BQ77915: Beginning new on-shore sourced BMS design. Would like some assistance if available

Erick Miller

Part Number: BQ77915
Other Parts Discussed in Thread: BQ34Z100-R2, BQ79616, BQ78706, BQ76952

Tool/software:

Hi there, I'm a new member of a very small team creating large tactical battery storage systems, and I was hoping to get some guidance towards products for my design. I'm pretty new to battery pack BMS design as well, so I'd love to be shown some good design resources on the topic.

I will be designing a BMS capable of protecting and balancing a few hundred 18650 Lithium battery cells stacked in a 14 cell series. This first design should be as simple as possible, so I was looking at the option of building a BMS based on the stackable bq77915. I was also thinking of using a separate fuel gauge chip to indicate state of charge and track state of health.

The current discharge will be quite large, about 150A draw, so I figure I'll need to use balanced power MOSFETs to facilitate all of that current.

Any resources on BMS design, Safety, or better chip set recommendations would be much appreciated!

Thank you!

7 months ago

0 Luis H. S. 7 months ago

TI__Mastermind 42060 points

Hello Erick,

For fuel gauge, you could use the BQ34Z100-R2. For protection/balancer, are you looking to read from it, or you would just like protections? The BQ77915 can work for protections. But we do not recommend to stack too many of them.

For stackable devices, I would advice to look at the BQ79616 or BQ78706. These are stackable devices with a robust daisy chain communication interface. These also have ASIL-B functional safety.

However, what is your system's architecture? A block diagram may be useful to understand how we can help!

Best Regards,

Luis Hernandez Salomon

0 Erick Miller 7 months ago in reply to Luis H. S.

Prodigy 10 points

Thank you for the part recommendation, Luis! I'm working on a block diagram now, so keep an eye out

0 Erick Miller 7 months ago in reply to Luis H. S.

Prodigy 10 points

Hi again Luis! Sorry for being slow to get this to you, I've been struggling with some sickness the past couple of weeks. Attached is an image of what I'm proposing for this V0

Some notes and questions:

1. I'm now considering the BQ76952 for this design, hopefully forgoing the complications of stacking cell management ICs. Thoughts?

2. After looking at the fuel gauge you recommended, BQ34Z100-42, I'm pretty convinced that it would be a good choice for this design

3. Since each cell stack of the design will have a maximum of 48 18650s in parallel, I figure the cell balancing will require external FETS. I checked out SLAUAA81A for suggestions on cell balancing, but I couldn't find much information on FET selection for extremely paralleled battery banks like what we're building. Some guidance here with reference material would be very helpful

4. The design that we're replacing with this BMS has discharge and charge gate FETS sized and rated for 50A, 12 FETS paralleled and balanced for a max output rating of 600A. I'm planning on using a similar topology using SLPA020 as reference, but more information on FET biasing would be much appreciated.

5. Please point out any questions, comments, concerns as I'm tackling this solo. I would also love some guidance on system testing for something like this.

Cheers, and thank you!

0 Luis H. S. 7 months ago in reply to Erick Miller

TI__Mastermind 42060 points

Hello Erick,

Understood and no worries! Hope you are feeling a bit better now.

Erick Miller said:
1. I'm now considering the BQ76952 for this design, hopefully forgoing the complications of stacking cell management ICs. Thoughts?

Now that I see your block diagram, I do definitely believe the BQ76952 would be best! If you want

Erick Miller said:
3. Since each cell stack of the design will have a maximum of 48 18650s in parallel, I figure the cell balancing will require external FETS. I checked out SLAUAA81A for suggestions on cell balancing, but I couldn't find much information on FET selection for extremely paralleled battery banks like what we're building. Some guidance here with reference material would be very helpful

Yes, I agree that with that many cells in parallel external cell balancing would be the best thing. The BQ76952 offers both host balancing (So you could have your own balancing algorithms) or autonomous cell balancing, where it performs voltage-based balancing based on your specific requirements.

FET Selection may depend on your cell balancing requirements. Additionally, because of the way cell balancing works, a MOSFET with a low enough turn-on voltage is required. For reference however, you can look at the schematic of the reference design, which uses the DMG2302UK-7 for external balancing FETs

Erick Miller said:
4. The design that we're replacing with this BMS has discharge and charge gate FETS sized and rated for 50A, 12 FETS paralleled and balanced for a max output rating of 600A. I'm planning on using a similar topology using SLPA020 as reference, but more information on FET biasing would be much appreciated.

I've referred to the SLPA020 many types before, so a good one to read! Another application note that will be useful here is the Multiple FETs with the BQ76952, BQ76942 Battery Monitors document. We have tested up-to 12 FETs in parallel as well with the BQ76952 and gives some good explanations as to how the driver works. Another application I find useful (although it is for a different type of device), is the Best Practices for Board Layout of Motor Drivers app note, this has some useful tips that can apply to many types of design and also provides MOSFET layout recommendations that I find are useful.

---

I believe looking at the reference designs is a good point to begin understanding the part, and looking at the EVM User Guide to see how it works. If you can get an EVM, you would also be able to configure a device and play with it to understand it better. Another good resource is the (+) [FAQ] BQ76952: Common questions for TI's BQ769x2 Family of Battery Monitors - Power management forum - Power management - TI E2E support forums FAQ post, which has some common questions and points to all of our training videos and documentations.

Do let us know if you have any more questions .

Best Regard,

Luis Hernandez Salomon