L822 battery

Analysis of on-board power L822 battery system and charger charging technology

Electric vehicles use electricity instead of fossil fuels as power, which is the only long-term solution for future transportation. As the heart of electric vehicles, the power battery system can only be promoted smoothly if it is fully understood. This article focuses on the development trend of the main on-board power batteries of electric vehicles at home and abroad, and analyzes the lithium-ion batteries and their battery management systems with relatively promising development prospects.

Unbalanced charging of lithium-ion battery packs can easily cause overcharge and discharge problems, seriously damaging their service life. This article proposes a new intelligent charger charging mode, which makes the battery pack safer and more reliable, can extend its service life, increase safety and reduce the cost of use.

1. On-board lithium-ion battery management system

As the monitoring "brain" of electric vehicle batteries, the battery management system (BMS) can monitor the remaining battery power in hybrid electric vehicles, predict the power intensity of the battery, and facilitate the understanding of the entire battery system and the control of the entire vehicle system.

In pure electric vehicles, BMS has intelligent adjustment functions such as predicting the remaining battery power, predicting mileage and fault diagnosis. BMS has a particularly significant effect on lithium-ion batteries. It can improve the battery's usage status, extend the battery's service life, and increase battery safety. BMS will be a key technology for the future development of electric vehicles.

Power battery system, car charger charging technology, electric vehicle charging technology

As shown in Figure 1, the data acquisition module in the BMS measures the voltage, current, and temperature of the battery pack, and then transmits the collected data to the thermal management module and the safety management module for data display. The thermal management module controls the temperature of the battery cells to ensure that the battery pack is within the optimal temperature range.

The safety management module judges the voltage, current, temperature, and state of charge (SOC) estimation results of the battery pack, issues a fault alarm when a fault occurs, and takes emergency protection measures such as circuit breaking in time. The state estimation module estimates the SOC and health state (SOH) based on the collected battery state data.

At present, it is mainly SOC estimation, and the SOH estimation technology is not yet mature. The energy management module controls the charging and discharging process of the battery, including battery charge balancing management, which is used to eliminate the problem of inconsistent power of each cell in the battery pack. The data communication module uses CAN communication to realize the communication between BMS and on-board equipment and non-on-board equipment.

The core functions of BMS are SOC estimation, balancing management, and thermal management. In addition, it also has other functions such as charge and discharge management, pre-charger charging management, etc. During the battery charging and discharging process, it is necessary to manage according to relevant parameters such as environmental status and battery status, and set the optimal charge and discharge curve of the battery, such as setting the charger charging current, charger charging upper limit voltage value, and discharge lower limit voltage value. The capacitive load in the high-voltage system circuit of electric vehicles is equivalent to a short circuit at the moment of power-on, so pre-charger charging management is required to prevent transient current shock when the high-voltage circuit is powered on.

2. Core functions of battery management system

2.1 SOC estimation

SOC is used to describe the remaining battery power and is one of the most important parameters in the battery use process. SOC estimation is the basis for judging battery overcharge and over-discharge. Accurate estimation can avoid overcharge and discharge problems of battery packs to the greatest extent, making it more reliable.

The estimation of battery SOC shows a very strong nonlinearity under the influence of changes in the internal working environment and the external use environment. There are many internal and external factors that affect battery capacity, such as battery temperature, battery life, battery internal resistance, etc., and it is very difficult to accurately complete SOC estimation.

The existing SOC estimation methods are as follows:

(1) Ampere-hour measurement method. The ampere-hour measurement method does not consider the changes in the internal structure and state of the battery, so it has the advantages of simple structure and easy operation, but the accuracy of this method is not high. If the current measurement accuracy is not high, then over time, the SOC cumulative error will continue to increase, affecting the final result. This method is suitable for measuring the battery SOC on electric vehicles. If the measurement accuracy can be improved, it is a simple and reliable SOC measurement method.

(2) Open circuit voltage method. The open circuit voltage of lithium-ion batteries has an approximately linear relationship with SOC and can be used to determine the internal state of the battery. However, due to the strict measurement requirements, the battery needs to be left standing for at least 1 hour, which is not suitable for online real-time detection of batteries in electric vehicles. In general, because the open circuit voltage method has a high accuracy rate in the initial and final stages of charging, the open circuit voltage method is often used in combination with the ampere-hour measurement method.

(3) Kalman filter method. The Kalman filter method is particularly suitable for hybrid batteries with severe current fluctuations due to its excellent error correction ability. The disadvantage of this estimation method is that it has high requirements for system processing speed.

