Battery Management System

Battery Management System (BMS) controls the battery pack and declares the status of the battery pack to the outside world. An introduction to the BMS gives a high level overview and connections to the system.

The Battery Management System (BMS) is the hardware and software control unit of the battery pack. This is a critical component that measures cell voltages, temperatures, and battery pack current. It also detects isolation faults and controls the contactors and the thermal management system. The battery management system protects the operator of the battery-powered system and the battery pack itself against overcharge, over-discharge, overcurrent, cell short circuits, and extreme temperatures.

SoC from OCV lookup

SoC Estimation Techniques

A look at the estimation of State of Charge (SoC) using voltage profiling and coulomb counting. These two methods give a good overview of the difficulty and errors associated in estimating this critical battery parameter.

cells in series recharge

Cell Balancing

If we look at the simple case of 3 cells in series we can see one of the requirements for the BMS. A need to balance the Ah between cells in a series string.

cell balancing

Fundamentally there are two approaches to cell balancing.

  1. Passive balancing
  2. Active balancing


Illustration of general decentralized BMS system structure.

Master and Slave BMS

Decentralized BMS Architecture is split into one main controller and multiple slave PCB boards. The advantages of decentralized BMS are less wiring costs and highly scalable due to its modular design; while the drawbacks would be requiring more slave PCBs.

Centralised BMS

The centralized BMS has embedded all general functions (cell Voltage/Temperature/Series Current sensing, cell balancing… ) in a single control module/board, and was widely applied on smaller battery packs for commercial vehicles.

In the Cloud

Cloud BMS is critical for improving battery lifetime, charging, and safety. Despite next-generation battery chemistries emerging, current battery technology has room for growth. Intelligent software, advanced models, and better data analytics within cloud BMS can unlock potential performance gains. This technology is crucial for optimizing battery operations, ensuring safety, and advancing the field of battery management and electric mobility. 


The output is only as good as the input signals, hence the need to get the right ones of the right quality:

BMS Hardware Suppliers

There are a number of suppliers of BMS worldwide who design and build systems for single cell all the way up to complex managed BMS systems that can control very large grid based battery systems.

Functional Safety

  1. detailed analysis of the intrinsic hazards of lithium-based batteries
  2. hazard and risk assessment of the automotive lithium-based battery
  3. address the specific risks deriving from the automotive application
  4. safety goals to required to keep the battery under control
  5. safety requirements for the hardware design 

Emergency Shutdown

The main reasons for an emergency shutdown are:

  • Cell Temperature
  • Cell Voltage
  • Over Current
  • Impact
  • Disconnected HV
  • User request

Cell Balance and End of Life

At some point the cell balance or unbalance will be too big to be corrected by the balancing circuit in a reasonable time window. Once this unbalance gets to a point where the cells have less energy than needed to meet the basic requirements this will then be end of life for the pack. Or at the very least a repair process will be required where the failing cells are replaced.


  1. Lelie, M.; Braun, T.; Knips, M.; Nordmann, H.; Ringbeck, F.; Zappen, H.; Sauer, D.U. Battery Management System Hardware Concepts: An Overview. Appl. Sci. 20188, 534.
  2. Marcos, D.; Garmendia, M.; Crego, J.; Cortajarena, J.A. Functional Safety BMS Design Methodology for Automotive Lithium-Based Batteries. Energies 202114, 6942.

BMS Definitions & Glossary – an A to Z page BMS terminology.