As the global climate deteriorates, all human beings discuss how to deal with climate change, energy conservation and emission reduction, and the search for new energy sources are important ways to reduce environmental damage. At present, the global automobile ownership has reached 1 billion. Automobile exhaust is an important factor that aggravates environmental degradation. To this end, many enterprises are developing non-polluting electric vehicles, and electric vehicle power batteries are the most important part of them. direction.
At present, most electric vehicle companies and research institutes use lithium-ion battery packs as their power batteries. Lithium-ion batteries have many advantages. Compared with traditional nickel-cadmium and nickel-hydrogen batteries, lithium-ion batteries are small in size, light in weight, high in operating voltage, and large in capacity. Lithium-ion batteries have high energy density, and their capacity is the same weight. Ni-MH batteries are 1.5 to 2 times higher, and have a very low self-discharge rate. In addition, lithium-ion batteries have almost no "memory effect" and do not contain toxic substances such as lead and cadmium, so they become the first choice for electric vehicle batteries.
The lithium-ion battery pack used in high-power electric vehicles is connected in series by a multi-cell lithium-ion battery to obtain a higher output voltage. However, both the lithium-ion battery pack and the single-cell lithium-ion battery are in use. There is a big difference. In the battery pack, there is inevitably individual difference in the production of each single-cell lithium-ion battery, and the degree of aging is different in use. If the measures are not taken during the charging process, the difference will be accumulated or even expanded, resulting in the entire battery pack. The performance is greatly reduced or the battery pack life is seriously shortened. In addition, due to the large capacity of the battery pack, the conventional charging method has low efficiency and long charging time, which seriously hinders the promotion of the electric vehicle.
Therefore, the fast, effective, safe and reliable charging system is currently the key technology developed in the electric vehicle industry.
1. Research on charging method of traditional lithium ion battery
Different charging methods are important factors affecting the performance and service life of single-cell lithium-ion batteries. A suitable charging method not only maximizes the capacity of the battery, but also extends the life of the battery. This effect is mainly reflected in three aspects:
(1) The effect of voltage. On the one hand, in the charging process, the battery voltage should not be strictly controlled to exceed the charging limit voltage. The charging limit voltage is called overcharge. If the battery is overcharged several times, the battery capacity will decrease and the battery will be deformed. The battery exploded. On the other hand, after the end of charging, the battery voltage should be as close as possible to the full charge voltage, otherwise the battery capacity will be greatly reduced.
(2) The effect of current. The acceptable charging current of a lithium ion battery is limited. If the charging current is higher than the upper limit value, a gassing reaction occurs in the electrolyte in the battery, and a large amount of heat is generated, so that the battery temperature rises sharply.
(3) The effect of temperature. If the temperature of the lithium-ion battery is too high, a series of reactions will occur inside the battery, and the battery may explode. Therefore, during the charging process, the battery temperature should be monitored and controlled.
Choosing the right charging method can improve charging efficiency and extend service life. There are many charging methods for lithium ion batteries. Common charging methods include constant current charging method, constant voltage charging method, constant current/constant voltage charging method, variable current charging method, pulse charging method, and intermittent charging method.
1.1 Constant voltage charging method
The constant voltage charging method refers to charging a battery at a constant voltage during charging. In this process, the charging current satisfies the formula: I = (U - E) / R (where: I is the charging current, U is the charging voltage, E is the battery voltage, and R is the charging loop resistance).
In the initial stage of charging, since the battery potential is low, the charging current is large, and as the charging process progresses, the battery potential gradually increases, and the charging current gradually decreases. The constant voltage control system has a simple structure, and the charging current is relatively close to the acceptable charging current, and has certain adaptability. However, in the initial stage of charging, the current is relatively large, which may cause the battery temperature to rise too fast, which may adversely affect the battery.
1.2 constant current charging method
The constant current charging method refers to charging at a constant current throughout the charging process. The charging current of lithium-ion battery can be exponentially decreased with the charging time, and the charging current remains unchanged. If the charging current is large, the charging current may exceed the acceptable current in the late stage of constant current charging, resulting in the analysis of the battery electrolyte. Gas reaction, but if the charging current is too small, it will prolong the charging time and reduce the charging efficiency.
Both of the above methods are relatively simple and have many shortcomings. In the initial stage of constant voltage charging, the current is too large and the current in the late stage of constant current charging is too large. Therefore, the charging method of constant current and constant voltage can be used to overcome the disadvantages of the two methods in these two stages.
1.3 constant current / constant voltage charging method
This charging method divides the charging process into three phases. As shown in Figure 1.
