3. Overview

Hereafter a general overview of the content of foxBMS can be found.

The foxBMS Master Unit consists of 3 boards: BMS-Master Board, BMS-Interface Board, BMS-Extension Board. Two ARM-based microcontroller units (Cortex-M4) are used on BMS-Master Board: MCU0 (also called primary MCU) and MCU1 (also called secondary MCU). The BMS software runs on MCU0, while MCU1 is used for redundant safety.

MCU0 communicates with the outside world via a CAN bus (CAN0). The current flowing through the battery system is measured via a current sensor connected via the CAN bus. The sensor is controlled via CAN by MCU0 and sends the resulting measurement via CAN.

The foxBMS Slave Unit (BMS-Slave Board) are used to measure cell voltages and cell temperatures in the battery modules. The foxBMS Slave Units are linked via a proprietary daisy chain from the company Linear Technology (i.e., isoSPI).

In order for the foxBMS Master Unit to communicate with the foxBMS Slave Units, an interface board (i.e., BMS-Interface Board) is needed. It converts the SPI signals from the BMS-Master Board into differential signals used by the daisy chain and vice versa.

Three power contactors are used to connect and disconnect the battery modules (or pack) from the load:

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

These contactors are driven by MCU0. Requests are made via CAN to the system to open and close the contactors. Based on the measurements and the algorithms implemented 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.

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

In case more inputs and outputs and further functions are required, an BMS-Interface Board is available and can be easily adapted to specific needs.

This description reflects the current state of foxBMS. Due to the open nature of the system, many other possibilities can be implemented, like for example:

  • Use of other types of current sensors (e.g., shunt-based or Hall-effect based)
  • No foxBMS Slave Unit needs to be used: a direct measurement of the cell voltages and cell temperatures can be performed by the foxBMS Master Unit
  • A higher number of contactors can be controlled (e.g., up to 9)
  • etc…