Electric vehicles _ Episode 4 - Battery Management Systems

In this video, we are going to explain the role of the battery management system,
also referred as BMS, in efficiently managing and controlling the batteries.
Hello, I'm Trung Bui. I'm a research fellow in Energy Innovation Centre at WMG.
Hello, I'm Kai Xu, a research fellow in Energy Innovation Centre at WMG.
Lithium-ion batteries are popular for a number of applications,
including electric vehicles and energy storage systems because of their high power density,
low cell discharge and relatively low cost.
However, individual battery cells alone cannot provide enough power to operate, say, a whole energy storage system or support an electric power grid.
So what we do is, we combine many cells together in series and parallels to create a module
and then connect many modules together to create a pack.
At pack level, lithium-ion battery performance becomes difficult to manage because the cells
can get charged and discharged at varying rates and can be operating at different conditions
due to their different operational states in terms of temperature, state of charge and state of health.
Complex electronics control system, known as the battery management system, is required
to monitor charge rates across the whole pack up to cell level to ensure peak performance and prolong the battery life.
Cells come in various formats which have different characteristics, cylindrical, pelt, coined and prismatic.
A module is formed by connecting multiple cells together, providing them with a mechanical
A pack is formed by connecting multiple modules together with sensors and a controller, all housed within the case.
Now that you understand about the cells, modules and pack, let's take a look at the key functions of a battery management system.
Within a battery pack, when multiple cells are connected in series, the cells with the
The lowest power will restrict the performance of the entire string.
Any failure in a single cell can adversely affect the overall capacity of the series.
The same is true of the cell's thermal characteristics.
Depending on their state of charge, cells in the module will be experiencing different temperatures,
affecting the performance of the pack as a whole.
There are two important aspects, which are the key elements that make up an efficient
battery management system, charge balancing and thermal management.
We are now going to explain each one in detail, starting with thermal management.
Lithium-ion batteries each have an acceptable operating temperature range.
A lithium-ion battery operating outside of its safe temperature range will cause performance
degradation and irreversible damage to the cells.
In extreme cases, it can even cause thermal runaway excessive overheating of the cell and possible combustion.
A battery management system controls the temperature of the battery through heating and cooling.
Temperature sensors are installed in the pack to provide cell temperature information.
The battery management system uses this information to distribute coolant where it's needed to maintain the ideal temperature range.
Common coolants used include air, water or glycol, dielectric oil and refrigerant.
Now let's look at passive and active balancing and why this matters.
Due to manufacturing inconsistencies, even brand new cells of the same type have slight
differences like capacity, impedance and self-discharge rate.
This causes the cells to age at different rates, exacerbating those differences and
resulting in significant energy imbalance among the cells in a module, hence the degradation of its performance.
Let's look at an example.
If we have a battery module with several cells in series, the current that flows through
each cell is the same during its operation.
If one cell has charge capacity lower than others, it will become fully charged before other cells.
Equally, when the battery is being discharged, some cells still have charge left in them and are never fully discharged.
This causes a waste of energy and reduces the module capacity.
Passive balancing is a conventional way to address this issue by removing excess energy in cells.
The system drains all of the cells to the same point so that they recharge fully and at the same rate,
but passive charging is not helpful when the battery is being discharged
as the capacity of a module is still limited by the weakest cell.
Since the excess energy is dissipated as heat on external resistors, an additional cooling system is needed.
This is why we need active balancing.
Active balancing is a more energy efficient way to balance cell energy compared with passive balancing.
It redistributes the energy among cells rather than dissipate and waste it.
Power electronics devices are used to move energy from the strong cells to weak cells,
maximizing the available energy and increasing the effects of capacity of the module.
Here is an example of one form of active balancing system, a cell to external storage topology.
The cells in series are connected to an external power source via bidirectional DC to DC converter.
Each cell can be charged and discharged individually to exchange energy with the external source.
To balance the cells, we take the energy from the strong cells in the module,
temporarily store it in the external storage and then give this energy to the weak cells.
Now that we have looked at the different functions of a battery management system, let's look
at the active balancing system and show you an example of the hardware.
There are several commercially available active balancing circuit designs.
However, they are not necessarily suitable for airing application and there is still
research to do to refine the technology.
Using this rig designed by WMG, there are five cells connected in series to emulate a battery module.
The bottom three cells are used as the external power source.
The TREM1402 power electronics board with switching metrics and bidirectional DC to
DC converter embedded is used as the balancing circuit.
A TI microprocessor board is used to control the power electronics.
We can also test experimental advanced control and optimization algorithms in the virtual world using simulation.
The COM communication module, which stands for Controller Area Network, in this system
allows communication to high-performance computers and rapid control prototyping units like Deep Space Galaxy.
This helps researchers and control engineers to quickly test algorithms with their MATLAB or Simulink models.
At WMG, we build test rigs to help businesses validate battery management system functions for their battery systems.
For example, we recently helped Jaguar Land Rover to develop a battery management system for an automotive application.
Here is another battery management system that WMG researchers developed for testing
and validating the performance of the features, functions, and control algorithms.
The battery management system developed at WMG contains most of the functions of a commercial product .
and additional features allowing the testing of any battery models regardless of cell chemistry,
developing functions, and control algorithms.
Furthermore, the real-time test rig allows the testing of the battery management system
functions and control algorithms against real batteries from cell to module level.
and the safety features ensure the battery is correctly connected and isolated before closing the
main contactors within the hardware-in-the-loop testing environment.
Through this video, you should now have learned about a battery management system, its functions,
and importance of managing and controlling batteries.
We hope you enjoyed the video and thank you for watching.