Last updated: Feb 2020 by Narasimhan Santhanam
This post is a part of EV Next’s EV Perspectives.
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The battery is the only power source in pure electric vehicles. Therefore, the BMS in this type of application should include battery monitoring and protection systems, a system that keeps the battery ready to deliver full power when necessary and a system that can extend the life of the battery. The BMS should include systems that control the charging regime and those that manage thermal issues. In a vehicle, the BMS is part of a complex and fast-acting power management system. In addition, it must interface with other on-board systems such as the motor controller, the climate controller, the communications bus, the safety system and the vehicle controller.
There are different types of BMSs used to improve battery lifetime and performance. The most common type is a battery monitoring system that records the key operational parameters such as voltage, current and the internal temperature of the battery along with the ambient temperature during charging and discharging. The system provides inputs to the protection devices so that the monitoring circuits could generate alarms and even disconnect the battery from the load or charger if any of the parameters exceed the values set by the safety zone.
Functions of BMS
Battery Management System performs the following functions:
- Discharging Control
- Charging Control
- State-of-Charge Determination
- State-of-Health Determination
- Cell Balancing
The primary goal of a BMS is to keep the battery from operating out of its safety zone. The BMS must protect the cell from any eventuality during discharging.
Batteries are more frequently damaged by inappropriate charging than by any other cause. Therefore, charging control is an essential feature of the BMS. For lithium-ion batteries, a 2-stage charging method called the constant current – constant voltage (CC-CV) charging method is used. During the first charging stage (the constant current stage), the charger produces a constant current that increases the battery voltage. When the battery voltage reaches a constant value, and the battery becomes nearly full, it enters the constant voltage (CV) stage. At this stage, the charger maintains the constant voltage as the battery current decays exponentially until the battery finishes charging.
One feature of the BMS is to keep track of the state of charge (SOC) of the battery. The SOC could signal the user and control the charging and discharging process. There are three methods of determining SOC: through direct measurement, through coulomb counting and through the combination of the two techniques.
To measure the SOC directly, one could simply use a voltmeter because the battery voltage decreases more or less linearly during the discharging cycle of the battery.
In the coulomb-counting method, the current going into or coming out of a battery is integrated to produce the relative value of its charge. This is similar to counting the currency going into and out of a bank account to determine the relative amount in the account.
In addition, the two methods could be combined. The voltmeter could be used to monitor the battery voltage and calibrate the SOC when the actual charge approaches either end. Meanwhile, the battery current could be integrated to determine the relative charge going into and coming out of the battery.
The state of health (SOH) is a measurement that reflects the general condition of a battery and its ability to deliver the specified performance compared with a fresh battery. Any parameter such as cell impedance or conductance that changes significantly with age could be used to indicate the SOH of the cell. In practice, the SOH could be estimated from a single measurement of either the cell impedance or the cell conductance.
Cell balancing is a method of compensating weaker cells by equalizing the charge on all cells in the chain to extend the overall battery life. In chains of multi-cell batteries, small differences between the cells due to production tolerances or operating conditions tend to be magnified with each charge-discharge cycle. During charging, weak cells may be over-stressed and become even weaker until they eventually fail, causing the battery to fail prematurely.
To provide a dynamic solution to this problem while taking into account the age and operating conditions of the cells, the BMS may incorporate one of the three cell balancing schemes to equalize the cells and prevent individual cells from becoming over-stressed , the active balancing scheme, the passive balancing scheme and the charge shunting scheme. In active cell balancing, the charge from the stronger cells is removed and delivered to the weaker cells. In passive balancing, dissipative techniques are used to find the cells with the highest charge in the pack, as indicated by higher cell voltages. Then, the excess energy is removed through a bypass resistor until the voltage or charge matches the voltage on the weaker cells. In charge shunting, the voltage on all cells would be leveled upward to the rated voltage of a good cell. Once the rated voltage of the cell is reached, the current would bypass the fully charged cells to charge the weaker cells until they reach full voltage
As the SOH is relative to the condition of a new battery, the measurement system must hold a record of the initial conditions or a set of standard conditions for comparison. An alternative method of determining the SOH is to estimate the SOH value based on the usage history of the battery rather than on certain measured parameters, such as the number of charge-discharge cycles completed by the battery. Therefore, the logbook function of the BMS would record such important data to the memory system.
