Ricardo Generic Battery Management System Designed to Provide Flexibility to OEMs

15 June 2009

A battery management system is a key component of advanced automotive electric energy storage systems. Engineering firm Ricardo has developed a generic battery management system (BMS) for Li-ion batteries that is independent of cell size or chemistry. The effort is designed to help OEMs in response to the wide variety of cell-level Li-ion chemistries, with different cost, reliability, life, safety and availability factors. Overall, the BMS is designed to be universal—suitable for other energy storage systems such as NiMH or ultracapacitors as well as Li-ion batteries.

The availability of such a generic BMS could reduce the cost to OEMs of changing a cell supplier or even cell chemistry. It would provide flexibility for dual sourcing—e.g., if a current supplier could not meet production volumes—and would reduce risk by more cost-effectively supporting future changes. Dr. Peter Miller, Director of Ricardo’s Electrical/Electronic Engineering efforts, provided an overview of the project at the recent Advanced Automotive Battery Conference 2009 (AABC 2009) in Long Beach.

Ricardo’s approach is to create a tools set to allow rapid pack design from cell data, and allow rapid calibration of the BMS from cell data. It is seeking to design the BMS to adapt to cell production variation and aging to minimize issues in production or in the field.

Ricardo’s BMS comprises multiple Voltage Temperature Balancing Modules (VTBM) and a Battery Control Module (BCM).

• VTBM boards measure the voltage and temperature of each cell and do cell balancing, with up to 16 cells supported for each board. VTBM boards can do active or passive balancing. They are self-powered from cells being monitored.

• The BCM measures pack current and pack insulation resistance and reads cell data from VTBM boards. The BCM communicates to the vehicle via CAN.

Testing results have show the VTBM cell temperature sensor measurement circuits are accurate within 0.5 °C. Cell voltage measurement shows accuracy <±0.4mV across the the full range.

On the tools side, the cell/pack data analyzer loads test data from a cell or pack tester and creates an accurate model capturing key characteristics. The pack designer uses data from the cell data analyzer to select the best series/parallel combination of cells that meets a number of constraints, including pack weight/volume; max/min voltages; cell SOC/voltage limits; and temperature limits. The designer either finds the smallest number of cells that meet the constraints(i.e., the cheapest) or the maximum Wh possible within the constraints (i.e., max electric range).

The cell/pack analyzer can directly create the information needed by the BMS, thereby reducing the time required for calibration. The best way to calculate SOC is automatically selected from a range of methods including using cell/pack voltage at rest; using cell/pack voltage in operation; and integrating pack current. Changes in the cells within the pack are automatically compensate for. This corrects for aging and unmodelled behavior. Active and passive balancing allows the best use of the cells.

Ricardo applied its toolkit to designing a replacement pack for the Efficient-C vehicle—a full diesel hybrid (earlier post). The replacement pack uses a new chemistry for the cells, and switches from cylindrical to pouch formats.