(4) Neural network method. Neural networks have the characteristics of distributed parallel processing, nonlinear mapping and adaptive learning, so they can be used to simulate battery dynamic characteristics and estimate SOC. However, this method requires a large amount of reference data for neural network learning, and the data and training methods have high requirements, otherwise it will cause unacceptable errors.

2.2 Balancing management

In the process of producing batteries, many processes must be passed, and differentiation will cause inconsistent states. The difference between battery cells is mainly reflected in the difference in internal resistance and capacity over time and temperature changes. Large differences between cells are more likely to cause overcharging or over-discharging, causing battery damage. Realizing battery balancing can maximize the effectiveness of power batteries, extend battery life and increase safety. The mainstream balancing methods at home and abroad at this stage are as follows:

(1) Resistance balancing method. This method is the main representative of energy dissipation balancing method. The method is simple and low cost, but the energy loss is relatively large and the efficiency is low. It is only suitable for systems with small current charging and discharging.

(2) Switched capacitor method. This method is the main representative of non-energy dissipative balancing methods, which makes up for the shortcomings of resistance balancing. However, its control circuit is complex, the balancing speed is slow, and it takes a long time, which is not suitable for high current use.

(3) Transformer balancing method. This method is an active balancing control method for series battery packs based on a symmetrical multi-winding transformer structure. Its disadvantages are complex circuits, many devices, and a large volume, which is not easy to expand the battery pack. It is generally suitable for large current charging and discharging.

(4) Centralized balancing. This method can quickly transfer energy from the entire battery pack to the battery cell, and the volume of the centralized balancing module is smaller. However, the balancing operation of multiple batteries cannot be carried out in parallel, and a large number of cables are required for connection, which is not suitable for battery packs with a large number of batteries.

2.3 Thermal management

Temperature affects all aspects of battery performance. The unevenness of the temperature field will aggravate the inconsistency of the battery pack, so it is very necessary to manage it. The purpose of thermal management is to maintain the temperature of the battery system within a certain range through heating or heat dissipation measures, and to try to maintain the temperature consistency within the battery pack.

Temperature management mainly completes the following four functions: (1) Rapidly heat the battery pack under low resistance conditions; (2) Ensure the uniform distribution of the battery temperature field; (3) Accurately measure and monitor the battery temperature; (4) Effectively evacuate heat when the battery pack temperature is too high. Common cooling methods include natural convection, forced air convection, liquid flow, phase change material and thermal management, and common heating methods include battery internal heating, heating plate, heating sleeve and heat pump.

3. Lithium-ion battery charger charging technology

3.1 Current status and development trend

In practical applications, choosing different charger charging modes according to the battery capacity limit is an inevitable choice to extend the battery life. There are many charging methods for lithium-ion battery chargers, and the simplest one is the constant voltage charger charging method. Lithium-ion battery packs are generally composed of a large number of monomers in series. Due to the differences in the manufacturing process of each monomer, there are inconsistencies in internal resistance, voltage, capacity and temperature, which can easily cause imbalance in the charging and discharging process, that is, shallow discharge of large-capacity monomers and over-discharge of small-capacity monomers, which will cause serious damage to the battery pack. Solving the problem of unbalanced charging and discharging is the research focus of lithium-ion battery packs.

The requirements of electric vehicles for battery charger charging technology include:

(1) Faster charging process. The low specific energy of power batteries leads to a short range of one-time charging, which has always been an important factor restricting the development of electric vehicles. As long as the battery can be charged faster and more efficiently, the short range of electric vehicles can be indirectly compensated.

(2) Universal charging equipment. In order to pursue relevant academic frontiers, optimize their own products and strive for as much market share as possible, various new types of batteries emerge in an endless stream and coexist in this market. In the case of the coexistence of batteries of different types and voltage levels, the charging equipment in public places needs to have a wider adaptability. On the one hand, the charger needs to be suitable for as many batteries as possible. On the other hand, for different voltage levels, the charger needs to meet customer requirements.

(3) Intelligent charging strategy. In order to achieve lossless charging of batteries as much as possible, monitor their charging and discharging status, avoid over-discharge, and achieve the purpose of both energy saving and delayed aging, a more intelligent charging strategy is needed. That is, different charger charging strategies are provided for different batteries to match the charging curve of the battery charger.

