Skip to end of metadata
Go to start of metadata

You are viewing an old version of this page. View the current version.

Compare with Current View Page History

« Previous Version 9 Next »

BMS carrier is the board which is responsible for monitoring and managing the main battery pack. It interfaces with the AFE (analog front end) board and the Current Sense board to get readings from the main pack.

Other Resources:
FWXIV BMS Carrier notes

BMS AFE Description

Components

  • Current Sense - connected via I2C, and provides current readings and cell temperature,

  • AFEs - 3 AFE boards, daisy-chained over iso-spi. These provide per-cell-unit voltage readings and Ambient temperature readings, as well as performing load balancing

  • Relays + killswitch monitoring - Positive and negative relays must both be closed for the battery to be connected to the rest of the car. Killswitch monitor reads state of manual killswitch to batteries

    • HV Neg relay

    • HV Pos Relay

    • HV Solar Relay

  • Fans - Used to provide cooling to batteries. Controlled via PWM and monitored for fault

Fault Conditions

These are all fault conditions which will trigger a BPS fault. They are split into Minor, in which the issue is likely to not be dangerous and the driver can take time coming to a stop, and Major in which the driver should exit the car as quickly as possible due to dangerous conditions or the potential for thermal runaway.

Note: Cell refers to individual input AFE measurements, which in reality is several physical cells. Cell group refers to an area managed by a single AFE.

AFE:

  • Undervoltage - One Cell Falls < 2.5V (minor)

  • Overvoltage - One cell > 4.5V (Major)

  • Balance - Across entire pack, if difference between max and min cells is > 0.05V (Major)

  • Temperature - if any thermistor reads > 60deg (Major)

    • Note: If thermistors read > 45deg, open solar relay to disable charging, but this is not a fault

Current Sense

  • Overcurrent - reading > 58.2A (Minor)

  • Temp > 60Deg (major)

KillSwitch

  • Will shutoff batteries by itself, but we will treat it as a BPS fault (minor)

Operation

BMS carrier sends/receives messages, which help it start up and operate properly.

Startup

  1. Car turns on to power main or power aux, BMS board receives power from Aux

  2. On startup, do all initial checks that are possible

  3. If everything checks out, start transmitting BPS heartbeat to indicate successful startup

  4. Wait for message from Centre Console to close relays

  5. Close relays

  6. Continue to monitor and send BPS heartbeat messages, data, in normal operation

Shutdown

  1. Centre console (when power off is pressed) will send a close relays message

  2. Centre console will cut power to BMS via Power distribution once it has confirmed that relays are closed

Faults

AFEs

Current Sense

Fan Faults

Data Output

(this is just documentation on what was done in FW14, and is subject to change)

BMS Outputs the following over the CAN bus:

  • cell voltages

  • cell temperatures

  • avg current

  • avg voltage

  • relay states

  • fan states

AFEs

The AFEs (analog front end) handle interactions with the main battery pack. There are multiple AFEs (LTC6811 chips) which handle reading cell voltages, reading thermistors to determine cell temperatures, and doing load balancing for the cells. These multiple AFE units are daisy-chained through one SPI interface. This SPI interface is interacted with the same as normal spi, but through hardware is translated to isoSPI and back to SPI for improved reliability.

AFE Configuration

The AFE operation revolves around two main data structures.

typedef struct LtcAfeSettings {
  GpioAddress cs;
  GpioAddress mosi;
  GpioAddress miso;
  GpioAddress sclk;

  const SpiPort spi_port;
  uint32_t spi_baudrate;

  LtcAfeAdcMode adc_mode;

  uint16_t cell_bitset[LTC_AFE_MAX_DEVICES];
  uint16_t aux_bitset[LTC_AFE_MAX_DEVICES];

  size_t num_devices;
  size_t num_cells;
  size_t num_thermistors;
} LtcAfeSettings;

typedef struct LtcAfeStorage {
  // Only used for storage in the FSM so we store data for the correct cells
  uint16_t aux_index;
  uint16_t retry_count;

  uint16_t cell_voltages[LTC_AFE_MAX_CELLS];
  uint16_t aux_voltages[LTC_AFE_MAX_THERMISTORS];

  uint16_t discharge_bitset[LTC_AFE_MAX_DEVICES];

  uint16_t cell_result_lookup[LTC_AFE_MAX_CELLS];
  uint16_t aux_result_lookup[LTC_AFE_MAX_THERMISTORS];
  uint16_t discharge_cell_lookup[LTC_AFE_MAX_CELLS];

