An Overview of the Battery Management System (BMS)

Battery Management System Definition

A Battery Management System also referred to as simply BMS is an electronic system that controls the charging of rechargeable batteries. It serves as both a regulator and monitor and performs these functions through varied mechanisms.  

In today’s world, electronic devices are taking over much of our everyday functions. And much of our move into a technologically advanced world is through battery advances. The efficiency of batteries has grown exponentially in just the last few years. Basically, a battery can give off more power than ever before.

To give an everyday example of how battery power has progressed, think of how your car battery works. A car battery is typically very clunky and heavy. However, with recent advances, it is possible to give your car a jump with a lithium-ion battery. This would use a battery system lithium ion. That battery itself is so small that you can likely hold it in the palm of your hand. And it doesn’t weigh more than a few pounds.

The uses of battery management systems are quite common and are used in nearly any device that makes use of rechargeable batteries. As a matter of fact, it is likely that you interact with products that use battery management systems on a daily, or near daily, basis. Common examples where a BMS is used include smartphones or MPS players. More sophisticated examples include cars, the most notable ones that run on electric.

Battery Management System Functions

There are several functions performed by a BMS. Some are quite simple while others perform a more sophisticated usage.

Overall, a battery management system can oversee and ultimately control a variety of factors. These include the following:

• Battery temperatures
• Overall health of the battery or batteries
• Charging rates
• Voltage of the main power
• Battery voltage

A BMS oversees all of these factors through various acts. Each of the BMS’s functions can be broken into the categories of monitoring, communicating, protecting, and computation.

Monitoring

A Battery Management System can monitor different facets of the battery’s overall state through a variety of ways:

• Temperature: temperature of the coolant intake, the output of the coolant, the temperature of individual cells, and the overall average temperature.
• Voltage: maximum and minimum cell voltage, overall total voltage, the voltage of periodic taps, and individual cells’ voltages
• State of power (SOP): how much power is available for a given interval of time with the current usage of power, overall conditions, and temperature
• Flow of coolant: for fluid or air-cooled batteries
• State of charge (SOC): Indicate the battery’s charge level
• Current: the current that moves in, out and through the battery
• State of health (SOH): Measures the capacity left over as it relates to the battery’s original power capacity

Communicating

The controller at the heart of the BMS communicates with its own hardware internally. It may also communicate with corresponding hardware externally. This hardware might include items such as a laptop. 

The communication that takes place could be very simple in nature and it might use a variety of different methods such as:

• Serial communication
• Wireless communication
• Serial communication over power-line
• CAN bus communications which are most prevalently used in environments that have to do with automotive

Protecting

One of the primary functions of a BMS is to protect its battery. It can do so through several different mechanism, the most common of which is making sure the battery does not operate outside of its safe operating area. Examples of what the BMS protects against include the following:

• Temperature that is too high or low
• Voltage that is too high during charging
• Voltage that is too high in discharging or charging modes
• Under-voltage
• Over-pressure

The BMS protects against these items through actions such as the following:

• Uses an internal switch which is opened if the battery stars to operate outside the designated safe area
• Controls the environment directly and actively. It does so through such items as fans, liquid cooling, heaters or air conditioning.
• Requests that the items using the battery end their use

Computation

A Battery Management System can calculate several different values including:

• Maximum charge and discharge currents
• Total number of cycles
• Charge stored or delivered
• Total energy and operating time expended since the very first use
• Energy expended since the last charge cycle or charge

Battery Management Systems can perform the simple tasks of measuring voltage or putting an end to charging when the voltage required has been met. In addition, a BMS can shut down the flow of power.

Battery System Topologies

A battery management system is usually made up of numerous functional blocks. These might include fuel-gauge monitor, state machine, real-time clock, cell-voltage monitor, cutoff field-effect transmitters, cell-voltage balance, and temperature monitors. The right architecture depends on a few factors including battery requirements and the goals you have for battery life. Battery management systems use technology that can be quite unique, and that overall varies quite substantially in complexity as well as performance.

There are several variations as to how the functional blocks are grouped. Some use a very straightforward analog front end while others use an autonomously running standalone integrated solution.

The topologies of Battery Management Systems are grouped into three categories:

• Distributed
• Modular
• Centralized

Distributed Topology

This topology uses just one cable to connect the battery with the controller. A BMS board is installed at each cell. This category of topology represents those that are the most expensive. They are also the easiest and most straightforward to install. In addition, their assembly is the simplest and cleanest.

Modular Topology

This BMS topology uses a few controllers. Each of them controls a designated number of cells. The communication takes place between the few controllers used. This topology is kind of a hybrid of the other two. It basically offers a mix of features, as well as drawbacks that make it a kind of middle choice. It is not the best but not the worst.

Centralized Topology

This topology uses just one controller, which is connected to the battery cells through a series of wires. This version is the most economical of the three categories highlighted here. They aren’t really able to be expanded upon and they have to use several different wires.

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How does the Battery Management System Work?

Through the many functions performed by a battery management system, there are essentially three primary objectives central to each BMS. They are:

• Extend the battery’s life
• Protect the battery from damage
• Keep the battery performing enough to fulfill its functions, whatever those may be.

The BMS achieves these objectives through its varied functions, the specifics of which will determine how that BMS actually works. For example, to extend the life of a battery in a power tool, communication is facilitated between the charger and the battery. The battery relays information related to its specification, the condition it is currently in, and its history of usage. The charger then takes that information and determines the ideal charging profile to help control its usage. 

Bottom Line

Battery management systems can be built using several different methods, techniques and functional blocks. They essentially manage a rechargeable battery and prevent it from working outside of its safe operating area. It does so through varied functions, all of which serve to monitor and regulate the battery.

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