Research on Fast Charge of Electric Vehicle Battery Pack

introduction

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, and automobile exhaust is an important factor that aggravates environmental degradation. To this end, many organizations are developing pollution-free electric vehicles, and electric vehicle power batteries are the core of them, and they are the focus of research.

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, and work. High-voltage, high-capacity, lithium-ion battery has a high energy density, its capacity is 1.5 to 2 times that of a nickel-hydrogen battery of the same weight, and has a very low self-discharge rate. In addition, lithium-ion batteries have almost no " Memory effect", does not contain lead, cadmium and other toxic substances, so it becomes 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 a certain individual difference in the production of each single-cell lithium-ion battery. The degree of aging is different in the use. If the measures are not taken during the charging process, the difference will be accumulated and even The expansion has led to a significant reduction in the performance of the entire battery pack or a serious shortening of the battery pack life. 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 electric vehicles.

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. Suitable charging methods not only maximize the battery capacity, but also extend the battery life. This effect is mainly reflected in three aspects. :

(1) Influence of voltage. On the one hand, it is necessary to strictly control the battery voltage during charging to exceed the charging limit voltage. Exceeding the charging limit voltage is called overcharging. A slight overcharging will cause the battery capacity to decrease and the battery to be deformed. When the battery is overcharged, it will directly cause the battery to explode. On the other hand, after the charging is completed, the battery voltage should be as close as possible to the full charge voltage, otherwise the battery capacity will be greatly reduced.

(2) The influence of current. The acceptable charging current of lithium-ion battery is limited. If the charging current is higher than this upper limit, it will cause the gassing reaction of the electrolyte in the battery, and a large amount of heat will cause the battery temperature to rise sharply.

(3) Effect of temperature. Lithium-ion battery temperature is too high, which will lead to a series of reactions inside the battery. The battery may explode. Therefore, during the charging process, the battery temperature should be monitored and controlled.

Selecting the appropriate charging method can improve the charging efficiency and prolong the 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, and variable current charging. Method, pulse charging method, intermittent charging method, etc.

1.1 Constant voltage charging method

Constant voltage charging refers to charging the 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, R is the charging loop resistance).

In the initial stage of charging, since the battery potential is low, the charging current is large. 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 acceptable charging. The current has certain adaptability, but the current is relatively large at the initial stage of charging, 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 the lithium ion battery can be exponentially decreased with the charging time, and the charging current remains unchanged. If the charging current is large, In the late stage of constant current charging, the charging current may exceed the acceptable current, resulting in a gassing reaction of the battery electrolyte. However, if the charging current is too small, the charging time will be prolonged and the charging efficiency will be reduced.

The above two methods are relatively simple and have many shortcomings. The current in the initial stage of constant voltage charging 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 two types. The drawbacks of the way 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.

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(1) Pre-charging stage. After the DC power is turned on, when the battery is detected, the charging chip starts and enters the pre-charging process. During this period, the charging controller charges the battery with a small current to restore the battery voltage and temperature. normal status.

(2) Constant current charging phase. At the initial stage of charging, the charging circuit charges the lithium ion battery with a constant current. Generally, the lithium battery mostly uses the standard charging rate. When the constant current is charged, the battery voltage will rise slowly, once the battery voltage reaches the set. The final termination voltage, the constant current charge is terminated, and the constant voltage charging process is entered.

(3) Constant voltage charging phase. During constant voltage charging, 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 to the top to cut off the charging. At this time, the charging controller is extremely small. The charging current is used to replenish the battery. In general, this process can extend the battery's 5% to 10% usage time.

In this charging method, in order to avoid excessive current, the battery temperature is too high. In the constant current phase, charging is usually performed with a small charging current, and the charging efficiency is still not high. To improve the charging efficiency, a variable current charging method can be used.

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 react to the gas, which will affect the battery life.

The ideal charging process is that the charging current is always close to the acceptable charging current value. As shown in Fig. 2, curve 2 is the variable current charging current curve. As can be seen in Figure 2, in the initial stage of charging, the charging current is larger and the efficiency is higher. It 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. Therefore, the original constant current charging phase can be charged by the segmented constant current charging process. Instead, to achieve improved charging efficiency.

