The class develops the BMS, which is responsible for battery management, and is critical in battery performance, life and protection.
Slide 2: Index.
- BMS- BMS-Battery Management System.
- BMS architectures.
- Wireless BMS.
- BMS Data Analytics-A tool for getting the most out of batteries.
Slide 3: BMS-Battery Management System.
- The battery management system is one of the most important components of the electric vehicle.
Because it manages the battery, protects the cells, and is fundamental in the performance of the battery.
The BMS is a differentiating component of electric vehicles, there are vehicle manufacturers that develop better BMS than others, and get more performance from the batteries.
It is an intelligent software whose algorithms may vary from one vehicle manufacturer to another and which is key to extending battery life. The BMS is accompanied by a communication system with the outside world, commonly known as the data BUS.
The BMS consists of an electronic system that, together with the appropriate software, is capable of collecting all the key information on the operation and life of the battery. This allows not only to manage the correct operation of the battery during use, but also provides a data bank to be exploited in order to optimize its operation in terms of performance, longevity and safety.
The BMS performs the following functions:
- Monitor voltages and currents.
The BMS constantly monitors the maximum charge current, maximum discharge current, energy delivered since the last charge cycle or last charge, total operating time, number of cycles, etc. to ensure that limits are not exceeded, ensuring that the cells do not operate in unsafe conditions by minimizing their degradation.
- Manage cell balancing.
Ideally, all cells in a battery pack should be kept in the same state of charge. If cells become unbalanced, individual cells can become stressed, leading to premature charge termination and a reduction in the overall battery life cycle.
No two battery cells are exactly alike, the differences can be in internal resistance, level of degradation, capacity, ambient temperature etc.
These unavoidable differences between cells can cause many problems, and the overall capacity can be drastically reduced. This imbalance causes individual cells to over-discharge or over-charge, which is dangerous to the battery as a whole, and therefore load balancing between individual cells is of utmost importance to maintain performance and extend battery life.
Some cells always have a slightly higher or lower capacity, battery cells that have a lower capacity are discharged faster and also destroyed faster, while the capacity of other cells remains unused. Similarly, during charging, the weakest cells are charged first and the others are only partially charged.
Balancing the charging and discharging of individual cells significantly increases the overall capacity, because it is not only determined by the weakest cells, but also protects these weaker cells from being damaged, shorted or leaking, which could damage the entire battery.
There are two methods of battery balancing:
1. Passive charging.
The passive method means that cells that have more energy than others are discharged by resistors, and the excess energy is dissipated as heat. By discharging the excess energy from the fuller cells, the battery can be easily balanced.
However, this method wastes too much energy and also complicates thermal control of the entire battery. Also, in this case, only overcharged batteries work, and if one cell of the battery is significantly weaker than others, more energy is discharged during balancing than during driving.
2. Active charging.
Active switching circuits are used for active balancing, which can transfer energy between individual cells. Unlike passive methods, only a small amount of energy is wasted. But to achieve this, more components need to be incorporated into the circuit, which not only leads to higher cost, but also to lower reliability.
Active balancing occurs through a variety of strategies called topologies, which take into account cost, complexity and reliability. As always, there is a trade-off. With the simplest, and therefore most reliable, topologies, one cell can only balance another cell that is right next to it, which is insufficient if the unbalanced cells are further apart.
To effectively connect all the cells so that their alignment can be flexible, too many components are needed, and therefore become unreliable and quite expensive.
The third method, which allows for a small number of components and flexibility, is too slow to be effective in practice, as two adjacent cells are gradually aligned until homogeneous loading is achieved.
As this is a fundamental function of the BMS, much effort is devoted to designing the most efficient topologies and using the best strategies, both passive and active balance, using as few components as possible.
Every design always involves the need to balance price, efficiency and longevity of each system.
The one value that is never discounted is safety.
All components must meet ISO 26262 safety standards, so each BMS must be fail-safe and contain redundant resources, such as processing units, each of which must have their own dedicated devices.
- Monitor cell temperature.
The BMS monitors the temperature inside the battery pack and regulates the cooling circuit to maintain temperature targets inside the pack. In general, the optimum operating range of a lithium-ion cell is between 15°C-40°C.
- Calculates the battery charge - State of Charge (SoC).
