Since the beginning of this century, lithium battery production and research have made great breakthroughs because of their many good advantages, such as stable discharge voltage, low self-discharge rate, wide operating temperature range, no memory effect, long storage life, and light weight , small size and other characteristics, it has slowly replaced traditional nickel-cadmium batteries and lead-acid batteries. It has become more and more widely used in social production and life, and has become the current mainstream power battery. Because the chemical reactions inside lithium batteries are very complex, while people are constantly improving the performance of the batteries themselves, they are also conducting continuous research on battery management technology and use to increase battery life, improve battery efficiency, and maximize the use of batteries. performance.

Battery Management System (BMS), which involves microcomputer technology and detection technology, implements dynamic monitoring of the operating status of battery units and battery packs, can accurately calculate the remaining power of the battery, and implement charge and discharge protection for the battery to promote It is in the best working condition, reducing operating costs and increasing service life. This paper combines some advanced results at home and abroad to design and implement a new lithium battery management system. The structure of this management system adopts a modular and distributed design. The system contains a two-level control structure, namely the local measurement module and the central processing module. Among them, the main function of the central processing module is to communicate with the host computer using the RS232 interface and connect to the local measurement module in the form of a CAN bus network; the main function of the local measurement module is data collection (mainly data collection of temperature, current and voltage ), charge and discharge control, power measurement, individual battery balancing and communication with the central processing module using CAN bus technology, etc.

1 Management system hardware design plan

The battery management system designed in this article is mainly used in electric vehicles and some underwater equipment. Therefore, the system design must have a reasonable structure, advanced technology, and strong scalability; the various parameters of the system must have high technical accuracy. Therefore, the battery management system is designed to achieve the following functions:

1) Collect battery information in real time, including total battery voltage, individual battery voltage, charge and discharge current, temperature and other parameters;

2) Measure and display remaining power;

3) It can provide a data transmission interface to complete communication with the CAN bus part and the host computer;

4) The human-computer interaction function is good, the system is safe and reliable, and has strong anti-interference ability.

The battery management system block diagram is shown in Figure 1.

As can be seen in Figure 1, this lithium battery management system includes a two-level control structure, which are the central processing module (Central Electric Control Unit, CECU), the local measurement module (Local Electric Control Unit, LECU), the central processing module and The local measurement module implements communication connection in the form of CAN bus. The structure of this battery management system is shown in Figure 2. In Figure 2, the main function of the local measurement module is to charge the battery pack. The components include: data acquisition module (for data collection of current, voltage, temperature, etc.), balancing module, charging module, and power measurement module etc.; the central processing module is mainly responsible for the management of the local measurement module, and uses CAN bus communication to send control information and receive battery status information. This article only introduces a few of the key modules.

2. Hardware design of local measurement module

2.1 Voltage acquisition module

The single cell terminal voltage is an important basis for calculating the remaining battery capacity, selecting charging and discharging methods, and evaluating operating status. Therefore, the prerequisite for monitoring the battery pack is to have a reasonable single cell terminal voltage measurement method. However, due to the large number of cells in the battery pack, the total voltage is relatively high, and the measurement accuracy is required, making it difficult to implement power measurement. The working principle of the voltage monitoring scheme is: in the first step, the multi-way switches Kn-1 and Kn-2 (n is between 1 and 7) controlled by the MCU synchronously connect the capacitor to both ends of the corresponding unit battery. Start capacitor charging to achieve the purpose of the capacitor voltage being the same as the unit battery voltage; in the second step, disconnect the MCU-controlled multi-way switches Kn-1 and Kn-2, close the switches K1 and K2, and connect them to A of the microcontroller. The /D module implements the measurement. During the measurement, in order to prevent the unstable battery terminal voltage from affecting the results, the module adopts the method of selecting the average value of multiple measurements. This solution can easily use the internal A/D unit of the microprocessor without adding additional A/D modules, which improves the efficiency of the design and saves costs. Usually in actual circuits, relays can be used to implement analog switches.

