Overview of Lithium Battery Charging Technology

　　Charging is the most important part of the use of lithium batteries, which directly affects the life and cycle performance of the battery. On the basis of the optimal charging curve of lithium batteries, various charging methods of lithium batteries are analyzed, their respective charging speeds, application ranges, battery life and cycle performance are compared, and finally the directions for future research are proposed.

　　Since the advent of lithium batteries in the 1970s, scholars have never stopped researching them and have made great progress. As an important member, secondary lithium batteries have the advantages of high single output voltage, long cycle life, large specific energy, small size, low self-discharge, no memory effect, no pollution and wide operating temperature range, and are widely used. to various fields, such as electronic instruments, digital and home appliances, electric vehicles, etc.

　　For lithium batteries, charging methods have a great impact on their performance. Reasonable charging methods can extend the life of lithium batteries and improve charging efficiency. This article analyzes the various charging methods of lithium batteries and compares their respective advantages and disadvantages in terms of charging speed, service life and implementation cost for your reference and communication.

　　1. Theoretical basis

　　In 1972, American scientist J.A. Mas proposed that there is an optimal charging curve for batteries during the charging process: I=I0-αt0 is the initial charging current of the battery; α is the charge acceptance rate; t is the charging time. The values of I0 and α are related to the battery type, structure and age.

　　The current research on battery charging methods is mainly based on the optimal charging curve. As shown in Figure 1, if the charging current exceeds this optimal charging curve, it will not only fail to increase the charging rate, but also increase the amount of gas evolution of the battery; if it is less than this optimal charging curve, although it will not cause damage to the battery, it will prolong the battery life. charging time and reduce charging efficiency.

　　2. Charging method

　　There are many ways to charge lithium batteries, which can be divided into conventional charging and fast charging according to charging efficiency. The conventional charging methods include: constant current charging, constant voltage charging, stage charging and intermittent charging, while fast charging includes pulse charging and Reflex charging. Finally, smart charging is also analyzed.

　　1.Constant current charging

　　According to the size of the charging current, constant current charging can be divided into fast charging, standard charging and trickle charging. During the entire charging process, the charging voltage of the power supply is generally adjusted or the resistance value in series with the battery is changed to maintain the charging current of the battery unchanged. The advantage of this method is that it is simple to control and is suitable for charging a battery pack with multiple batteries connected in series. The disadvantage is that the ability of the lithium battery to accept charging will gradually decrease as the charging progresses. Excessive charging current in the later stage of charging will cause bubbles inside the battery, causing damage to the battery. Therefore, constant current charging is often used as a link in stage charging.

　　2. Constant voltage charging

　　Constant voltage charging means that the charging voltage remains constant during the entire charging process, and the charging current automatically adjusts as the battery status changes. As charging proceeds, the charging current gradually decreases. Compared with constant current charging, its charging process is closer to the optimal charging curve, with simple control and low cost. The disadvantage is that the charging time is long, and the battery charging current is too large in the early stage of charging, which directly affects the life and quality of the lithium battery. Therefore, the constant voltage charging method is rarely used alone, and is only used when the charging power supply voltage is low and the current is large.

　　3. Constant current and constant voltage charging

　　Figure 2 shows the constant current and constant voltage charging curve. Before starting to charge, first detect the battery voltage. If the battery voltage is lower than the threshold voltage (about 2.5V), trickle charge the battery with a small current of C/10 to slowly increase the battery voltage; when the battery voltage reaches the threshold voltage , enter constant current charging. At this stage, the battery is quickly charged with a larger current (0.5C ~ 1C). The battery voltage rises quickly, and the battery capacity will reach about 85% of its rated value; when the battery voltage rises After reaching the upper limit voltage (4.2V), the circuit switches to constant voltage charging mode. The battery voltage is basically maintained at 4.2V. The charging current gradually decreases and the charging speed slows down. This stage is mainly to ensure that the battery is fully charged. When the charging current drops to When 0.1C or 0.05C, the battery is judged to be fully charged.

　　Constant current and constant voltage charging avoids the problem of excessive charging current at the beginning of constant voltage charging, and overcomes the phenomenon of overcharging that is prone to occur in the later stages of constant current charging. It has a simple structure and low cost. It is currently widely used in lithium battery charging methods. . But it cannot eliminate the polarization phenomenon during battery charging and affects the charging effect.

　　4. Pulse charging

　　It mainly includes three stages: precharging, constant current charging and pulse charging.

　　In the constant current charging process, the battery is charged with a constant current, and most of the energy is transferred to the inside of the battery. When the battery voltage rises to the upper limit voltage (4.2V), it enters the pulse charging mode: the battery is charged intermittently with a pulse current of 1C. The battery voltage will continue to increase during the constant charging time Tc, and the voltage will slowly decrease when charging stops. When the battery voltage drops to the upper limit voltage (4.2V), charge the battery with the same current value, start the next charging cycle, and cycle charging until the battery is fully charged.