This is one of the most important functions, thanks to which the BMS can tell the driver the remaining range of the vehicle. But determining the state of charge is not as simple as it sounds. In fact, it is one of the most complicated problems in the development of BMS systems.
The current state of charge is defined as the ratio of the available capacity to the total capacity of the battery, and can therefore take any value from 100% to 0%. And since the discharge of the battery is a flow of electrons, it would seem that the amount of charge or discharge can only be measured.
However, consumption is affected by many more variables, such as current temperature, temperature change during discharge, current charge etc., so taking all of these into account is complex, and the current state of charge is an estimate using a mathematical model-algorithm.
- Manage battery discharge and recharge.
Lithium-ion cells have two critical design issues; if overcharged, they can be damaged and cause overheating and even an explosion or flame, so it is important to have a battery management system that provides surge protection.
Lithium-ion cells can also be damaged if they are discharged below a certain threshold, approximately 5% of full capacity. If the cells are discharged below this threshold, their capacity can be permanently reduced.
To ensure that the charge of a battery does not exceed or decrease its limits, the BMS has a safety device called a dedicated lithium-ion protector.
Each battery protection circuit has two electronic switches called "MOSFETs". MOSFETs are semiconductors that are used to turn electronic signals on or off in a circuit.
A battery management system typically has a discharging MOSFET and a charging MOSFET.
If the protector detects that the high voltage between the cells exceeds a certain limit, it will interrupt the charge by opening the Charge MOSFET chip. Once the load has dropped to a safe level, the switch will close again.
Similarly, when a cell is drained to a certain voltage, the protector will cut off the discharge by opening the discharge MOSFET.
The parameters of the battery itself change over time such as oxidation on the terminals, changes in the capacity of the battery cells, etc. the BMS manages that the charge always adapts to the state of the battery.
- Calculate the battery state of health (SoH).
The BMS estimates the state of charge (SoC) and state of health (SoH) of the battery by using a built-in algorithm to maximize battery capacity, life and performance.
Battery health is defined as the ratio of the current total capacity to the total battery capacity at 0 kilometers. When a battery is purchased, it has 100% health, which deteriorates with charge/discharge cycles.
Battery health is affected by temperature, battery charging current, number of charge cycles and other primary factors. However, not all battery processes are fully understood, so there are no accurate methods for determining battery health status.
As with determining the state of charge, approximate mathematical models-algorithms that take into account: internal resistance, conductivity, self-discharge rate, capacity, energy received during charging, temperature during use, age, number of cycles, etc., are necessary.
Currently, there are three basic estimation methods: the ampere-hour method, the open circuit voltage (OCV) method, and mathematical model-algorithm-based methods.
The use of intelligent logarithms, fuzzy logic, artificial neural networks, and other possibilities for estimating battery condition are currently being investigated.
- Data recording and communication.
The BMS needs to store data from previous battery characteristics in order to be able to compare them with new values, allowing the evaluation of battery performance as well as its diagnosis.
Communication between the BMS and other parts of the vehicle, such as the on-board charger or charging station, is important.
The BMS also ensures that the driver's display shows how far he has traveled or when the car needs to be recharged.
Slide 4: BMS Architectures.
There are two main types of BMS architecture:
- Centralized.
In a centralized architecture, all cells are connected to the same BMS board.
This is an effective solution for battery packs containing a limited number of cells, up to 100.
- Distributed.
The distributed architecture consists of two types of boards: a single master board, and multiple boards located near the cells that monitor anywhere from six to a few dozen cells. They are then connected to the master board through a digital communication chain.
This distributed architecture is preferable for medium to large batteries, where more than 100 cells are connected in series.
Slide 5: Wireless BMS.
- There is a maximum number of cells that can be connected to the BMS.
When you increase the number of cells inside a battery to provide it with more energy capacity, and therefore offer more range in an electric vehicle, you also increase the number of components needed to monitor each cell inside a BMS because there is a maximum number of cells that can be connected to these systems.
The BMS can be enhanced with smarter algorithms to monitor more cells, but is limited by the number of cells that can be in contact through the wiring harness.
- The connection between the cell monitors and the microcontroller units- MCUs has generally been made using wires and connectors.
But when there are too many of them, the likelihood of failures such as wire breaks and bad contacts increases.