2.2 Current acquisition module

For the measurement of dynamic current during the charge and discharge process, this article uses the LTSR25-NP current sensor of LEM Company to achieve this. This component is a compensated closed-loop multi-range current sensor based on the Hall effect. It is powered by a unipolar voltage. It has good measurement accuracy, no insertion loss, excellent linearity, and relatively good current overload capability. Below 25 degrees Celsius, its measurement accuracy can reach ±0.2%. Its rated current is 25 A and the maximum measurable current is 80 A, which can well meet the system design requirements. This current sensor can convert the charge and discharge current into a voltage signal of 0 to 5 volts, and then connects to the A/D unit of the microcontroller to measure the charge and discharge current.

2.3 Temperature acquisition module

The temperature acquisition module is implemented through the DS620 programmable intelligent digital temperature sensor of Dallas Semiconductor Company in the United States. The chip contains registers, A/D converters and interface circuits, which can directly output digital signals. Its interface circuit with the microcontroller is relatively simple, its transmission distance is long, its control function is good, and its ability to resist external interference is strong. It is especially suitable for micro temperature measurement systems with low power consumption. The DS620 digital temperature sensor can provide low-voltage temperature measurement of 1.7 to 3.5 volts. In an environment of 0 to 70 degrees Celsius, the measurement accuracy can reach ±0.5 degrees Celsius. The sensor can work in the range of minus 55 to above zero 125 degrees Celsius. . It can be used in distributed sensing systems to connect multiple points. One bus can connect 8 DS620s at the same time and work at the same time. This article uses the IOA2 and IOA3 interfaces of SPCE061A to simulate the I2C bus to communicate with the DS620.

2.4 Balance module

When charging a series-connected battery pack, due to the differences in the chemical characteristics of each unit in the battery pack, if some unit cells are fully charged, but other unit cells have not been fully charged, this will result in fully charged battery units. Overcharging occurs, which will have a great impact on the battery. On the contrary, if those batteries cannot be fully charged for a long time, the internal resistance will increase and the battery capacity will be reduced, causing the battery to be easily damaged. One of the most effective ways to solve some undercharging and overcharging problems of batteries during charging is to implement balanced charging of batteries so that all batteries can reach a balanced and consistent state. The balancing scheme adopted by this battery management system is based on the principle of the bidirectional reversible DC/DC dynamic balancing method. Through the DC/DC switching power supply, the required voltage value of each single cell is detected during the charging and discharging process. The charged single battery is dynamically balanced and charged, and the battery pack's power is used to perform additional balancing charging on the battery. The DC/DC switching power supply uses Nova's DOM-24D15S5 chip. Its input voltage is between 18 and 36 volts, and its output voltage is between 4.6 and 5.5 volts.

2.5 charging module

Currently, most charging curves are a combination of constant voltage and constant current charging curves. In the later stage of charging of lithium batteries, based on the consideration of ensuring battery safety, constant voltage charging is required for battery charging. The ordinary charging method divides the battery charging process into three parts, namely: precharge, constant current and constant voltage. The principle and control process are relatively simple. In the early stage of charging, the charging speed is faster and the charging efficiency is higher. However, this method of charging generates a lot of heat. In order to solve this problem, this article changes precharging and constant voltage charging into intermittent charging. The constant current charging method relies on the current limiting control of the charging power adapter. The timing diagram of intermittent charging is shown in Figure 3.

When charging a lithium-ion battery pack, if the battery pack is equipped with a battery management system, it must be connected to an external constant-voltage and current-limiting power adapter that can match it. The expression for calculating the constant voltage value U is: U=4.2*N+loss voltage; in the above formula, N represents the number of cells of the battery, and the loss voltage is obtained through experiments. In this system, the lithium battery used is the TS-LCP50AHA type of Shenzhen Leitian Company. The current limit value Ic of this type of battery is between 0 and 0.5C, and C represents the battery capacity. When calculating, take the optimal charging current of TS-LCP50AHA battery as 0.3C. Before charging the battery, the system must be initialized first, and then the battery is charged in three steps: precharging, constant current charging and constant voltage charging.

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