(1) Pre-filling stage. After the DC power is turned on, when the battery is detected, the charging chip starts and enters the pre-charging process, during which the charging controller charges the battery with a small current to restore the battery voltage and temperature to a normal state.
(2) Constant current charging phase. At the beginning of charging, the charging circuit charges the lithium ion battery with a constant current. Generally, the lithium battery mostly uses a standard charging rate. When the constant current is charged, the battery voltage will rise slowly. Once the battery voltage reaches the set termination voltage, the constant current charge is terminated and the constant voltage charging process is entered.
(3) Constant voltage charging stage. During the constant voltage charging process, the charging current is gradually attenuated. When the charging current is detected to fall below the set value, or the full charging time expires, the charging is turned into the top end charging. At this time, the charging controller replenishes the battery with a very small charging current. In general, this process can extend the battery life by 5%~10%.
In this charging method, in order to avoid excessive current, the battery temperature is too high, and in the constant current phase, charging is usually performed with a small charging current, and the charging efficiency is still not high. In order to improve the charging efficiency, a variable current charging method can be employed.
1.4 variable current charging method
The acceptable charging current of the lithium-ion battery decreases exponentially with the charging time. If the charging current curve is above the acceptable charging current curve of the battery (curve 1 in Figure 2), the battery electrolyte will have a gassing reaction, which will affect the battery life.
The idealized charging process is that the charging current is always approaching an acceptable charging current value. Curve 2 in Figure 2 is a variable current charging current curve. As can be seen in Fig. 2, in the initial stage of charging, the charging current is large and the efficiency is high, which overcomes the shortcoming of the initial charging current setting in the constant current/constant voltage charging phase. As the charging continues, the charging current is gradually reduced. The original constant current charging phase can be replaced by a segmented constant current charging process to improve the charging efficiency.
In the variable current charging phase, by detecting the state of the battery, the initial current is determined to perform constant current charging, and the battery state is detected. When the battery state reaches the constant current charging segment termination criterion, the segment is ended, and the current is adjusted. Value, enter the next segment, until the battery state reaches the set standard, terminate the converter charging, enter the constant voltage charging to ensure that the battery is fully charged.
The main difficulty of the variable current charging method is to determine the constant current charging current value at each stage, and select the appropriate parameters as the basis for judging the termination of the phase constant current charging.
In the variable current charging method, the purpose of fast charging is achieved by increasing the charging current in the initial stage. However, in fact, excessive charging current causes the battery to be polarized and shortens the life. In order to reduce the polarization reaction as much as possible under the premise of ensuring efficiency, the researchers compared the polarization of the battery by different charging waveforms, and proposed a method of intermittent charging and pulse charging.
1.5 intermittent charging method
The intermittent charging method refers to increasing the intermittent time after charging for a period of time to reduce the polarization phenomenon. In the intermittent phase, the hydrogen and oxygen generated by the electrolyte gassing reaction have time to be recombined, which can effectively slow down the voltage rise in the battery, eliminate the ohmic polarization, reduce the internal resistance, and enable the battery to be charged in the next charging process. Accept more power.
1.6 pulse charging method
In the pulse charging process, on the basis of the charging current approaching the acceptable current of the battery charging, the battery is charged with the pulse current, and the charging current is sometimes absent, and the charging state and the suspended state alternate with each other. The pulse charging mode has two modes: positive pulse charging and positive and negative pulse charging. In the positive pulse charging mode, the ions in the electrolyte are freely diffused during the positive pulse idle time, which reduces the polarization increase; charging in positive and negative pulses In the mode, a negative pulse is added during the positive pulse idle time, and ions in the electrolyte are moved in opposite directions by the reverse force, which effectively suppresses the polarization phenomenon.
2, the charging method used in this article
Comparing the characteristics of the above various conventional charging methods, it can be seen that each method has its own advantages and disadvantages. Electric vehicle battery packs require high charging process. They must require safe charging, extend battery life as much as possible, and must meet the characteristics of fast charging. Fast, efficient, safe, long service life must be achieved. The advantages of each charging method, this paper has been deformed and improved in the traditional charging method, and a voltage-limiting variable-current pulse charging method is proposed.
The characteristic of the voltage limiting variable current pulse charging method is that the constant current charging phase of the constant current/constant voltage charging mode is changed to the variable current pulse charging, and the current current is in accordance with the characteristics of the variable current charging, and the initial current is large, try to Approaching charging can accept current to ensure that the battery obtains sufficient current. At the same time, the pulsed charging current is used to effectively suppress the polarization effect, slow down the internal pressure of the battery, and reduce the internal resistance. In the later stage, a constant voltage charging method is adopted to obtain an overcharge amount.
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