BQ77915: Cell Balancing circuit

Ravi Kumar Sutrai

Prodigy 120 points

Part Number: BQ77915

Hi TI team,

Need support on cell balancing circuit values. My max charge current is 2.1A from external charger.

1. Please refer below schematics, I have used RINI = 1.5K, CINI = 0.1uF. Can i use these values?

2. For ESD protection, instead of capacitor I have used TVS diode (part number : VLIN1616-02G-E3-18).

My pack max voltage is 14.6V. I have used TVS diode with Working value of 16V.

Thank you,

Regards,

Ravi

over 1 year ago

0 Max Verboncoeur over 1 year ago

TI__Mastermind 25980 points

Hi Ravi,

A few comments:

The recommended value for R2 is 1k.
The recommended value for the input resistors when using internal cell balancing is 33-Ohms.

With 1.5k, you're going to get next to no balancing current. ( 3.2V / (1.5k+1.5k+12) = ~1mA ), internal FET resistance is 12-Ohms typical.
Keep in mind the abs max balancing current for one cell is 50mA, so the absolute minimum resistor value you can use for a 3.2V cell is ~26-Ohms.
( 3.2V / (26+26+12) = 50mA )
The recommended value for the input filter capacitors when using internal cell balancing is 1uF.
The TVS diodes should be fine to use.

Regards,

Max Verboncoeur

0 Ravi Kumar Sutrai over 1 year ago in reply to Max Verboncoeur

Prodigy 120 points

Hi Max Verboncoeur,

Thanks for your email.

I have few more questions.

1. I am using external charger to charge my LiFePo4 cells. The charger specification 14.6V, charge current 2.1A. Do I need external cell balancing or Internal cell balancing? Please confirm.

2. When cell balancing will perform ? while charger connected or after charger removed.

Regards,

-Ravi.

0 Max Verboncoeur over 1 year ago in reply to Ravi Kumar Sutrai

TI__Mastermind 25980 points

Hi Ravi,

1. The required balancing current is decided by the end application. If your applications requires more than 50mA of balancing current, then you will need to implement external balancing circuits. This previous thread explains in a little more detail (it's for a different device, but the premise is the same): (+) BQ79606-Q1: how to select the battery Balance current? - Power management forum - Power management - TI E2E support forums.

2. I recommend watching this video, which explains how the cell balancing algorithm for the bq77915 works: https://www.ti.com/video/5976525922001.

Regards,

Max Verboncoeur