In addition, too many wires can also make battery pack assembly more complex and costly.
- Wireless BMS.
All of these drawbacks are causing battery designers to turn to the new wireless battery management systems wBMS.
A wireless BMS is a type of distributed BMS in which communication between boards is done wirelessly instead of using a wired daisy-chain connection to reduce the number of wires and connectors.
Most of the components used are the same as those of a conventional distributed BMS. The only thing that changes is the communication interface which is realized by a wireless integrated circuit coupled to an antenna instead of a wired communication transceiver.
- Advantages.
In a wireless system, wiring weight, complications and costs are eliminated, while the number of cells that can be managed is increased.
Implementing wBMS eliminates traditional wiring, saves up to 90% of the wiring and up to 15% of the volume in the battery pack, and improves design flexibility and manufacturability, without compromising range and accuracy over the life of the battery.
The reduced weight due to the elimination of connectors enables greater energy efficiency, thus increasing vehicle range on the same charge.
The solution also opens up space in the battery pack to allow larger batteries to fit, ensuring versatility, scalability and optimization.
- Data security.
Wireless communication is less secure than wired communication because anyone can access the data within the propagation range of radio waves. However, there are software-implemented measures, such as encryption, that force data authentication and protect transmission.
- Performance.
The most important factors for wireless technology to provide good performance are several: data transmission rate (megabits per second), signal strength and power consumption.
Excellence in wireless BMS technology is achieved by optimizing these factors for fast, long-range communication speeds and acceptable power consumption.
Slide 6: BMS Data Analytics-A Tool for Getting the Most Out of Batteries.
Batteries generate large amounts of data during their life cycle. Most of this data is currently unused because the BMS is not designed to translate this data into actionable information, forecasts and predictions.
By using a BMS based on data stored in the cloud and managed by appropriate algorithms, we would get more performance out of electric vehicle batteries.
By collecting data such as voltage, lithium loss and complex impedances from field data, they allow to detect anomalies with the potential to cause incidents such as fires, predict future performance, aging and optimize operation, also help to determine the value of retired batteries to find suitable second life applications for them.
- New "Big Data" and data mining solutions open up a new range of opportunities for the battery industry.
The use of such solutions makes sense especially because of the wide variety of key parameters and critical indicators associated with batteries, which themselves need to be tracked and monitored to achieve improved performance.
Aspects such as energy density, c-rate, cyclability, temperature, geometry, etc. are elements to be analyzed throughout the battery life cycle in order to optimize them, both from an individual and global perspective.
- Allows traceability of the entire battery cycle.
The collection of data throughout the life of the battery makes it possible to carry out a complete monitoring of results for all the key indicators identified, as well as for all the components that make up a battery.
This makes it possible to know everything that happens in a battery from its manufacture to the end of its life, thus being able to understand the "what", "when" and "where" in order to deepen the knowledge of the batteries and carry out the necessary adjustments to optimize them.
- It will allow to have a theoretical formulation capacity of the batteries.
From the exploitation of the available information, thus being able to define a priori the solutions that can give us the best results taking into account their final application.
- It takes battery management during operation one step further.
By establishing a "bidirectional" communication system between the battery, specifically the BMS and its software, and the data analysis and exploitation system, corrections and adjustments can be made in real time to optimize the use of the battery and its operation according to the parameters and needs observed at any given time.
- Understanding what has worked and what needs to be improved in the use of a device that has already fulfilled its first life.
It is a very valuable information to give rise to a greater residual value of its application in a second life or use.
Slide 7: BMS Data Analytics-A tool to get the most out of batteries.
- The challenges to achieve this model:
1. Succeeding in developing BMS systems that have the right software for information capture and exploitation.
It is necessary to incorporate new software solutions that allow the BMS to go beyond the current state of the art, making this system a real "brain" that not only manages the information, but is capable of understanding it and making the most of it through the system.
2. The development of advanced BMS capable of adapting to any generation of batteries.
In other words, this type of solution should advance at the same pace as the energy storage technology itself, being able to adapt to different configurations, chemistries or approaches, such as those based on solid state.
- There are already different players in the market that are working to promote this type of solution.
Slide 7: Thank you for your time.
The class has developed why the BMS is one of the most important components of the electric vehicle, see you soon.