In the variable current charging phase, the battery current is detected to determine the initial current for constant current charging, and the battery state is detected. When the battery state reaches the constant current charging segment termination criterion, the segment ends and the current is adjusted. Value, enter the next segment, until the battery state reaches the set standard, terminate the variable current 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, the charging current is too large, causing polarization of the battery and shortening the life. In order to reduce the efficiency as much as possible under the premise of ensuring efficiency The small polarization reaction, the researchers compared the polarization of the battery generated by different charging waveforms, proposed the 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 ohmic polarization and reduce internal resistance, so that the battery can accept more power during the next charging process.

1. 6 pulse charging method

In the process of pulse charging, 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 pause state alternate with each other. The pulse charging mode has positive pulse charging and positive In the positive pulse charging mode, during the positive pulse idle time, the ions in the electrolyte are freely diffused, which reduces the polarization; in the positive and negative pulse charging mode, the positive pulse idle time is added. When a negative pulse is applied, the ions in the electrolyte are moved in opposite directions by the reverse force, effectively suppressing the polarization phenomenon.

2 The charging method used in this article

Comparing the characteristics of the above various traditional charging methods, each method has its own advantages and disadvantages. The electric vehicle battery pack requires a high charging process, which requires safe charging and prolongs the battery life as much as possible. Must meet the characteristics of fast charging, fast, efficient, safe, long service life, these indicators must be achieved, to combine the advantages of each charging method, this paper has been deformed and improved in the traditional charging method, proposed a pressure limiting Stream pulse charging method.

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 magnitude is in accordance with the characteristics of the variable current charging, and the initial current is large, try to Approximating the charging to accept the 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. Later, the constant voltage charging method is adopted to obtain overcharge. the amount.

3 Study the significance of battery pack equalization charging

Lithium-ion battery packs for electric vehicles are connected in series by multi-cell lithium-ion batteries to obtain high output voltage. Since the battery characteristics of the individual cells are inevitably different during manufacturing and use, the reasons for this difference are: Due to the processing technology and other reasons, the capacity and internal resistance of the same batch of batteries may be different; the battery usage is unbalanced due to temperature differences during use; the discharge conditions between different batteries are different, accumulating for a long time, causing the battery Unbalanced state. Long-term use will inevitably lead to imbalance of parameters such as capacity, internal resistance and terminal voltage of each battery, thus affecting the actual capacity of the entire battery pack, shortening the life, causing waste and increasing costs.

Compared with other batteries, lithium-ion batteries have almost no overcharge resistance. When the battery state is full, if the battery continues to charge, the battery voltage will continue to rise. If the voltage is too high, lithium ions will accumulate in the negative electrode. Metal lithium is used to lose the battery's storage capacity, and this process is irreversible. At the same time, the electrolyte is electrolyzed, and hydrogen and oxygen are evolved. With a large amount of heat, the battery temperature gradually rises, and hydrogen and oxygen may be generated. Explosion. Therefore, the lithium battery must not be overcharged, otherwise it will cause fatal damage to the battery or cause a safety accident. Experimental data proves that overcharge will seriously reduce the number of charge cycles of the battery.
Therefore, when charging the series battery pack, the battery cells cannot be balanced by overcharging. If the battery is filled with a large capacity, the battery with a small capacity will inevitably be overcharged and damage the battery; If the battery with a small capacity is full, it will inevitably lead to under-charging of the large-capacity battery. These two methods can not reach the equilibrium state. Also in the discharge process, if the battery is discharged based on the large-capacity battery, it will inevitably lead to a battery with a small capacity. If the battery with a small capacity is discharged, the battery with a large capacity will retain most of the power and cannot be fully utilized. The battery pack of the electric vehicle needs frequent charging and discharging. As the number of charging and discharging increases, Forming a vicious circle, the imbalance of each battery will be intensified, causing the performance of the entire battery pack to deteriorate significantly.
In order to cope with the above problems, in the charging process, through some method, all the batteries of the battery pack can be fully charged when the charging is completed, and the equilibrium state is achieved, and the equalization control is realized.

4 battery pack charging equalization control research

At present, the method of equalization control is mainly divided into energy dissipation type, energy conversion type and energy transfer type according to energy consumption.