  LtcAfeSettings settings;
} LtcAfeStorage;

The settings struct holds all the configuration for the hardware being used with the AFEs, including the number of AFEs, number of cells, and number of thermistors. The cell_bitset and aux_bitset indicate which of the total set of cells/thermistors we should read as inputs.

The storage struct holds the information from the settings, as well as memory for all of the readings taken.

AFE Readings

AFE readings are taken via the daisy-chain method, meaning that a read of a certain register on one AFE will be replied to with the values of all of the AFEs connected. (For example, the reading of the voltage register A of one of the AFE will be replied to with x 64 bit readings, where x is the number of AFEs connected)

Cell Sense

Cell sense readings happen in ltc_afe_impl_read_cells. Essentially, we iterate through the number of voltage registers (4) and for each we read the 3 uint16_t voltages and one 16-bit Packet Error Check from each of the AFEs at this register. These results are then placed at the correct position in the cell_voltages array based on the cell_bitset and cell_result_lookup

Aux Sense

Aux sense, used for the thermistors, is done on a per-cell basis. Essentially, each read is done for an individual input thermistor. Due to the daisy-chain, this result will be the width of readings*x afes.

This reading must be initiated repeatedly for the number of thermistors connected to the afes

AFE FSM

The LTC AFE FSM is responsible for handling the different states required for generating cell sense and a data (cell voltages) as well as aux data (thermistor readings). Once it is initialized, it will start reading cell sense values from the ltc6811, processing these values, and then doing the same for thermistor values in a continuous cycle.

LTC AFE Driver FSM Structure:

States:

  • LTC_AFE_IDLE (TBD)

    • Only needed if we need to execute error functionality while not running commands on the ltc6811

  • LTC_AFE_TRIGGER_CELL_CONV

    • Trigger conversion of voltage values

    • Output:

      • Send command to start conversion of cell voltage values

    • Input func:

      • Check to see if 10ms has passed

      • Transition to read cells

  • LTC_AFE_READ_CELLS

    • Output function:

      • SPI exchange to read/store all cell values, store in results array

      • Check for faults in readings

      • Input function transitions to aux conversion assuming no faults

  • LTC_AFE_TRIGGER_AUX_CONV

    • Output:

      • Send command to start read temperature values of a cell (we should maybe change to set of cells)

    • Input

      • Transition to read aux if 6 ms have passed

  • LTC_AFE_READ_AUX

    • Output:

      • Read current cell, increment cell number

    • Input

      • if cell number == number of cells, aux is done reading → transition to complete

      • else transition back to trigger aux conversion for next conversion

  • LTC_AUX_COMPLETE

Fault Behaviour:

Just transition to Idle for now. We will signal main BMS if an error occurs

Transitions:

LTC_AFE_TRIGGER_CELL_CONV → LTC_AFE_READ_CELLS

LTC_AFE_READ_CELLS → LTC_AFE_TRIGGER_AUX_CONV

LTC_AFE_READ_CELLS → LTC_AFE_READ_CELLS

LTC_AFE_TRIGGER_AUX_CONV → LTC_AFE_READ_AUX

LTC_AFE_READ_AUX → LTC_AFE_TRIGGER_AUX_CONV

LTC_AFE_READ_AUX → LTC_AFE_AUX_COMPLETE

LTC_AFE_READ_AUX → LTC_AFE_READ_AUX

LTC_AFE_AUX_COMPLETE → LTC_AFE_TRIGGER_CELL_CONV

Transitions continue in this loop format for the entirety of the time that BMS is operational.

The states should also be able to transition to and from LTC_AFE_STATE_IDLE if this is how we decide to operate our error states.

AFE Discharge/Load Balancing

Based on the results received, we iterate through and determine what the minimum and maximum voltage values are per cell. We are checking for a difference

  • No labels