With the increasing number of electric vehicles, in addition to seeking
batteries with high energy density and high safety, the importance of battery
energy management systems is also increasing. Different power batteries have
different properties, and even batteries of the same type have inconsistencies
in their properties, which may increase the risk of accidents during use.
Therefore, it is very important to effectively manage the power battery system
to ensure the safety of electric vehicles. It is also necessary to ensure the
performance of the battery system, extend battery life, and improve battery
efficiency.
Battery management system composition and principle
The battery management system (BMS), also known as Battery Management
System, determines the status of the entire battery system by detecting the
status of each single cell in the battery pack, and makes corresponding control
adjustments and strategy implementation for the power battery system based on
their status to achieve power management. The charge and discharge management of
the battery system and each cell ensures the safe and stable operation of the
power battery system.
A typical battery management system topology diagram
A typical battery management system topology structure is mainly divided
into two major blocks: the master control module and the slave control module.
Specifically, it consists of a central processing unit (main control module),
data acquisition module, data detection module, display unit module, control
components (fuse device, relay), etc. Generally, data information communication
between modules is achieved by using internal CAN bus technology.
Based on the functions of each module, BMS can detect the voltage, current,
temperature and other parameters of the power battery in real time, implement
thermal management, balance management, high voltage and insulation detection of
the power battery, and calculate the remaining capacity, charge and discharge
power of the power battery, and SOC&SOH status.
Basic functions of the battery management system
The basic functions of the battery management system can be divided into
three major parts: detection, management, and protection. Specifically, it
includes functions such as data collection, status monitoring, equalization
control, thermal management, and safety protection.
(1) Data collection
As the basis and premise for other functions in the battery management
system, the accuracy and speed of data collection can reflect the quality of the
battery management system. Other functions of the management system, such as SOC
status analysis, balance management, thermal management functions, etc., are all
analyzed and processed based on the collected data.
The objects of data collection are generally voltage, current, and
temperature. In actual use, the electrochemical properties of the battery are
different at different temperatures, resulting in different energy released by
the battery. Lithium-ion power batteries are sensitive to voltage and
temperature, so the effect of temperature must be considered when evaluating the
battery's SOC.
(2) Status analysis
The analysis of battery status mainly includes two aspects: battery
remaining power and battery aging degree, namely SOC evaluation and SOH
evaluation. SOC allows drivers to obtain direct information and understand the
impact of remaining power on cruising range. Much of the research at this stage
focuses on SOC analysis and continuously enhances its accuracy. The analysis of
SOC will be affected by SOH. The SOH of the battery is continuously affected by
temperature, current, etc. during use and needs to be continuously analyzed to
ensure the accuracy of SOC analysis.
In the analysis of SOC, there are mainly charge measurement method, open
circuit voltage method, Kalman filter method, artificial neural network
algorithm and fuzzy logic method. Here we briefly introduce the two methods of
charge measurement method and open circuit voltage method.
(1) Charge measurement method
The charge measurement method calculates SOC by counting the charges
charged and discharged from the battery over a period of time, that is, the
accumulation of current over time. Although it is the most commonly used
measurement method, it is affected by many factors, including data collection
accuracy, self-discharge issues, etc. For example, due to the insufficient
accuracy of the current sensor, there is an error between the current used for
integral calculation and the real value, making the SOC result more and more
biased. Therefore, when using the charge measurement method, some correction
algorithms need to be used to correct various influencing factors and reduce the
errors in calculation and analysis results.
(2) Open circuit voltage method
The open circuit voltage method is to calculate the SOC of the battery by
measuring the open circuit voltage of the battery when the battery is in a
static state. However, it should be noted that when using the open circuit
voltage method, it is generally believed that SOC and electromotive force have a
certain linear relationship, and any SOC value only corresponds to one
electromotive force value. When using the open circuit voltage method, the
voltage rebound effect must be taken into account. When the voltage does not
rebound to a stable value, the calculated SOC will be too small. Compared with
the charge measurement method, the open circuit voltage method cannot be used
when the battery is working normally. This is its biggest problem.
There are great difficulties in measuring SOC very accurately during the
implementation phase. For example, inaccuracies in sampling data caused by
sensor accuracy and electromagnetic interference cause deviations in state
analysis. In addition, battery inconsistency, historical data, and ambiguity in
usage conditions also have a great impact on the calculation of SOC.
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