General Background

 

The most common BMS architectures are: distributed, centralized and modular. Contemporary BMSs, regardless of system architecture, provide the following core functions:

  • high accuracy cell voltage measurement
  • cell/module temperature measurement
  • redundant, comparator-based, voltage monitors
  • cell balancing hardware
  • cell balancing algorithm for cell charge equalization
  • current sensing
  • under voltage, over voltage and over temperature fault handling
  • contactor control
  • insulation measurement (for safety)
  • State-of-Charge (SOC) algorithm
  • State-of-Health (SOH) algorithm
  • current limits/discharge and charge control

 

While there are other functions, these are the big hitters and the modular BMS that Infortronix has built provides the user with the components and tools necessary to accomplish all of these functions in a variety of ways depending on their needs.

At the highest-level, the differences between distributed, centralized and modular architectures are driven by differences in how the cells are monitored and balanced. More specifically, which integrated circuits (ICs) are used to monitor cell voltages, cell/module temperatures and balance the cells and how are they related or interconnected cell-to-cell (remembering these systems often handle 100s and even 1,000s of cells arranged in series and parallel to each other.)

Notes:

  • As stated above, battery packs are often formed not only by connecting individual cells in series to increase/sum voltage but by connecting parallel strings of cells in series to increase voltage, pack current carrying capability and energy capacity.  For diagram and description simplicity the BMS architectures are described using the term cell or referring to individual cells connected in series.  For pack and BMS architecture construction purposes individual cells connected in series are synonymous with parallel strings of cells connected in series.
  • Balancing refers to a mechanism through which charge is dissipated from an individual cell or transferred between cells. Balancing refers to dissipative or passive removal of charge by switching a resistance across a cell’s positive and negative terminals using a transistor. Cell balancing or cell charge equalization is critical over time to maximize usable capacity.

 

Distributed BMS Architecture

In a distributed system, there is a dedicated monitoring and balancing (circuit) board (MBB) attached to each cell and all communicate to a system controller (BMSC) via a digital communications bus (e.g. CAN, SPI, I2C) as shown in the figure below.

Figure 11: Distributed BMS ArchitectureDistributed BMS Architecture

The circuits are generally simple, but as there is a dedicated balancing board for each cell, the overall cost can be high and some cell types are difficult if not impossible to mount to and integrate against in a clean and robust fashion.

For these reasons, OEM BMS and battery pack manufacturers DO NOT use distributed architectures.

 

Centralized BMS Architecture

In a centralized system, there is a single, monolithic system controller which has all of the hardware and software features necessary to perform required BMS functions. This means wires need to be run from the negative terminal of the bottom/first cell and the positive terminals of all cells, as shown in the figure below, back to the BMSC.

Figure 12: Centralized BMS Architecture

Centralized BMS Architecture

Beyond wire routing and labeling complications, this system architecture promotes cell connection wires of various lengths; as it can be difficult to keep the BMSC centrally located to all cells. Wire length variance (and potentially architecture-unique long lengths) can lead to:

  • cell voltage measurement inaccuracy
  • increased cell voltage measurement calibration complexity
  • increased losses and voltage drops during balancing operations
  • increased likelihood of interference and/or connection issues

 

For these reasons, OEM BMS and battery pack manufacturers DO NOT use centralized architectures.

 

Modular BMS Architecture 

The Infotronix BMS is an example of a modular architecture. In this architecture MBBs which can support multiple cells are distributed throughout the pack and connected to the BMSC via a digital communications bus as shown in the figure below.

Figure 13: Modular BMS Architecture

Modular BMS Architecture

BMS functional distribution (across the BMSC and MBBs) is similar in this architecture as was documented for the distributed architecture. Cell connections are once again short, lending to more accurate and reliable (in terms of noise immunity and electrical connection robustness) cell voltage measurements and overall operation. Support for multiple cells per MBB offers per-cell cost reduction through part consolidation. For example, all cells can share a common high precision analog-to-digital converter and isolated power and communication circuits.

In this type of system the MBBs implement the first four to five core BMS functions listed at the beginning of this section:

  • high accuracy cell voltage measurement
  • cell/module temperature measurement
  • redundant, comparator-based, voltage monitors
  • cell balancing hardware
  • cell balancing algorithm for cell charge equalization

The remainder of the functions listed are handled by the BMSC in a coordinated fashion via the digital communications bus.

In the modular system, MBBs with increased on-board memory and processing capabilities and isolated CAN communications can be offered at a per-cell or per-channel cost similar to that of less capable MBBs associated with competitor distributed or centralized systems. Not only does the additional “horsepower” and CAN communications provide application flexibility, the Infotronix MBBs can operate as part of a modular system or as a standalone BMS depending on user needs, but this architecture is the same as adopted by OEM electric vehicle manufacturers.

This is evidenced by inspecting the various battery teardown reports found online, including those for the Tesla Model S and Chevy Volt. In fact, the Infotronix MBBs use the exact same cell monitoring and balancing IC as is used in the Tesla Model S  Texas Instruments’ BQ76PL536A-Q1.