BQ76940: Cell Balancing for 13s,14s

Pranit Pawar

Part Number: BQ76940

Hi,

I've implemented external balancing on BQ76940 using P-ch MOSFETs. I'm planning to use odd-even type of cell balancing ie. balancing all odd numbered cells at once, first, and then balancing all even numbered cells at once.

This odd-even balancing isnt an issue when 15 cells are used. However, when 13 or 14 cells are used, how will the cells be numbered?

Looking at the 'Top 10 Design Considerations' app note, it looks like adjacent VCx pins of same group must not be turned on simultaneously. Lets consider the case of 13s BMS. Pins VC8-VC9 and VC13-VC14 will be shorted. So the cell numbering+type will be

Cell1: VC1 - odd
Cell2: VC2 - even
Cell3; VC3 - odd
Cell4: VC4- even
Cell5: VC5 - odd

Cell6: VC6 - even
Cell7: VC7 - odd
Cell8: VC8-VC9 - even/odd?
Cell9: VC10 - even/odd?

Cell10: VC11 - odd
Cell11: VC12 - even
Cell12; VC13-VC14 - even/odd?
Cell13: VC15- even/odd?

In the above case, in group two can I balance Cell6-Cell8 at once and then Cell7-Cell9 at once?
Similarly in group three, Cell10-Cell12 at once and then Cell11-Cell13 at once?

over 6 years ago

0 Matt Sunna over 6 years ago

TI__Guru* 81034 points

Hi Pranit,

The important thing is to balance every other cell in a group (not every other VCx pin). So, in your case Cell8 is even, Cell9 is odd. Here is another thread that also addresses this question: e2e.ti.com/.../701628

Best regards,
Matt

0 Pranit Pawar over 6 years ago in reply to Matt Sunna

Intellectual 520 points

Alright! So a short is considered as one cell

0 Pranit Pawar over 6 years ago in reply to Matt Sunna

Intellectual 520 points

which bits of CELLBAL2 should I set to enable balancing of C8 and C9 in my case? Same question regarding C10-13

0 Matt Sunna over 6 years ago in reply to Pranit Pawar

TI__Guru* 81034 points

Hi Pranit,

The BQ76940 has no way to know how things are connected externally, so the bits in the CELLBAL registers correspond to the VCx pins. For the shorted pins, you can use either of the corresponding bits CB8 or CB9 for Cell 8. CB10 for Cell 9, etc.

Matt

0 Pranit Pawar over 6 years ago in reply to Matt Sunna

Intellectual 520 points

Matt, just to verify, 'adjacent cells' means physically adjacent cells and not adjacent VCx pins right?

And, "don't balance adjacent cells" means don't turn on balancing for physically adjacent cells, right?

In my case, to balance cell 8, I have to set either of CB8, CB9 bits of CELLBAL2 register.

Tbh, I'm really worried about cell balancing.

0 Matt Sunna over 6 years ago in reply to Pranit Pawar

TI__Guru* 81034 points

Pranit,

Yes, your understanding is correct. The best way to verify the controls is to connect an oscilloscope and observe the cell balancing waveform as you enable the bits.

Matt

0 Pranit Pawar over 6 years ago in reply to Matt Sunna

Intellectual 520 points

Alright will do that Matt! I'll post the results in a week

0 Pranit Pawar over 6 years ago in reply to Matt Sunna

Intellectual 520 points

Hi Matt,

A thought came in my mind. VCx pins are either pulled up or pulled down depending on their CBx state (0 or 1).

In my case, VC8 and VC9 are shorted. So if I write a 1 to CB8 and 0 to CB9, VC8 will be pulled up to VCC and VC9 will be pulled down to GND. Now because both the VC pins are shorted,
it means that VCC and GND will get shorted. Isnt that a problem? Even if I write a 1 to CB8 and leave CB9 as it is, by default, CB9 will be 0. This will again short VCC and GND.

To avoid a short, shouldn't a 1 (or 0) be written to both the shorted VC pins' corresponding CB bits?

0 Matt Sunna over 6 years ago in reply to Pranit Pawar

TI__Guru* 81034 points

Hi Pranit,

The CB bits control internal FETs that connect the VCx inputs to allow the cells to balance. For example, VC8 and VC7 are connected together when the CB8 bit is set. When the CB9 bit is set, it connects VC9 to VC8 which is okay because they are also already connected on your board.

You also need to be careful on the external connections since you are using external balancing. A couple of good resources are Section 4 of the Top Design Consideration app note (www.ti.com/.../slua749a.pdf) and there is a 13S reference design using BQ76940 in which VC8 and VC9 are shorted as well as VC13 and VC14 - www.ti.com/.../TIDA-010030. This reference design should have many similarities to your board.

Best regards,
Matt

0 Pranit Pawar over 6 years ago in reply to Matt Sunna

Intellectual 520 points

Yes it works just like Matt said!

Although VC8-VC9 are shorted, setting CB8 bit doesn't affect the state of CB9 bit. To balance cell 8, I set the CB8 bit, left CB9 as it is.

All 13 cells are balanced :)

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BQ76940: Cell Balancing for 13s,14s