BQ77915: Cell balancing current limit

Shivani Ghorpade

Part Number: BQ77915
Other Parts Discussed in Thread: BQ76940, BQ76930, CSD13381F4

Hello,

Thank you for your response, It solved many of my open queries. But I have one more question here regarding the cell balancing current.

As you mentioned, the cell balancing current of 20mA in your example and we can use internal cell balancing in this case. But In my case, my battery pack is 10Ah with charge current of 0.5C which is 5A. So if I consider the termination current of 10%, my balancing current shall be 500mA which is high for internal cell balancing and external cell balancing FETs shall be used. Please correct my understanding if wrong. The example considers 0.05C charge current which is very low. Is this feasible.

Regards,

Shivani Ghorpade

over 4 years ago

0 Matt Sunna over 4 years ago

TI__Guru* 77793 points

Hi Shivani,

For higher balancing current, external cell balancing FETs are needed. The power dissipation would be too high for the internal FETs at higher currents.

You might be able to refer to one of the TI Reference designs for the BQ76930 or the BQ76940 for example circuit implementations using external cell balancing FETs (these references may be helpful even though they are for a different device).

Best regards,

Matt

0 Shivani Ghorpade over 4 years ago in reply to Matt Sunna

Prodigy 190 points

Hello,

How do I select these external FETs. what are the recommendations.

The datasheet just mentions that these MOSFET must be selected with threshold voltage less than 1.7 V.

Can you brief over this...?

Regards,

Shivani Ghorpade

0 Matt Sunna over 4 years ago in reply to Shivani Ghorpade

TI__Guru* 77793 points

Hi Shivani,

This thread might be helpful https://e2e.ti.com/support/power-management/f/196/t/793033. It also points to an application note (for a different device) that gives some detailed discussion on the external balancing circuits: http://www.ti.com/lit/an/slua749a/slua749a.pdf

There are also a few TI Reference Designs that use external balancing FETs that might be helpful (search for BQ76930 and BQ76940 to see 5-6 different reference designs). For example, https://www.ti.com/tool/TIDA-00449 uses the BQ76930 with CSD13381F4 for the external balance FETs.

Best regards,

Matt

0 Shivani Ghorpade over 4 years ago in reply to Matt Sunna

Prodigy 190 points

Hello,

If my balancing current is 500mA as specified in the initial query, what specifications are recommended for external FETs and the resistors, capacitors.

Kindly request to guide.

Regards,

Shivani Ghorpade

0 Matt Sunna over 4 years ago in reply to Shivani Ghorpade

TI__Guru* 77793 points

Hi Shivani,

You will want to find a FET with a Vgs threshold less than 1.7V and higher than 500mA drain current. The example I pointed you to before (www.ti.com/.../TIDA-00449 uses the BQ76930 with CSD13381F4 for the external balance FETs.) meets these requirements.

There are many FETs available and I am not a FET expert. Other things to consider are the Rdson at the target Vgs.

Matt

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BQ77915: Cell balancing current limit