1. The difficulty in promoting and applying at present is that the duration is short and the charging time is long.
1) One continuous mileage is short. At present, the current mileage of electric vehicles is relatively reasonable between 200 km and 300 km. But it is still more than half of the fuel car. Businesses repeatedly advertised that a charge can run 500 kilometers, 800 kilometers, is misleading consumers. The actual situation is that how many kilometers an electric car can run on a single charge is related to how much the car battery is linear. The farther you have to run, the more you have to pack the car battery.
The amount of vehicle battery in the design value of automotive engineering must consider its technical feasibility and economic rationality. It is the same as the fuel tank volume design of the fuel car, and it is not arbitrarily infinite. At present, the mileage of an electric vehicle cannot be as long as that of a fuel vehicle. The main reason is that the specific energy of the power battery is more than 100 times worse than that of the gasoline and diesel. We can hear the progress of power battery technology every day, but scientifically, in the short term, it is impossible for a new battery of a power battery to catch up with a fuel vehicle.
2) The charging time is as long as 6-8 hours, which is more scientific. In other words, in fact, the convenience of charging electric vehicles is unacceptable to everyone. First, use the home 220V AC to charge, charging time is 6-8 hours; Second, to the charging station with DC charging, in addition to queuing, the charging process must also take more than 30 minutes; the third is to buy a replacement battery Basically, there is no sale.
Second, the basic way to improve the convenience of charging
1) If you can charge with civilian AC, if the charging time is less than 3 hours, you can accept it;
2) If the charging station is charged with DC, the charging time is less than 1 hour to accept;
3) If you change the battery, you must use the battery for your own use and it is acceptable.
Third, the charging mode of the lithium ion power battery pack
The following is the charging curve, that is, the charging mode adopts the "current limiting, voltage limiting" two-stage charging mode.
1) Current limiting process:
The charging at this stage is called "current limiting" charging, and the charging current is kept at the current limiting value. With the continuation of charging, the battery electromotive force continues to rise, and the charging voltage of the battery also rises. Constant voltage charging is maintained when the battery voltage rises to the highest allowable charging voltage. Manufacturer's suggestion: Generally use the best charging rate (lithium ion battery is 0.3CA) for current limiting charging.
2) Pressure limiting process:
At this stage, as the battery electromotive force continues to rise and the charging voltage remains unchanged, the charging flow of the battery is in a hyperbolic trend and continues to drop to zero. The charging at this stage is called constant "constant voltage" charging, and the charging voltage at this stage: U = E + IR = constant voltage value.
(Practical: Stop charging when the charging current is reduced to 0.015CA.)
Third, charging the ion power battery pack is actually a dynamic process.
The charging system must have automatic adjustment of charging parameters, automatic control and automatic protection:
1) Automatically adjust the charging parameters
a) For example, during charging, especially during the constant voltage charging phase, if the charging voltage of one of the battery cells in the battery pack exceeds the allowable charging voltage (depending on the characteristics of different lithium batteries, it is generally set to 3.9V to 4.3V. When charging, the charging system should automatically reduce the charging voltage and current according to the signal output from the battery management system (BMS), so that the charging voltage of the battery does not exceed the allowable charging voltage to prevent overcharging of the battery.
b) For example, at the beginning of charging, the battery management system (BMS) monitors that the voltage of a single battery is too low, and the charging system should be able to automatically change the initial charging current. After the battery voltage is normal, turn it again. Enter normal charging.
Therefore, during the charging process, the charging system must be able to automatically adjust the charging parameters and automatically control the charging according to the state of the battery pack, the information output by the battery management system (BMS), and the information of the vehicle monitoring output.
2) Automatic control and automatic protection
During the charging process, when the battery pack and the electric vehicle are in any abnormal condition, such as short circuit, open circuit, high temperature, fire and battery damage, the charging system should be able to cut off the power quickly and stop charging.
3) Communication with the whole vehicle CAN bus
During the charging process, the information of the charging system can communicate with the vehicle CAN bus.
