With the increasing number of electric vehicles, battery energy management system is becoming more and more important in addition to looking for batteries with high energy density and high safety. Different power batteries have different properties, and even if the same type of battery properties are inconsistent, in the process of use, there will be the possibility of accidents caused by expansion. Therefore, it is very important to effectively manage the power battery system to ensure the safety of electric vehicles. At the same time, it is necessary to ensure the performance of the battery system, prolong the battery life and improve the battery efficiency.

Structure and principle of Battery Management system.

The battery management system (BMS), namely Battery Management System, determines the state of the whole battery system by detecting the state of every single battery in the battery pack, and carries on the corresponding control adjustment and strategy implementation to the power battery system according to their state, to realize the charge and discharge management of the power battery system and each unit to ensure the safe and stable operation of the power battery system.

A typical topology structure of a battery management system is mainly divided into two parts: master control module and slave control module. Specifically, it is composed of the central processing unit (main control module), data acquisition module, a data detection module, display unit module, control components (fuse device, relay), and so on. Generally, the data information communication between modules is realized by using internal CAN bus technology.
Based on the functions of each module, BMS can real-time detect the voltage, current, temperature, and other parameters of the power battery, realize the thermal management, balanced management, high voltage, and insulation detection of the power battery, and calculate the residual capacity, charge and discharge power and SOC&SOH status of the power battery.

Basic functions of Battery Management system.

The basic functions of a battery management systems can be divided into three parts: detection, management, and protection. Specifically, it includes functions such as data acquisition, condition monitoring, balanced control, thermal management, security protection, and so on.
(I) data collection.
As the basis and premise of other functions in the battery management system, the accuracy and speed of data acquisition can reflect the advantages and disadvantages of the battery management system. Other functions of the management system, such as SOC state analysis, equilibrium management, thermal management, and so on, are based on the collected data for analysis and processing.
The objects of data acquisition are generally voltage, current, and temperature. In the process of practical use, the electrochemical performance of the battery is different at different temperatures, resulting in different energy released by the battery. Lithium-ion power battery is sensitive to voltage and temperature, so the effect of temperature must be taken into account when evaluating the SOC of the battery.

(II) State analysis.

The analysis of battery status mainly includes two aspects: the remaining charge of the battery and the degree of battery aging, namely SOC evaluation and SOH evaluation. SOC allows drivers to get direct information about the impact of remaining power on mileage. At the present stage, a lot of studies are focused on the analysis of SOC, constantly improving its accuracy. The analysis of SOC will be affected by SOH. The SOH of the battery is continuously affected by temperature and current in the process of use, so it needs to be analyzed continuously to ensure the accuracy of SOC analysis.
In the analysis of SOC, there are the charge metering method, open circuit voltage method, Kalman filter method, artificial neural network algorithm, and fuzzy logic method, and so on. The charge metering method and the open circuit voltage method are briefly introduced here.
(1) charge metering method.
The charge metering method calculates the SOC through the statistics of the charge charged and discharged by the battery over some time, that is, the accumulation of current in time. Although it is the most commonly used measurement method, it will be affected by many factors, including data accuracy, self-discharge, and so on. For example, due to the lack of accuracy of the current sensor, there is an error between the current used for integral calculation and the real value, which makes the deviation of the result of SOC larger and larger. Therefore, when using the charge measurement method, we need to use some correction algorithms to correct various influencing factors to reduce the error of the calculation and analysis results.
(2) Open circuit voltage method.
The open-circuit voltage method is to measure the open-circuit voltage of the battery when the battery is in a static state to calculate the SOC of the battery. However, it should be noted that when using the open-circuit voltage method, it is generally considered that there is a certain linear relationship between SOC and EMF, and any SOC value corresponds to only one EMF value. The voltage spring back effect must be taken into account when using the open-circuit voltage method, and the calculated SOC will be too small when the voltage does not rebound to the stable value. Compared with the charge metering method, the open-circuit voltage method can not be used when the battery is working normally, which is its biggest problem.

It is very difficult to measure SOC accurately at this stage, for example, the inaccuracy of sampling data caused by sensor accuracy and electromagnetic interference leads to the deviation of state analysis. In addition, the inconsistency of the battery, the historical data and the uncertainty of the operating conditions also have a great impact on the calculation of SOC.

(III) balanced control.
Due to the influence of manufacturing and working environment, the cell unit is inconsistent, and there are differences in voltage, capacity, internal resistance and other properties, resulting in different effective capacity and charge and discharge capacity of each single cell in the actual use process. Therefore, in order to ensure the overall performance of the battery system and prolong the service life, it is very necessary to balance the battery in order to reduce the difference between single cells.
Balanced management contributes to the maintenance of battery capacity and the control of discharge depth. If there is no balanced control of the battery, due to the protection function setting of the battery management system, there will be a phenomenon that when a single battery is fully charged, other batteries are not fully charged, or when the discharge of a single battery with minimum power is cut off, the other batteries have not reached the discharge cut-off limit. Once the battery is overcharged or overdischarged, some irreversible chemical reactions will occur in the battery, which will affect the properties of the battery, thus affecting the service life of the battery.
According to the circuit structure and control mode in equalization management, the former is divided into centralized equalization and distributed equalization, and the latter is divided into active equalization and passive equalization. Centralized equalization means that all battery units in the battery pack share a single equalizer for equalization control, while distributed equalization is an equalizer dedicated to one or more battery cells. The former has the advantages of simple and direct communication and fast equalization speed. However, the arrangement of the wire harness between the battery unit and the equalizer is complex, so it is not suitable for the battery system with a large number of units. The latter can solve the harness problem of the former, but the disadvantage is the high cost.
Active equilibrium, also known as non-dissipative equilibrium, image theory is the energy transfer between battery units. The energy in the cell with high energy is transferred to the monomer with low energy to achieve the purpose of energy balance. The passive type is dissipative equilibrium, which consumes the energy of the high-energy monomer to a state of equilibrium with other monomers by means of parallel resistance. Active equilibrium is efficient and energy is transferred rather than consumed, but the complex structure leads to an increase in cost.