(4) Efficiency of electric energy conversion. The energy loss of electric vehicles is closely related to the operating cost. In order to further promote electric vehicles, it is necessary to balance its cost-effectiveness and reduce energy consumption as much as possible.

(5) Integration of charger charging system. With the requirements of system miniaturization and multi-functionality, as well as the improvement of battery reliability and stability, the charger charging system will be integrated with the electric vehicle energy management system as a whole, integrating current detection and reverse discharge protection and other functions, without external components, to achieve a smaller and more integrated charger charging solution, thereby saving layout space for the rest of the electric vehicle components, greatly reducing system costs, and optimizing the charger charging effect and extending battery life.

3.2 Intelligent charger charging technology

Based on the above analysis of the current status of lithium-ion battery packs and their chargers, and in view of the imbalance and safety problems that are easy to occur during the charging process of lithium-ion battery packs, this paper summarizes an intelligent charger charging mode based on electric vehicle BMS, as shown in Figure 2.

Power battery system, car charger charging technology, electric vehicle charging technology

During the entire charger charging process, the BMS system mainly monitors the battery voltage and current signals and detects the temperature and connection status of the lithium-ion battery pack; the intelligent management system in the charger monitors the output mode of the charger in real time. The BMS system and the charger intelligent management system realize intelligent communication, perform real-time mode comparison between the battery pack and the charger status, and select the optimal charger mode for the battery pack.

During the initial charging process of the charger, the BMS estimates the maximum allowable charger charge of the lithium-ion battery pack, that is, evaluates the SOC of the cells of the entire battery pack and measures the maximum allowable charger charge of the battery pack. Combined with the pre-set charger charge safety factor, the maximum allowable charger charge of the battery pack is calculated.

During the charger charging process, the lithium-ion battery pack is charged according to the maximum allowable charger charge. Make full use of the energy management module of the BMS to perform charger charge balancing control on the battery pack cells to ensure the consistency of cell parameters. At the same time, during the charging process of the charger, the SOC value needs to be periodically detected (the detection cycle is determined according to the increasing gradient of the battery charge).

The state estimation function of the BMS system is used in combination with safety management to prevent the battery pack from overcharging. After the maximum charger charge of the battery pack is reached, the BMS and the charger charging equipment intelligent management system can intelligently control the charger charging controller to end the charger charging process. At the same time, the BMS disconnects the communication with the charger intelligent monitoring system.

The intelligent charger charging method can not only solve the problem of unbalanced charging of lithium-ion battery packs, but also maximize the safety of battery pack charger charging, extend the service life of lithium-ion battery packs, and ensure its safety in use.

4. Lithium-ion battery detection technology

my country has vigorously developed the electric vehicle industry and actively promoted the construction of related charger charging facilities. However, these demonstration devices have found many problems in operation, such as battery screening and matching, equipment heating, and poor contact of the plug-in interface of the connection device. If these problems that occur in a small number of devices cannot be solved, after the large-scale application of electric vehicles, there will be a situation of being overwhelmed, which will inevitably have an adverse impact on its development.

With the large-scale construction of electric vehicle infrastructure, relevant supporting detection solutions are urgently needed. Tianjin Electric Power Company has carried out the project "Research on Detection Technology of Key Charging Equipment for Mobile Electric Vehicle Chargers", among which the most important thing for electric vehicle battery swap stations is the detection of battery packs.

The electric vehicle battery swap station mainly includes battery fault diagnosis, screening maintenance and charging technology of sub-box chargers based on BMS monitoring, and will focus on the performance of battery screening devices and chargers. The study and mastery of lithium-ion battery characteristics is conducive to judging the accuracy of screening devices in battery swap stations and improving battery life.

By investigating a large number of key charging equipment for chargers that have been put into operation, it is helpful to master their operating characteristics and fault characteristics, improve detection efficiency, and form a simple and fast mobile detection solution. This will be a strong core technology guarantee and contribute to the comprehensive development of electric vehicles.

5. Conclusion

This paper analyzes the lithium-ion battery system, focuses on the composition and core functions of BMS, and proposes an intelligent charger charging mode for the problem of unbalanced charging of battery pack chargers.

A complete intelligent charger charging system can coordinate the supply and demand relationship between the charger and the battery pack, provide a safer and more reliable charger charging mode for the battery pack, extend its life, increase the reliability of the battery pack and reduce the operating cost, which will become the research focus of future electric vehicle technology. The research and development of a convenient and fast "mobile" charger key equipment detection device that matches the intelligent charger charging technology is imperative.

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