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Battery Management System

Basic Description

A Battery Management System (BMS) is an electronic system that controls the charge level and monitors the health of a battery. The main functions of a battery management system are:

  1. Protecting battery cells from damage
  2. Satisfying real-time vehicle energy demands
  3. Prolonging the life of the battery, and
  4. Maintaining an appropriate battery state.

Battery management systems monitor key operational parameters, such as voltages, currents, and temperatures, during charging and discharging of a vehicle's batteries. Lithium-ion batteries, in particular, require careful monitoring to ensure that they do not become over-charged or under-charged and that they don't overheat. In electric or hybrid-electric vehicles, the battery management system interfaces with other on-board systems such as engine management, climate control, communication and safety systems.

Basic Battery Management System

The image above shows a basic layout of a battery management system. This particular unit consists of an electronic control unit that monitors real-time conditions of the battery, and an equalizer that maintains the optimum charge necessary for proper battery operation. The ECU typically measures individual cell voltages, selected temperatures and battery current. It controls the battery cooler, heater, equalizer and charge disconnect switch. The equalizer keeps the battery pack in balance by charging the weak cells and/or discharging the strong cells. This helps to increase the battery life.

How the BMS system operates is entirely dependent on the type of battery it is programmed to protect and control. There are several advanced battery technologies under study for electric and hybrid/electric vehicles today. The two most common are Lithium-Ion batteries and Nickel-Metal Hydride Batteries. While both are plausible solutions, they both have their pros and cons. Lithium-Ion batteries for example have a very high power to weight ratio, are energy efficient, have a low self-discharge, and operate well even under high temperatures (although too high and they become unsafe and could cause fires or even explosions). Nickel-Metal Hydride Batteries on the other hand, have reasonable specific power capabilities (not as high as Li-Ion), have a long life cycle, and are safe and abuse tolerant (will not explode). However, Nickel-Metal Hydride Batteries have a very high self-discharge and are relatively inefficient at high temperatures. Both the Li-Ion and the Nickel-Metal Hydride Batteries share in one major drawback, they are currently relatively expensive. However, data shows that battery costs are steadily decreasing. For example, the price per kWh using Li-Ion was over $900 in 2010, whereas now in 2015, the price is under $500/kWh.

Temperature sensors, voltage sensors, current sensors
AC/DC fan motor, relays, alarm and instrument panel indicators
Data Communications
Adverc, Ashwoods Energy, Clayton Power, Delphi, Elithion, EV-Power, Frazer-Nash, Johnson Controls, Johnson Matthey, Lithium Balance, Metricmind, Navitas, Nuvation, Orion, Preh, Torqeedo, Tritium, Reap Systems, Valence, Vecture, Voltronix
For More Information
[1] Battery Management Systems (BMS),
[2] Battery Management Systems, LG Chem website.
[3] Battery Management Systems,
[4] Tesla Roadster (BMS) Battery Management System, YouTube, July 30, 2008.
[5] Battery Management Design for High Power Lithium Battery Stacks, John Day Automotive Electronics News, Aug. 11, 2009.
[6] Battery Management Systems Overview, YouTube, Oct 5, 2013.
[7] Battery Management System (BMS) for 750Ah Forklift Battery, YouTube, Jan. 24, 2014.
[8] What is a Battery Management System / BMS?, YouTube, Nov 19, 2014.
[9] Challenges in Battery Management for Hybrid/Electric Vehicles, EETimes Europe, Feb. 9, 2015.