The communications function of a BMS may be provided through a data link used to monitor performance, log data, diagnostics or set system parameters. The function may also be provided by a communications channel carrying system control signals. The choice of the communications protocol is not determined by the battery; instead, it is determined by the application of the battery. The BMS used in electric vehicles must communicate with the upper vehicle controller and the motor controller to ensure the proper operation of the vehicle. There are two major protocols used by the BMS to communicate with the vehicle: through the data bus or the controller area network (CAN) bus. Data buses include the RS232 connection and EIA-485 (also called the RS485 connection). The industry standard for on-board vehicle communications is the CAN bus, which is more commonly used in vehicle applications.
Components of BMS
Safety circuitry has long been used in BMS. However, with more sensors being used in the latest BMS, improvements in current safety circuitry designs can be implemented, such as the addition of accurate alarms and controls to prevent overcharge, over-discharge and overheating.
The sensor system consists of different sensors to monitor and measure battery parameters including cell voltage, battery temperature and battery current. Some researchers have proposed adopting EIS (Electrochemical Impedance Spectroscopy) to monitor internal impedance. However, both space constraints and device costs hinder the feasibility of these measurements outside laboratory environments. Thus, current, voltage, and temperature should be measured to improve the capability of state tracking in real life applications.
Data acquisition (DAQ) and data storage are critical parts of the software in the BMS to analyze and build a database for system modeling. Charge control is a subsystem governing the charge-discharge protocol. Batteries are often charged by the constant current/constant voltage method (CC/CV) and will thus need to include a potentiostat and a galvanostat.
A variable resistor may be necessary to help balance cells or perform internal resistance measurements. Cell balancing control is still a critical design feature with room for improvement in order to equalize the battery pack and estimate the battery status in an efficient way.
Most subsystems in a BMS are stand-alone modules, and hence, data transfer throughout the BMS is required. Communication through a CAN Bus is a prominent way to transfer data within the BMS. With the development of smart batteries, more data can be collected to communicate with the user and the charger through the microchips incorporated within the battery.
In addition, wireless and telecommunication techniques are gradually being incorporated into charging systems that facilitate communication between the battery and the charger. A module for thermal management is critical because temperature differences have an impact on cell imbalance, reliability and performance. Thus, Pesaran pointed out that it is important to reduce the temperature difference among cells, which must be monitored and operated under proper temperature conditions.
The software of the BMS is the center of the whole system because it controls all hardware operations and analyses of sensor data for making decisions and state estimations. Switch control, sample rate monitoring in the sensor system, cell balancing control and even dynamic safety circuit design should be handled by the software of a BMS.
Determination of SOC and SOH will be integrated into a capability assessment, which also presents the life status of the battery and sets the operating limits according to state-of-the-art algorithms, such as fuzzy logic, neural networks, state-space-based models and so on. The objective of cell balancing is to maximize battery performance without overcharging or over-discharging. Its nature is to make the SOC levels of cells closer to each other. The controller will control the charge process based on a comprehensive strategy that depends on the SOC of each cell. Thus, the accurate SOC estimation of each cell is the basic for improving the balancing.
Most soft faults will be discovered through online data processing. An intelligent data analysis is required in order to provide battery fault warning and indicate out-of-tolerance conditions. Historic data will be recorded and provide the pre-alarm condition before the possible faults. The user interface should display the essential information of the BMS to the users. The remaining range should be indicated on the dashboard according to the SOC of the battery. Additionally, abnormal alarming and replacement suggestions are needed to inform the users in terms of the estimation and prediction of the battery.
Read more on the EV Battery ecosystem from: EV battery Innovations | Components of BMS | FCEV Trends | FCEV Indian Efforts | Anode/Cathode R&D | Li-ion Battery Trends | BMS Innovations | Indian Battery Manufacturers | Cost of Li-ion Batteries | Anode Materials in 2020-2030 | Key Drivers shaping Battery Chemistry |
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