Fourth, charging the battery, actually "electric" can be converted into a "chemical" process
Charging the battery must be equipped with a charging device - the charger. The charger is after the AC power supply is rectified and filtered, and after a series of regulation, the DC power is output, and the power battery pack is supplied for charging, that is, the DC power received by the battery pack. Chargers are divided into three categories:
a) The most used charger in the early stage is a power frequency charger using a thyristor as a rectifying control element. A large and bulky power frequency transformer must be configured. Moreover, the efficiency is low, the power factor is low, and it is easy to cause pollution to the power grid.
b) High frequency switching power supply charger. It has the advantages of small size, light weight, reliable operation, high efficiency, high power factor, no fear of grid fluctuations and no grid fluctuations, no fear of grid interference and no interference to the grid, low power and large size, and easy realization of intelligence. It is especially suitable for high-frequency switching power supply chargers with low power to high power and extra high power. It can be made into a high-voltage charger through the series connection of the modules; it can also be made into a large-current charger through the parallel connection of the modules; it can also be made into a high-power, high-current high-power charger through the series and parallel connection of the modules. The switching power supply charger also has the function of automatically adjusting the charging parameters and automatically protecting the battery according to the information of the battery management system (BMS).
c) Kind of pulse charger, the structure is more complex, the technical difficulty is greater. Pulse charging can reduce the polarization phenomenon caused by the battery during charging, thereby improving the charging efficiency of the battery, reducing the charging time, and achieving fast charging.
As explained above, the conversion of electrical energy into chemical energy is a very complicated process. It is necessary to have a charger and BMS work together to complete this work.
5. Long charging time is a difficult problem in the promotion and application of electric vehicles.
1) One of the problems: charging time for charging electric vehicles according to the best charging requirements, at least 3 hours. Therefore, the charging method of an electric vehicle cannot be charged at a charging station like a gas station like a fuel car refueling at a gas station.
2) The second problem: the charging system of electric vehicles is independent. It is not possible to charge in parallel in the charging station. In other words, it is not possible to share a single charger with several electric vehicles.
Sixth, battery fast charge (quick change) is the inevitable requirement of the market for electric vehicles
1) Technical difficulty of battery fast charging: It can fully charge electricity in 20 minutes, and the market competitiveness of electric vehicles will be significantly improved. The difficulty is:
a) Have a battery that can accept fast charging. For the former lithium iron phosphate lithium ion battery, it is possible to charge the 3CA charging rate, and the lithium titanate battery can be charged with a higher charging rate of 10CA.
b) Fully charged for 20 minutes, then at least the charging rate of 3CA should be used for charging. The capacity of the battery harness and connector should be increased by at least 5 times. How to solve it?
c) The power required to charge the device is large. Take an electric bus as an example. It is equipped with 120 batteries of 600Ah. The charging current of 3CA is 1800A and the charging voltage is 510V. The output power of the charger is 918kw, and the power provided by the power grid is 1150kw. How to solve it?
2) Technical difficulty of battery quick change: It takes about 10 minutes to change the battery time. The public is satisfied, but its technical difficulties are:
a) When using the quick change method, the capacity of the battery pack is required to be halved. As a result, the battery's discharge rate is doubled, which will reduce the battery life.
b) When using the quick change method, the standardized design of the battery pack and the battery consistency and charging control requirements are higher, and the operating cost of the electric vehicle is increased.
c) If the battery is still properly configured, then double the battery reserve, which will naturally increase the operating cost of the electric vehicle.
Seven, opinions and suggestions
1) Compared with fuel vehicles, electric vehicles have no way to compare the convenience of one-off mileage and energy filling.
2) Conduct a detailed analysis of the use of electric vehicles, carry out comprehensive considerations, and carry out a combination method to improve the convenience of charging (changing) electricity. The basic principle is that electric vehicles are not charging, not charging, charging is the mainstay, and charging is supplemented by quick replenishment. The time for regular charging is about 4 hours. We advocate charging the charging rate of 1CA or so quickly, and charging it for more than 80%.
3) For electric buses, the charging station is located in the bus terminal. Use the low-altitude electricity to charge after work in the evening, and use the time for 5 to 6 hours. For vehicles that operate during the day, when the driving range is insufficient, the intermediate rest time can be used for supplementary charging. The bus with a height of more than 10 meters is best charged with overhead contact nets, supplemented by gun filling.
4) Other vehicles are mainly gun-filled, equipped with AC charger and DC charging. The two modes should be standard.
5) The charging rate of fast replenishment is 1CA, charging is more than 80%, the fastest can not exceed 3C, and it is necessary to have more than 20 minutes of patience.
6) With the portable charger below 15Kw, where to go.
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