorted to GND to enable balancing. If you want to disable balancing, you need to pull it high. As of now it seems that you can either leave it floating or pull it low. If it is floating it may or may not start balancing (The pin state is undefined between 1-V and 2.2-V). If you always want balancing, I would advice to simply short to GND. I don't believe this has nothing to do with this particular issue, but it is something to note.

Now, do the cells become this greatly mismatched after charging? What was the battery voltage before and after the charge?

Best Regards,

Luis Hernandez Salomon

Hi Luis,

As you said is not related to the issue but is interesting what do you think: I'am using the internal pull up, do you tink it isn't not shure this pull up ? Effectively is not showed ther value of this pull up.

However the enablig status is shure by Vin signal that became active during charge and became 0V when the charger is disconnected.

I,m using this circuit to avoid balancing during discharge and avoiding current loss.

The test is performed with two battery:

- 728mA/h@3,8V and 700mA/h@4.0 
- charging at 100mA for 3h

At the end of charge one battery is 4V and the other is 4,4; CB_EN is low during charge and more or less 8 mA is flowing in balance resistor.
What do you think ?

Thank an BR

Stefano 

Hello Stefano,

The internal pull-up is a high-impedance current source, it would not work well as a pull-up. I mentioned to use an external pull-up resistor in our previous e2e thread: https://e2e.ti.com/support/power-management-group/power-management/f/power-management-forum/1175228/bq29209-cb_en-pin-pull-up-resistor

I linked to a thread that talked more about the pull-up, the resistor symbol is not accurate and should've been changed.

Are you saying that you are using two different types of batteries? Different batteries may have different charge/discharge curves, which could also be affected by the age of the battery. You will want to use batteries that are closely matched as possible. Could you repeat the test with two batteries from the same factory and aged condition?

Best Regards,

Luis Hernandez Salomon

Hi Luis,

I see for the pull-up.

The batteries are the same makers and same capacity (700mA +-10%)  but the charge voltage is not garanteed and I chek some different charge condition.. If the charge is matched we have not balancing problem; if the cherge are different (200mV) the circuit is not able to compensate.

Best Regards

Stefano 

Hello Stefano,

Did you confirm that the cells are indeed balancing? If so, balancing of the cells works as long as there's a voltage difference less than 200-mV, but if the difference is larger, it is not able to balance. Correct?

Have you tested using external cell balancing (which would allow a larger balancing current), to see if the cells are able to become balanced?

Best Regards,

Luis Hernandez Salomon

Hello Luis,

I have check both internal and external balance and the current is flowing as axpected.

With the internal balance (no external mos): with 100mV of difference the batteries became balanced; with 200mV of differente are not able to balance

With the external balance (external mos): also  with 200mV of difference the batteries became balanced

Remember that the balance is enabled only during charge; what do you think about always enable the balancing ?

Best Regards

Stefano Sivera

Hello Stefano,

Always balancing ON may be okay as long as it is not uncharged for long periods of time.

It may be that the internal cell balancing current alone isn't sufficient, so using external cell balancing could be the best option. Would that be reasonable for you?

Best Regards,

Luis Hernandez Salomon