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Goals
- Design a BMS that is robust to electrical noise found from sources in and out of vehicle
- The BMS is robust and reliable to vibration, elements, and users
- The BMS provides test points for scrutineering
- The BMS is calibrated appropriately
- Provides at least the minimal active protection requirements for the Li-ion battery pack
- Over-voltage
- Under-voltage
- Over-temperature
- Over-current (either direction)
- Transmit voltages, current, temperatures, and SOC to telemetry/display
- Consider manufacturability and flexibility
Background and strategic fit
The Battery Management System (BMS) is responsible for monitoring the state our Li-ion pack, ensuring that the battery is operating within its Safe Operating Area, calculating secondary data, reporting that data, and balancing it. It consists of Analog Front-End chips that allow the board to grab pack data, and then take appropriate actions.
MSXI's BMS was a single board that had 6 AD7280As and a hall-effect current sensor. This approach was difficult to manufacture and test, as bringing up a board was expensive and error-prone. It was also physically large, which made it difficult to fit in the battery box. Also, all the wires needed to return to the single board, making wiring a challenge.
Requirements
| # | Title | User Story | Importance | Notes |
|---|---|---|---|---|
| 1 | Minimize wiring | We'd like to reduce the amount of wiring that needs to be run back to the controller board. | Must Have | Look into splitting AFEs into their own boards |
| 2 | Flexible setup | We haven't fully decided on our battery box layout, so the voltage and number of AFEs required is subject to change. | Must Have | Daisy chained AFEs would make this nice |
| 3 | Cell balancing | It'd be nice to be able to support cell balancing. | Optional | We're building a new pack and we should be testing our modules to match capacity |
Questions
Below is a list of questions to be addressed as a result of this requirements document:
| Question | Outcome |
|---|---|
| What are we using for the AFE? | LTC6804 - 12-cell AFE, add a mux to support up to 12 temp sensors? |
| Daisy chained or addressable? | Daisy-chained - addressable would require per-board configuration |
| What architecture are we using? | Single controller board, carrier board reads current and uses LTC6820 isoSPI interface, AFE boards contain AFE, balancers, and thermistor inputs Master battery box contains controller board, slave battery box is connected by isoSPI |
| Active vs. passive balancing | Passive balancing - See follow-up discussion in BMS Architecture Discussion for details |
| How many cells should we balance across? |
No longer relevant? |
| What current sensor are we using? | LEM bidirectional hall effect sensor (looking into DHAB or similar)- Planning on using the HO 150-S with a 24-bit sigma-delta ADC. |
| Will we protect the aux battery? | Rev1 will have the aux protection/charging circuits on a separate board, may look into combining the boards. Note that the aux battery faulting technically counts as a battery fault, but it probably doesn't matter. |
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