　　During the pulse charging process, the battery voltage will gradually slow down and the charging stop time T0 will become longer. When the constant current charging duty cycle is as low as 5% to 10%, the battery is considered to be fully charged and charging is terminated. Compared with conventional charging methods, pulse charging can charge with a larger current. During the charging stop period, the concentration polarization and ohmic polarization of the battery will be eliminated, making the next round of charging go more smoothly, with fast charging speed and The temperature change is small and has little impact on battery life, so it is currently widely used. But its disadvantage is obvious: it requires a power supply with current limiting function, which increases the cost of pulse charging.

　　The pulse charging studied by C.K. Leong et al. Each charging cycle lasts about 1 second. The battery is first charged forward, then stopped charging and reverse discharged for 20 to 30ms each. The positive pulse current charges the battery, while the negative pulse current reduces the precipitation of gas from the electrode, allowing the battery to be charged quickly using a larger current.

　　5. Intermittent charging method

　　Lithium battery intermittent charging methods include variable current intermittent charging methods and variable voltage intermittent charging methods.

　　Variable current intermittent charging method

　　The variable current intermittent charging method was proposed by Professor Chen Ti of Xiamen University. Its characteristic is to change constant current charging to limited voltage variable current intermittent charging. As shown in Figure 4(a), in the first stage (also the main stage) of the variable current intermittent charging method, a larger current value is used to charge the battery, and charging is stopped when the battery voltage reaches the cut-off voltage V0. At this time, the battery voltage drops sharply. decline.

　　After maintaining a charging stop period, use the reduced charging current to continue charging. When the battery voltage rises to the cut-off voltage V0 again, charging is stopped. Repeatedly several times (usually about 3 to 4 times), the charging current will reduce the set cut-off current value. Then it enters the constant voltage charging stage, charging the battery at a constant voltage until the charging current decreases to the lower limit, and charging ends. In the main charging stage of the variable current intermittent charging method, under limited charging voltage conditions, an intermittent method of gradually decreasing current is used to increase the charging current, which speeds up the charging process and shortens the charging time. However, this charging mode circuit is relatively complex and expensive, and is generally only considered for high-power fast charging.

　　Variable voltage intermittent charging

　　On the basis of the variable current intermittent charging method, someone also studied the variable voltage intermittent charging method. The difference between the two lies in the charging process in the first stage, which replaces intermittent constant current with intermittent constant voltage. Comparing Figure 4(a) and Figure 4(b), it can be seen that constant voltage intermittent charging is more in line with the charging curve of optimal charging. In each constant voltage charging stage, due to the constant voltage, the charging current naturally decreases exponentially, which is consistent with the characteristic that the battery current acceptance rate gradually decreases as charging proceeds.

　　6.Reflex fast charging method

　　The Reflex fast charging method is also called the reflection charging method or the "burp" charging method. Each working cycle of this method includes three stages: forward charging, reverse instantaneous discharge and stop charging. It solves the battery polarization phenomenon to a great extent and speeds up charging. But reverse discharge will shorten the life of lithium battery.

　　Sheng-YuanOu and Jen-HungTian studied the Reflex fast charging method. The charging curve is shown in Figure 5. In each charging cycle, a 2C current charging time of 10s Tc is used, and then the charging stop time is 0.5s. Tr1, reverse discharge time is Td of 1s, charging stop time is Tr2 of 0.5s, and each charging cycle time is 12s. As charging proceeds, the charging current will gradually become smaller. Experiments have proven that this charging method can increase the charging time of a single lithium battery to 40 minutes, the battery temperature only increases by 1.1°C, and the charging efficiency reaches 87.51%.

　　7.Smart charging method

　　Smart charging is currently a more advanced charging method, as shown in Figure 6(a). Its main principle is to apply du/dt and di/dt control technology to determine the battery charging status by checking the increment of battery voltage and current. Dynamic Track the battery's acceptable charging current so that the charging current is always near the battery's acceptable maximum charging curve. In this way, the battery can be fully charged quickly with little gas evolution.

　　By combining neural network and fuzzy control, a fuzzy neural network controller and neural network model were developed, and an intelligent charging control system was designed. It not only has the characteristics of the fuzzy controller, which is good at expressing human experience knowledge and has strong reasoning ability, but also has the characteristics of the neural network controller, which learns knowledge directly from the control data and has strong learning ability. Experiments have verified that the intelligent charging method has stable voltage changes and short charging time during the charging process. Therefore, as one of the fuzzy adaptive control schemes, it will receive more and more attention in the future.

　　3. Conclusion

　　This article analyzes various charging methods for lithium batteries. According to the theory of J.A.Mas, there is an optimal charging curve. When this curve is infinitely close, the charging speed is the fastest and the efficiency is highest. Beyond this curve, the battery will be damaged. . Compared with constant current and constant voltage charging, smart charging and pulse charging make better use of J.A.Mas theory, combine the advantages of various methods, and are widely used. The problem of battery polarization during the charging process has always affected the charging effect. If battery polarization can be eliminated in the future, the battery charging speed will reach a new level.

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