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Hi
I’ve posted a few times about designing a BMS for a LiFePO4 battery (BQ78350-R1: bqStudio is not showing cell voltages, BQ76930: Design help requested for BQ76930).
I’m now at the stage where I could really use some help with the detailed design. As was suggested during an earlier post on this forum, I went and purchased the BQ76930 Evaluation board (and BQ2400 interface module). After a few false starts, this is now running and I’m able to query and set registers as well as observe the fuel gauge behaviour when the board is connected to my battery pack.
Here are the details of my 8s8p battery pack.
Constructed using Headway 38120 cells which have the following specifications:
10C discharge
Voltage: 3.2V nominal, 2.5V min, 4.2V max
Charging Voltage: 3.65V
Cut-off Discharge Voltage: 2.0V
Charging Method: CC/CV
Maximum Discharge Current (continuous): 100A
Maximum Charge Current (continuous): 50A
Using the above values, the 8s8p battery pack has the following characteristics:
Voltage: 25.6V nominal, 20V min, 33.6V max
Charging Voltage: 29.2V
Cut-off Discharge Voltage: 16.0V
Charging Method: CC/CV
Maximum Discharge Current (continuous): 800A
Maximum Charge Current (continuous): 400A
Battery Pack is nominal 24V, 80AH.
The largest current draw will be a starter motor that draws approx. 200A. (4.8KW), so I'm thinking I should design the BMS to handle 300A.
Now that I’m designing my BMS, based on what I have understood from using the evaluation board, I have some questions.
Cheers,
Mike
Hi Mike,
You are correct that the external FETs are needed for higher cell balancing currents. The BQ76930 internal cell balancing current is not very high, so most of the reference designs also use external FETs.
Since this is very high current, the sense resistor selection is very important to make sure the power can be handled. Some users use multiple sense resistors in parallel for higher currents.
I'm not sure if your design will include the BQ78350, but if you are using this controller, you will need to download the BQ78350-R2 firmware which includes current scaling. The BQ78350-R1 maxes out at around 32A.
We can always help review the schematic when you're ready. I'm not sure if I pointed you to this already, but this video may be helpful thinking through the steps in the schematic: https://training.ti.com/how-create-schematic-bq76920-bq76930-and-bq76940
Best regards,
Matt
Hi Matt,
Thanks for your comments and reading suggestions.
I'll work through the tutorials and take up your offer to review my schematics as they progress.
Cheers,
Mike
Sounds good, Mike. I will go ahead and close this thread. If you have more questions or have a schematic to review, you can either start a new thread or reply to this one and it will reopen.
Best regards,
Matt
Hi Matt
As indicated in my earlier post, I am now working on the design for my BMS. I would appreciate your feedback on the wiring of my cell connections shown in the attached schematics. Although I will most likely not use the built-in balance FETs, I am trying to understand the simpler circuitry before I move on to having external balancing FETs.
I've attached 2 partial schematics, with the focus solely on the battery cell connections for the purpose of this post. The first schematic shows a 10 cell battery while the second shows the changes I've made for my 8 cell battery, namely the shorting of cells 3,4 and cells 8,9.
Does this look correct?
Thanks for your feedback,
Mike
Hi Mike,
Looks good! The cell connections for reducing the cell count are correct.
I noticed there are two capacitors from VC0 to GND, so you can remove one of these.
There also needs to be a diode from VC5 to VC5x and from VC10 to BAT, but I know you may not have gotten to this part of the schematic yet.
Best regards,
Matt
Hi Matt
Thank you for your feedback and confirmation that I am on the right track with this design.
I hadn't noticed the unwanted cap, so thanks for spotting that. My next step is to introduce external balancing FETs, so I will study how these operate with the BQ76930 and once I've added these to the design I'll share it with the forum and look for feedback.
Thanks very much for your ongoing support, its much appreciated.
Cheers,
Mike