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The major differences between the LTC6813 and LTC6811 can be summarized in the table below:

System Level Changes

There will not be any major changes to accommodate for the switch to an 18 cell battery monitor. The AFE connects directly to the battery module connector boards through connectors and cabling. No modifications are necessary to the board but the harness will need to be changed. Figure 1 and 2 shows a block diagram outlining the communication and harnessing between the AFE boards, BMS carrier, and the battery pack.

Figure 1: LTC6813 Block Diagram

Figure 2: LTC6811 Block Diagram

The LTC6811 features 12 cell taps while the LTC6813 features 18. This means that our current design, which requires 36 taps, can be reduced from three boards to just two but the boards may need to be expanded to accommodate the balancing network. Currently it is sized to 100 x 70 mm. The SPI bus communicating with the MUX still makes use of GPIO 3, 4, 5, leaving 6 extra GPIOs. However, a new way to account for the total thermistors is needed, as each AFE board monitor 30 thermistor cell measurements, for a total of 90 thermistors. Switching to two boards means that 30 thermistors will not be accounted for. We could accommodate this by utilizing two addressable MUX ICs (one 16 channel and one 32 channel for a 48 channel MUX) per board that would monitor 45 thermistor cells each. The extra GPIOs can be used for on-board thermistors so that the MUXs will monitor the cells exclusively. The conversion times of commands, such as the ADCV and ADSTAT, from both datasheets were found to take similar amounts of time, with differences in the range of microseconds, so there are minimal latencies in comparison. No further firmware changes are required as the LTC6811 has cross compatibility with the LTC6813. The major changes to the firmware would be the addition of the Group E and F battery cell commands for reading and balancing. Will need to talk to firmware to see how the driver for this will be implemented and how feasible this task is.

The daisy chain can be used for reversible IsoSPI but the benefit that this provides may not be helpful for our requirements This is because we would need to add an additional redundant path for the MUX which would require further modifications to the BMS carrier board, prolonging development time and debugging.

Cost

The cost breakdown will assume that the quantity of passive components will remain the same. Both systems will require the same number of thermistors, balancing resistors, and capacitors. Therefore, the price difference for the entire system will only need to look at the price difference of the different parts of the block diagram, namely the voltage regulator, MUXs, connectors, and ICs.

Cost

The cost breakdown will assume that the quantity of passive components of the balancing and voltage taps will remain the same. Both systems will require the same number of thermistors, balancing resistors, FETs, and associated capacitors. Therefore, the price difference for the entire system will only need to look at the price difference of the different parts of the block diagram, namely the voltage regulator, MUXs, connectors, and ICs.

Based off the following spreadsheet, the 12-cell layout will cost the overall AFE system approximately $86.76 while the 18-cell layout will cost $57.84, allowing us to save $28.92 on the entire system. The spreadsheet does not include the prices of major ICs such as the battery cell monitors or MUXs as there is the possibility for us to acquire samples through the manufacturers.The LTC6811 costs $34.93 while the LTC6813 costs $44.47. Also, the MUXs have yet to be chosen so their associated costs have yet to be determined. The cost breakdown does not include the connectors, though they are included in the spreadsheet, as final harnessing and the necessary connectors are undecided and can differ substantially in comparison to the current revision.

 Pros and Cons

Pros

Cons

LTC6811

  • Firmware development is mostly complete

  • More compact board sizes

  • Lower total measurement error (1.2mV)

  • More debugging and assembly required which can increase error likelihood

  • Increased isoSPI distances which can increase error, though impact may be negligible

LTC6813

  • Reduces the number of boards to two

  • Connections can be robust

  • Less assembly required

  • More GPIO pins available

  • Need to develop firmware for extra battery groups and MUX

  • Greater overall board size

  • Greater planning required for wiring

  • Higher total measurement error (2.2mV)

Overall, the LTC6813 can be a good alternative for the BMS AFE board for a future revision. It would introduce less error potential while also improving the costs by a considerable amount.

Gerald has commented that the firmware changes would not take long wrt register calls but may require more for balancing. Furthermore, it would be best to choose MUXs that are compatible with the current drivers if possible.

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