IV) Thermal management.
Battery system in different operating conditions because of its own internal resistance, in the output power, electric energy at the same time to generate a certain amount of heat, resulting in heat accumulation to increase the battery temperature, different space layout makes the battery temperature is not consistent. When the battery temperature exceeds its normal operating temperature range, the power must be limited, otherwise the battery life will be affected. In order to ensure the electrical performance and life of the battery system, the power battery system is generally designed with a thermal management system. The battery thermal management system is a set of management system used to ensure that the battery system works in a suitable temperature range, which is mainly composed of battery box, heat transfer medium, monitoring equipment and so on.
The main function of the battery management system in thermal management is to accurately measure and monitor the battery temperature. When the battery temperature is too high, the effective heat dissipation and ventilation are used to ensure the uniform distribution of the battery temperature field. Under the condition of low temperature, the battery pack can be heated quickly to achieve a normal working environment.

(V) Security and protection.
As the most important function of the whole battery management system, safety protection is based on the first four functions. It mainly includes overcurrent protection, overcharge and discharge protection, overtemperature protection and insulation monitoring.
(1) overcurrent protection.
Because the battery has a certain internal resistance, when the battery is working, the current flow will cause the internal heat of the battery, and the increase of heat accumulation leads to the increase of the battery temperature, which leads to the decrease of the thermal stability of the battery. For lithium-ion battery, the de-intercalation ability of positive and negative electrode materials is certain. When the charge and discharge current is greater than its de-intercalation capacity, the polarization voltage of the battery will increase, and the actual capacity of the battery will decrease and affect the service life of the battery. In serious cases, it will affect the safety of the battery. The battery management system will judge whether the current value exceeds the safety range, and if it exceeds it, it will take corresponding safety protection measures.
(2) overcharge and overdischarge protection.
In the charging process, when the charging voltage exceeds the battery cutoff charging voltage, the positive lattice structure will be destroyed and the battery capacity will become smaller. And when the voltage is too high, it will cause the hidden danger of explosion in the positive and negative pole short circuit. Overcharging is strictly prohibited. BMS detects the voltage of a single battery in the system, and when the voltage exceeds the charging limit, BMS disconnects the charging circuit to protect the battery system.
In the process of discharge, when the discharge voltage is lower than the battery discharge cut-off voltage, the metal collector on the negative electrode of the battery will be dissolved, causing irreversible damage to the battery. When charging an overdischarged battery, there is the possibility of internal short circuit or leakage. When the voltage exceeds the discharge limit voltage, the BMS will open the circuit to protect the battery system.
(3) overtemperature protection.
For over-temperature protection, it needs to be combined with the above thermal management functions. Battery activity varies at different temperatures. When exposed to high temperature for a long time, the structural stability of battery materials will become worse and shorten the battery life. The limitation of battery activity at low temperature will reduce the available capacity, especially the charging capacity will become very low, and may cause safety risks. The battery management system can prohibit charging and discharging when the battery temperature exceeds the high temperature limit or is lower than the low temperature limit.
(4) Insulation monitoring.
Insulation monitoring function is also one of the important functions to ensure the safety of battery system. The voltage of the battery system is usually several hundred volts, and once the leakage occurs, it will be dangerous to the personnel, so the insulation monitoring function is very important. BMS will monitor the insulation resistance of the total positive and negative to the body iron in real time. If the insulation resistance is lower than the safe range, the fault will be reported and the high voltage will be disconnected.
System design and technical requirements.

When designing the battery management system, we first need to determine the function of BMS according to the design requirements of the whole vehicle, and then determine its topology, by the software and hardware design of the main work. After the completion of the above basic work, we need to carry out the BMS unit test and the overall test of the power battery pack. Before the software and hardware design, the charge and discharge, capacity, resistance and other characteristics of the single battery need to be tested in order to better protect the circuit design, algorithm design and so on.
Hardware design should be combined with the requirements of software algorithms, and attention should be paid to voltage insulation, anti-electromagnetic interference, electromagnetic compatibility, communication isolation, ventilation and heat dissipation in circuit board development and component design. The general software design functions include voltage detection, temperature acquisition, current detection, insulation detection, SOC estimation, CAN communication, discharge equalization function, system self-test function, system detection function, charge management, thermal management and so on.
The related hardware design supports the functions of the software design, such as the MCU module is used to collect and analyze data, send and receive control signals, and the current detection module is to collect the charge and discharge current of the battery pack during the charge and discharge process.