Research and Analysis on the Charging System Technology of Electric Vehicles
1. Development of charging system standards
Because of the small scale and small number of electric vehicle charging stations, most of the related technologies of electric vehicle charging stations are still in the preliminary stage of practical application. The international standard for electric vehicle charging systems is mainly IEC61851:2001 issued by the IEC. The standard includes three parts: general requirements, requirements for connection of electric vehicles to AC/DC power supplies, and electric vehicles and AC/DC charging stations.
According to the development status of domestic electric vehicles, China formulated three standards in 2001. These three standards are equivalent to adopting the three parts of IEC61851. In recent years, with the rapid development of electric vehicles and power technology, these standards can no longer meet the current development needs, and these standards lack communication protocols, monitoring systems, and other aspects. At present, in order to regulate the application of internal electric vehicles, the State Grid Corporation has promulgated 6 enterprise standards related to electric vehicle charging stations.
At present, the lack of integrated technology for power supply, charging and battery system application and related standards and specifications is still the main weak link in the promotion and application of electric vehicles, which brings great difficulties to the next development of electric vehicles and the unified planning of charging facilities . There is no mature product for the charging station monitoring system that can ensure the normal operation of large-scale charging stations. There is no unified standard for the communication protocol and communication interface between the charging station monitoring system and the charger, and there is no information connection between charging stations.
2 Commonly used charging modes for electric vehicles
According to the technology and usage characteristics of electric vehicle power battery packs, there are certain differences in the charging mode of electric vehicles. Regarding the selection of charging schemes, there are generally three modes of conventional charging, fast charging and quick battery pack replacement systems.
2.1 Regular charging
1) Concept: After the battery is discharged, it should be charged immediately (in special cases it should not exceed 24h), and the charging current is quite low, about 15A. This kind of charging is called regular charging (ordinary charging). Conventional battery charging methods all use small current constant voltage or constant current charging, and the general charging time is 5-8 hours, or even more than 10 to 20 hours.
2) Advantages and disadvantages: Because the power and current ratings used are not critical, the charger and installation costs are relatively low; it can make full use of the low power period for charging, reducing charging costs; it can improve charging efficiency and extend battery life. The main disadvantage of the conventional charging mode is that the charging time is too long, which is difficult to meet when there is an emergency operation demand.
2.2 fast charging
Fast charging, also known as emergency charging, is to provide short-term charging services for electric vehicles within 20 minutes to 2 hours of parking for a short period of time with a relatively large current. The general charging current is 150-400A.
1) Concept: The charging method of conventional batteries generally takes a long time, which brings a lot of inconvenience to actual use. The emergence of fast rechargeable batteries provides technical support for the commercialization of pure electric vehicles.
2) Advantages and disadvantages: short charging time; long life of the rechargeable battery (more than 2000 rechargeable times); no memory, large capacity charging and discharging, 70% to 80% of the electricity can be charged within a few minutes; In a short time (about 10-15 minutes), the battery storage capacity can reach 80%-90%, which is similar to the refueling time. Therefore, the construction of the corresponding charging station does not need to be equipped with a large-area parking lot. However, compared with the conventional charging mode, fast charging also has certain shortcomings: the charging efficiency of the charger is low, the corresponding work and installation costs are high, and the charging current is large, so special consideration is given.
2.3 Mechanical charging
1) Concept: the battery pack quick replacement system. The purpose of charging is achieved by directly replacing the battery pack of the electric vehicle. Due to the heavy weight of the battery pack, the professional requirements for battery replacement are relatively strong, and professionals must be equipped to quickly complete the battery replacement, charging and maintenance with the help of professional machinery.
2) Advantages and disadvantages: electric vehicle users can rent fully-charged batteries and replace the depleted batteries, which is beneficial to improve the efficiency of the vehicle, and also improves the convenience and speed of the user; the replacement battery can use the low period of time Recharging reduces the cost of charging and improves the economy of vehicle operation; it solves the problems of charging time and even the amount of stored charge, battery quality, long driving range and price; it can detect the problems of single cells in the battery pack in time and perform maintenance Work, it will have a positive meaning for the maintenance of the battery, and the reduction of the depth of discharge of the battery pack will also help to improve the life of the battery.
3 charging station monitoring system
The electric vehicle charging station is an energy infrastructure service facility that guarantees the normal use of electric vehicles. The electric vehicle charging station monitoring system is a centralized monitoring system for the equipment in the charging station. It is very necessary to research and develop the electric vehicle charging station monitoring system. At present, the charging station monitoring system developed in China basically has the following major functions.
3.1 Charge monitoring function
1) Monitoring of the charging pile: monitor the status of the AC output interface of the charging pile, such as current, voltage, switch status, protection status, etc.; collect the basic information of the electric vehicle connected to the charging pile; control the AC output interface of the charging pile Switch off.
2) Monitoring of the charger: as the monitored object, the charger is sent to the monitoring system mainly including two types of data, the charger status information and the battery status information.
3.2 Power distribution monitoring function
Realize the monitoring of power distribution equipment in electric vehicle charging stations, facilitating unified management and data sharing. It can realize the monitoring and control of the total power, total current, total electricity, power factor, main transformer status, switch status, reactive power compensation and harmonic control equipment of the entire station.
3.3 Battery maintenance monitoring function
In a large integrated charging station, after the battery has been used for a period of time, due to the consistency problem of the battery module in the battery pack, it is necessary to maintain and evaluate the battery through special battery maintenance equipment to determine whether to reconfigure the battery. Group.
3.4 Quickly replace equipment monitoring function
In charging stations with fast battery replacement equipment, the charging station monitoring system can collect information such as the current track position and equipment status of the fast replacement equipment, and the charging station monitoring system can issue specific battery replacement commands to the fast battery replacement equipment: let fast replacement The device replaces the battery pack at the specified position on the battery rack at the specified track position.
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