Research on key technologies of BMS for electric vehicles

Background

BatteryManagementSystem

BMS

As an important bridge connecting the battery pack, vehicle system and motor of electric vehicles (electric energy vehicles), the battery management system performs real-time detection of the battery voltage, current, temperature, etc. through sensors closely integrated with the power battery, thus achieving comprehensive management of the vehicle's electric system. The key role of BMS is to avoid overcharging and overdischarging of batteries in applications, improve the asymmetry of each single cell in the battery pack, improve the efficiency of the battery pack and extend its service life. BMS detects the operating parameters of individual batteries and the entire battery pack (such as voltage, current and temperature, etc.), and conducts real-time bus communication with the vehicle monitoring system and the on-board charger, which is very important for predicting the safety performance of the entire vehicle battery. In short, as the core of the battery system, BMS plays an important role in electric vehicles, and the exploration of BMS key technologies is of great significance.

BMS Category & Function Overview

1. Basic categories of BMS

BMS can be classified into centralized, modular, master-slave, and distributed types according to the topological connection structure. (1) All cells of the centralized BMS are located in a package module, and a bundle of wires extends from the inside of the package module (N cells are N+1 wires), and then each wire is connected to each cell. The centralized BMS has the advantages of compact structure, low price, and convenient maintenance. (2) The modular BMS is divided into multiple identical sub-modules, and each packaged wire is connected to different modules inside the battery. In addition to the advantages of the centralized structure, the modular BMS also has the advantages of simple connection structure between BMS sub-modules and cells, close distance between sub-modules and batteries, and easy expansion of BMS sub-modules. (3) The master-slave BMS consists of a master module and multiple identical sub-modules (i.e., slave modules). The master module is mainly responsible for calculation and communication, and each sub-module is responsible for measuring the voltage of the cell. The master-slave topology has low manufacturing cost and combines most of the advantages of the modular topology. (4) Distributed BMS refers to electronic devices and the single battery to be tested are directly installed on the circuit board. Distributed BMS also has the advantage of easy connection. The centralized, modular, master-slave and distributed structures are shown in Figure 1:

Figure 1

BMS can also be divided into BMS analog system (simple system) and BMS digital system (complex system) according to different processing systems.

(1) The BMS analog system refers to the use of analog circuits (such as analog comparators, amplifiers, differential circuits or similar components) to process the voltage of single cells. This system is simple in design and easy to implement, but it cannot detect the power level of single cells. In other words, it can only detect that the voltage of a single cell is too low, but cannot know the specific value of the voltage of the single cell.

(2) The BMS digital system can accurately detect the voltage, temperature and other conditions of each single cell, and process the above-mentioned conditions of the single cell voltage into digital signals. The basic control principles of the analog system and the digital system are shown in Figure 2 and Figure 3 respectively:

Figure 2: Basic control principle of the simulation system

Figure 3: Basic control principle of digital system

(II) BMS basic functional modules

1. Constant current and constant voltage charging section

The constant current and constant voltage charging module is a device used to charge the battery in a standard and normalized manner, where "constant voltage" and "constant current" represent two working modes, namely constant current mode (CCmode) and constant voltage mode (CVmode). CCmode means that after the battery pack starts charging, the charging device will output a fixed charging current, and the voltage across the battery gradually increases during the entire charging process; CVmode means that when the battery pack is close to full charge and the battery voltage is close to constant, the charger maintains the constant charging voltage, and in the subsequent charging process, the charging current of the charger will decay exponentially until the battery is fully charged.

2. Diversion plate

The function of the shunt plate is to balance the battery pack. The shunt plate is connected in parallel with the single battery. When the single battery is in a fully charged state, the shunt plate will bypass part or all of the incoming current to prevent the battery from being overcharged.

3. Detection plate

The detection panel is used to detect parameters, but it does not have the ability to actively control charging and discharging. Its functions generally include:

●Measure the voltage of each single battery;

●Measure the current and temperature of the battery pack;

Compile data;

●Calculate or evaluate the state of the battery pack, such as the state of charge (SOC);

●Display results;

●Warning function.

4. Control Panel

The control board can realize closed-loop control after receiving the voltage of each single battery. The control board cannot cut off the battery pack current, but can only issue instructions to other devices (such as chargers, loads) to reduce or cut off the battery pack current.

5. Balanced Plate

The balancing module maximizes the performance of the battery pack by improving the asymmetry of each single cell in the battery pack. It has communication functions and can transmit data to other parts of the system. The connection method of the balancing module enables it to control the charging power supply and discharging load.

6. Protective plate

The protection plate has similar functions to the balancing plate, but has an additional switch to shut off the current. It is more suitable for the management of small batteries.

7. The technical summary of the functions is shown in Figure 4:

Figure 4

BMS key technology analysis

1. Measurement technology

1. Measurement technology: voltage measurement

The primary function of BMS is to collect data and measure signals, including: single cell voltage, single cell temperature, or battery module temperature, battery pack current. The voltage signal is collected by the analog multiplexer, read by the analog-to-digital converter and transmitted to the processor.

There are generally three detection structures for voltage detection, namely discrete voltage detection structure, unipolar multiplexing voltage detection structure and differential multiplexing voltage detection structure. Specifically, in the discrete voltage detection structure, the distributed BMS can directly measure the voltage of the single cell. When measuring the single cell voltage, the battery board is generally powered by the single cell itself; in the unipolar multiplexing voltage detection structure, the BMS can measure the tap voltage in the battery and calculate the voltage difference between the two taps as the single cell voltage value; in the differential multiplexing voltage measurement structure, the BMS can use two methods at the same time to measure the taps at both ends of the single cell and calculate the voltage difference as the single cell voltage. The three voltage detection structures are shown in Figure 5:

Figure 5

2. Temperature measurement of measurement technology Temperature

measurement technology is of great significance to ensure the stable operation of various types of batteries. The discharge capacity of most single cells will be limited when the external environment is in a certain temperature range. Therefore, the temperature of single cells needs to be monitored in some mobile application scenarios where the temperature cannot be controlled to ensure its normal use; the battery itself may also become hot due to internal problems (such as single cell damage) or external problems (poor power supply contact). At this time, a temperature measurement device is needed to send a warning signal to the system; in the distributed BMS, the placement of sensors in each submodule is easy to operate. It can not only measure the temperature of the single cell, but also monitor whether the load balancing function module is working.

Different types of BMS have different temperature measurement technology applications and requirements. Digital BMS has no absolute requirements for temperature monitoring; distributed BMS can measure the temperature of each single cell; non-distributed BMS can only measure the temperature of the battery or battery module; in terms of the arrangement of temperature measurement locations, if the BMS has only limited sensor probes, they should be distributed in locations where temperature changes are most obvious.

The single cell level structure is shown in Figure 6:

Figure 6

The battery level structure is shown in Figure 7:

Figure 7

3. Measurement technology: current measurement

Current detection technology enables BMS to have more functions: preventing the single cells in the battery pack from exceeding the safe area due to continuous current flow, calculating the internal DC resistance of the single cell and the single cell terminal voltage, etc. At present, there are two main devices for measuring current, namely shunt-based current sensors and Hall effect current sensors. The functional principle of the shunt-based current sensor and the working principle of the large current Hall effect sensor are shown in Figures 8 and 9: