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Forest Zhou

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Previous iterations of the AFE had a buck -> LDO supply for Vreg for better noise performance. The datasheet makes no mention of doing this and it appears not to be required as the IC has it’s own internal LDOs to use for the ADC reference (Vref1). At -75dB PSSR, nearly 900mV AC is required on the V+ pin to cause a 1 bit change, thus it’s determined that the LDO is not needed.

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Boost Controller

Boost converters with integrated FETs are pretty pricey, so we have decided to go with a discrete solution instead. Two low cost options were considered: the HV9150 hysteretic mode controller and MCP1632 PWM controller.

image-20240221-225741.pngImage Addedimage-20240221-230044.pngImage Added

Thermistor

Connection

Low side because blah

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Powered off 3V0 VREF2 supply from LT6811. Estimated current consumption for 8x thermistors (10k pullup + 3k thermistor @ 60C) is 1.84mA. VREF2 is expected to droop < 400ppm, which will cause 400ppm error, well within the acceptable range for temperature measurements. VREF2 is only used to drive thermistors, the internal ADC is referenced off VREF1.

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Change Log

Revision 2.0

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titleChanges from MSXIV

List of possible changes:

  • Change of IC - ex two 18 cell or one 36 cell BMS

  • Smaller pack won’t need as many thermistors

    • Is there any way of checking/approximating how many we will actually need? If we are changing this we also may want to change out the multiplexer that the thermistors work with. (probably not necessary as we can just tie them to down to ground or check with code)

  • Active balancing? (a challenger vehicle may benefit more than cruiser) (maybe stick to passive imo)

    • From MSXII:

      • After more research, we've found that most EV manufacturers (including Tesla) only use passive balancing. This whitepaper from Li-Ion BMS is a good comparison between active and passive cell balancing.

        Overall, it seems like the practical benefit between active and passive cell balancing is minimal for our situation, as we're planning on building a matched pack and will be charging for an extended period of time. Since we'll be charging whenever we're coasting and we're looking at 5kW chargers, we should have plenty of time to use a slower balancing cycle. If we're able to implement an SOC-based balancing algorithm, we'll be able to balance anytime we want instead of just at the top of the charge curve, reducing the required cell balance current further. This whitepaper on balancing current is interesting. The biggest concerns with active balancing are that hardware and software are complex and expensive, and that it may not even be necessary depending on cell tolerance, which is tested in every shipment.

      • Resources:

      • https://www.analog.com/en/technical-articles/active-battery-cell-balancing.html

      • https://www.nxp.com/docs/en/application-note/AN4428.pdf