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Purpose

Measure and balance cells, monitor cell temperature.

Monitoring IC

The LTC6811 is the same IC that was to be used on MSXIV, so that we can leverage our existing firmware stack and greatly reduce development time.

Power Supply

LTC6811 required power rails:

  • V+ >= voltage of cell string (12s * 4.25V = 51V)

  • Vreg = 5V

Previous vehicles had V+ supplied directly from the cells, and a buck to generate 5V from V+. This technically violates regs as it is a load connected to the pack before the primary fuse and main relays, although it was accepted in the past.

The simplest method found to bring our system in line with regs is to supply the AFEs with isolated 5V from carrier using a multi-output isolated supply, and boosting 5V to 51V for the V+ pin.

Supply Noise Sensitivity

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).

Thermistor

Connection

Low side because blah

Power Supply

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.

 Changes 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

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