Energy dissipation refers to the discharge of a battery with a higher voltage in the battery pack to achieve equalization. The commonly used method is the current shunt method. The shunt method is to connect each battery in parallel with a shunt resistor and control it through the switch. In the process, when a battery voltage is high, its shunt switch is closed, and the battery is shunted. This method is simple and easy to implement. The disadvantage is that the shunt resistor is always losing power, low energy efficiency, and generating heat. Electric vehicles need Considering the problem of energy efficiency, otherwise it cannot be promoted on a large scale. Obviously, this method of equalization is not applicable. There are two main types of energy conversion, one is to supplement the battery pack's overall voltage to the starved single cell, and the other is The cell voltage is converted to the overall voltage. The coil energy conversion method is commonly used, and the energy is supplemented to the starved battery by the transformer. There are three types of switches: a shared type, a shared type, and a free type. The coil energy conversion method equalizes the charging current. The charging rate is very fast, but when a large number of batteries are charged in series, the shortcomings are highlighted. At this time, a large number of transformers and magnetic wires are required.

The energy transfer type uses energy storage elements such as capacitors or inductors to transfer energy. During the charging process, the battery capacity of the battery pack with a low capacity is higher than other batteries. At this time, the low capacity battery charges the high capacity battery. Commonly used are the capacitance method and the inductance method.

The capacitive equalization circuit controls the switching of the switch through the MCU. First, the battery with the highest voltage is charged to the capacitor. After the battery is fully charged, the switch is switched. The capacitor is charged to the battery with low voltage, and the process is repeated many times. The advantage of this equalization circuit is that The structure is simple and the volume can be made small. The disadvantage is that a large number of power electronic switching devices are required, and the loss is large and the time required is long, so it is not suitable for charging the electric vehicle battery pack.

The principle of the inductive adjacent equalization circuit is that each battery is connected in parallel with an inductor to detect and compare the voltages of two adjacent batteries B1 and B2. If the B1 voltage is high, the MCU controls the duty ratio of the PWM output signal to control the switching device. , the B1 energy is stored in the inductor, and the common capacitor is charged, and then the energy is transferred to B2; conversely, if the B2 voltage is higher, the duty ratio of the PWM output changes, so that the energy flows from B2 to B1. Features of the equalization circuit It is efficient, fast, and small. The disadvantage is that the control algorithm is more complicated.

By comparing the characteristics of the above various equalization methods, this paper considers that the inductive equalization circuit is more suitable for the charging balance of electric vehicle battery packs. The electric vehicle battery pack has the characteristics of large-scale series connection. Therefore, the basis of the inductor adjacent equalization circuit In the above, the inductive equalization control circuit of large-scale series battery pack (Fig. 3) is designed to realize the power transfer between the batteries in the battery pack to achieve the purpose of balance.

In the system, the status information of each battery in the battery pack is detected in time, sent to the MCU, and processed by an algorithm to control the output of the PWM to drive the switching device to be turned on and off. When a battery voltage is high, the corresponding switch When the device is turned on, the battery stores energy into the corresponding inductor. When the switch is turned off, the energy is transferred to the downstream battery, and the excess energy is transferred to the common capacitor and fed back to the charging circuit.

5 Lithium-ion battery pack charging circuit design

Based on the above research, this paper designs a charging system with equalization function based on the voltage-limiting variable-current pulse charging method. The system uses MCU to control, and samples the voltage, current, temperature and other parameters of the battery. In the MCU, and through a certain algorithm, control the duty cycle of the PWM output signal, control the pulse charging current, control the switching device of the equalization circuit, and achieve balanced charging. The block diagram of the system is as follows.


6 Summary

In this paper, through the study of a variety of traditional charging methods, combining the advantages of various methods, a voltage-limiting variable-current pulse charging method is proposed, which makes the actual charging current close to the charging acceptable current, shortens the charging time, and effectively prevents polarization. At the same time, in response to the problem of unbalanced state of individual batteries in the battery pack, the balance control research is carried out to make the power in the battery packs mutually transfer, ensuring that the batteries reach an equilibrium state at the end of charging, and design on the basis of this. The structure of the battery pack charging system.