Technical Specifications

Overview of the Technical Specifications


foxBMS Master and Slave Overview


Battery System Block Diagram

Hereafter a general overview giving the technical specifications of the foxBMS ecosystem is presented. The complete detailled documentation is available on Read The Docs.

The foxBMS consists of 4 board types:

  1. BMS-Master Board
  2. BMS-Extension Board
  3. BMS-Interface Board
  4. BMS-Slave Board

The battery modules (or pack) connect and disconnect from the load via three power contactors (i.e., relays):

  • Main Contactor Plus
  • Main Contactor Minus
  • Pre-charge Contactor

MCU0 drives these contactors. Requests are made via CAN to the system to open and close the contactors. Based on the measurements and the algorithms in the software, MCU0 decides if the contactors should be closed or opened. It sends information via CAN so that the user knows the state of the system.

An interlock line is also present. If it is opened, either by MCU0, MCU1 or somewhere else (e.g., emergency stop), all contactors will immediately open.


foxBMS Master Unit Technical Specifications

foxBMS BMS-Master Board

The BMS-Master Board implements two ARM-based microcontroller units (ARM Cortex-M4F MCUs): MCU0 (also called Primary MCU) and MCU1 (also called Secondary MCU). The BMS software runs on MCU0, while MCU1 provides the required redundant safety during the software development process.

MCU0 communicates with the outside world via a CAN bus (CAN0). A shunt based current sensor connected to the CAN bus measures the current flowing through the entire battery system. MCU0 controls the current sensor via the internal CAN and sends the resulting measurements over the external CAN.

A secondary ARM-based microcontroller is present on the BMS-Master, called MCU1 (or secondary MCU). It monitors the Slave Units via a second and independent daisy chain. Like the MCU1, it can open the interlock in case something goes wrong with the system.

Parameter / ConnectorDescription
Board Size160mm x 120mm
Operating Temperature Range-40°C to +85°C
Power Supply12V-24V
150 mA @ 12V
110 mA @ 24 V
Primary USBGalvanically isolated USB connector to flash the MCU0
Secondary USBGalvanically isolated USB connector to flash the MCU1
CAN0First galvanically isolated CAN interface.
Contactors (6x)6 contactors with auxiliary contacts acting as contactor state feedback can be connected to the BMS-Master
MCU0ARM Cortex-M4 STM32F429IIT6 (prima-
ry)
Runs the applications.
Can open the interlock.
MCU1ARM Cortex-M4 STM32F429VIT6 (sec-
ondary)
Used for redundant safety software, has a dedicated daisy-chain to communicate
independently with the Slave Units.
Can open the interlock.
Memory256kB EEPROM with ECC for the storage of safety critical battery parameters
4kB of Backup-SRAM storing battery state parameters and diagnostic messages, supplied by a battery coin cell.
8MB SDRAM

foxBMS BMS-Extension Board

In case more inputs and outputs and further functions are required,

Because of the smart partionning developed for foxBMS, the BMS-Extension Board provides more inputs and outputs, thus enabling further functions and rapid adaption to specific needs.

This description reflects the free and open source version of foxBMS. Due to the open nature of the system, many other possibilities exist, like for example:

  • Use of other types of current sensors (e.g., shunt-based or Hall-effect based)
  • Use of no foxBMS Slave Units: the BMS Master Unit performs a direct measurement of the cell voltages and cell temperatures
  • Control of a higher number of contactors (e.g., up to 9)
Parameter / ConnectorDescription
Board Size160mm x 120mm
Operating Temperature Range-40°C to +85°C
CAN1Second galvanically isolated CAN interface.
RS485 (1x)Galvanically isolated differential RS485 series interface to be used with RS485 compliant equipment.
Isolated GPIOs (8x)Galvanically isolated general purpose inputs (4x) and outputs (4x).
Contactors (3x)Additionnally to the 6 contactor connectors on the BMS-Master, 3 more contactors with auxiliary contacts acting as contactor state feedback can be connected to the BMS-Extension.
NOC (6x)Galvanically isolated normally open contacts
Analog Inputs (5x)Analog inputs to sample voltages (not isolated galvanically)
Memory CardConnector on which a NAND Flash based memory card can be connected in SPI mode

foxBMS BMS-Interface Board

An interface board called BMS-Interface performs the signal conversion between the SPI interfaces of both microcontrollers on the BMS-Master and the foxBMS Slave Units. The BMS-Interface converts the SPI signals from the BMS-Master Board into differential signals used by the daisy chain and vice versa.

