Research on fast charging solutions for electric vehicle 18650 battery 3500mah lithium

　　As the global climate worsens, all mankind is discussing how to deal with climate change. Energy conservation, emission reduction, and finding new energy sources are important ways to reduce environmental damage. At present, the number of cars in the world has reached 1 billion, and car exhaust is an important factor exacerbating environmental degradation. For this reason, many companies and institutions are developing pollution-free electric vehicles, and electric vehicle power batteries are the core part and are the focus of research. direction.

　　Currently, most electric vehicle companies and research institutions use lithium-ion 18650 battery 3500mah lithium 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 working voltage, and large in capacity. The energy density of lithium-ion batteries is very high, and its capacity is the same as its weight. It is 1.5 to 2 times that of nickel-metal hydride batteries and has 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 have become the first choice for electric vehicle batteries.

　　The lithium-ion battery pack used in high-power electric vehicles is composed of multiple single-cell lithium-ion batteries connected in series to obtain a higher output voltage. However, the use of both lithium-ion 18650 battery 3500mah lithium and single-cell lithium-ion batteries is different. There is a big difference. In the battery pack, there are bound to be individual differences in the production and manufacturing of each single lithium-ion battery, and the degree of aging during use is also different. If no measures are taken during the charging process, these differences will be accumulated or even expanded, causing the entire battery pack to be damaged. 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 traditional charging method has low efficiency and long charging time, which seriously hinders the promotion of electric vehicles.

　　Therefore, a fast, effective, safe and reliable charging system is currently a key research and development technology in the electric vehicle industry.

　　1Research on traditional lithium-ion battery charging methods

　　Different charging methods are important factors affecting the performance and service life of single lithium-ion batteries. Proper charging methods can not only maximize the battery's capacity, but also extend the battery's service life. This impact is mainly reflected in three aspects:

　　(1) Influence of voltage. On the one hand, during the charging process, the battery voltage must be strictly controlled not to exceed the charging limit voltage. Exceeding the charging limit voltage is called overcharging. Slight overcharging multiple times will lead to a reduction in battery capacity and deformation of the battery. Severe overcharging will directly lead to The battery exploded. On the other hand, after charging is completed, the battery voltage should be kept as close to the full charging voltage as possible, otherwise the battery capacity will be greatly reduced.

　　(2) Influence of current. The acceptable charging current of lithium-ion batteries is limited. If the charging current is higher than this upper limit, it will cause a gas evolution reaction in the electrolyte in the battery, generate a lot of heat, and cause the battery temperature to rise sharply.

　　(3) 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 must be monitored and controlled in a timely manner.

　　Choosing the appropriate charging method can improve charging efficiency and extend service life. There are many charging methods for lithium-ion batteries. Commonly used charging methods include constant current charging, constant voltage charging, constant current/constant voltage charging, variable current charging, pulse charging, intermittent charging, etc.

　　1.1 Constant voltage charging method

　　The constant voltage charging method refers to charging the battery with a constant voltage during the charging process. 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 early stage of charging, because the battery potential is low, the charging current is large. As the charging process proceeds, 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, so it has a certain degree of adaptability. However, the current is relatively large in the early stages of charging, which may cause the battery temperature to rise too quickly, causing adverse effects on the battery.

　　1.2 Constant current charging method

　　The constant current charging method refers to charging with a constant current throughout the charging process. The acceptable charging current of lithium-ion batteries decreases exponentially with charging time, while the charging current remains unchanged. If the charging current is large, the charging current may exceed the acceptable current in the later stage of constant current charging, causing the battery electrolyte to decompose. Gas reaction, but 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, but also have many shortcomings. The current is too large in the early stage of constant voltage charging and the current is too large in the later stage of constant current charging. Therefore, the charging method of constant current first and then constant voltage can be used to overcome the disadvantages of these two methods in these two stages.

　　1.3 Constant current/constant voltage charging method

　　This charging method divides the charging process into three stages. As shown in Figure 1.

　　(1) Precharge stage. After the DC power supply is connected, when the battery is detected, the charging chip starts and enters the precharge process. During this period, the charge controller charges the battery with a smaller current to restore the battery voltage and temperature to normal conditions.

　　(2) Constant current charging stage. In the early stage of charging, the charging circuit charges the lithium-ion battery at a constant current. Generally, most lithium batteries use a standard charging rate. During constant current charging, the battery voltage will rise slowly. Once the battery voltage reaches the set termination voltage, constant current charging will be terminated and the constant voltage charging process will be entered.

　　(3) Constant voltage charging stage. During the constant voltage charging process, the charging current gradually decreases. When the charging current is detected to drop below the set value, or the full charge time times out, the charge controller switches to top end charging. At this time, the charge controller replenishes energy for the battery with a very small charging current. Generally, this process can extend the battery life by 5% to 10%.

　　In this charging method, in order to avoid excessive current and high battery temperature, a smaller charging current is usually used for charging during the constant current stage, but the charging efficiency is still not high. In order to improve the charging efficiency, the variable current charging method can be used.

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