foxBMS BMS-Slave Board


foxBMS Slave Unit LTC6811

The foxBMS Slave Unit (BMS-Slave Board) measures the cell voltages and cell temperatures in each battery module. The foxBMS Slave Units connect to each other via a proprietary daisy chain from Analog Devices (former Linear Technology) called isoSPI. In the following table, you can find the detailed technical specifications:

Parameter / ConnectorDescriptionDescription
Monitoring ICLTC6811-1
(formerly LTC6804-1)
LTC6813-1
Communication interface between BMS-Slave and BMS-Interface on the Master UnitisoSPI (proprietary differential daisy-chain communication interface from Analog Devices, formerly Linear Technology) at 1Mb/s with functional isolation up to 1600Vdc working voltage

Currently not enabled for reverse isoSPI
isoSPI (proprietary differential daisy-chain communication interface from Analog Devices, formerly Linear Technology) at 1Mb/s with functional isolation up to 1600Vdc working voltage

Enabled for reverse isoSPI to provide a backup path in case a daisy-chain connection is broken
Cell voltage measurements12 cell voltage sensing in the 0-5V range18 cell voltage sensing in the 0-5V range
Voltage measurement accuracy±2.8mV on cell level (in filtered mode with 16bit sampling in the -40°C to +125°C temperature range)

±0.25% on module level
±4.2mV on cell level (in filtered mode with 16bit sampling in the -40°C to +125°C temperature range)

±0.35% on module level
Temperature measurements8 temperature sensors (NTC 10kOhms) preconfigured (8 more can be added via implemented multiplexer for a total of 16 temperature sensors per BMS-Slave)8 temperature sensors (NTC 10kOhms) preconfigured (8 more can be added via implemented multiplexer for a total of 16 temperature sensors per BMS-Slave)
Measurement speed20ms for a pack with 8 modules with 12 cells per module (i.e., 50Hz cell voltage refresh rate with a pack with 96 cells) which corresponds to 2.5ms per module with 12 cellsTo be investigated (expected: approximately 3.75ms per 18 cells module)
Power supplyAt least 11V are needed if supplied only by the module itselfAt least 16V are needed if supplied only by the module itself
Number of cells that can be monitored with one BMS-Slave4 to 12 cells if the module self supplying is used
1 to 12 cells if the external power supply connection is used
6 to 18 cells if the module self supplying is used
1 to 18 cells if the external power supply connection is used
Optional external power supply voltage range (instead of using the battery module energy)12V-24V12V-24V
BalancingPassive balancing (36 Ohms / 100mA) using discrete PMOS and with global opto-coupler feedback functionPassive balancing (36 Ohms / 100mA) using discrete PMOS and with global opto-coupler feedback function
Module local data storage256kB EEPROM M95M02 (2Mbit, I2C interface) with ECC function storing min and max values of cell voltages, temperatures and other log data256kB EEPROM M95M02 (2Mbit, I2C interface) with ECC function storing min and max values of cell voltages, temperatures and other log data
Current consumption at 25°C<50mA (ON) ; <30µA (SLEEP)<50mA (ON) ; <30µA (SLEEP)
Fuse protection in case of wrong connection in the laboratoryEvery sense line (13) and power line (2) is fused (15 fuses)Every sense line (19) and power line (2) is fused (21 fuses)
Functional safety levelsConcept allowing adapted designs to reach safety levels up to SIL3 / ASIL-D / DAL-B even in 24/7 applications, including fault detection and compensation (these functional safety levels are not reached with the free online versions!)Concept allowing adapted designs to reach safety levels up to SIL3 / ASIL-D / DAL-B even in 24/7 applications, including fault detection and compensation (these functional safety levels are not reached with the free online versions!)
Additionnal functionsMultiplexers and port expander offering more flexibility for your applicationsMultiplexers and port expander offering more flexibility for your applications