What is the limit on the charging speed of the power battery?

by:CTECHi     2021-08-14

Lithium Grid News: When discussing the charging speed, the battery's own endurance is definitely the most inescapable factor. No matter how powerful the peripheral charging equipment is, how powerful it is, how strong the charging capacity is, if the battery itself has shortcomings in the acceptable charging capacity, then the charging speed will definitely not rise. In addition, if the battery capacity is relatively large, the natural charging time will be longer.


   If you have learned electrochemistry knowledge in high school, you will understand the process of battery charging and discharging. The essence is to realize electrons through a series of redox reactions inside the battery. Directional transfer between the positive electrode and the negative electrode. Take the current mainstream lithium batteries as an example. Although there are various types, the general structure includes nothing more than positive electrode materials, negative electrode materials, separators, electrolytes, etc. The charging process basically involves the extraction of lithium ions from the negative electrode, passing through the separator and Electrolyte, the process of diffusion to the positive electrode-the diffusion speed naturally becomes the key to the charging speed.


   In theory, it is indeed possible to increase the charging speed by increasing the current. However, if the current is too large, the diffusion speed of lithium ions inside the battery will not keep up with the electron diffusion speed, which will cause the electron-ion transport to be disconnected, which will affect the battery performance, and the achievable charging capacity will be correspondingly reduced. The battery life will be even more terrible. , There is even a danger of fire and explosion.


   So generally speaking, we recommend slow charging as much as possible when you are not in a hurry, which will help extend the life of the battery.


   And the diffusion rate of lithium ions is closely related to the temperature and the material and structure of the positive electrode.


   First of all, it is temperature. Generally speaking, the higher the temperature, the faster the diffusion speed, but if the temperature is too high, it will also cause problems such as reduced battery life and decreased charging safety. If the temperature is too low, the same will not work. If the temperature is too low, the metal lithium in the battery will be deposited, which will cause the internal short circuit of the battery, especially the lithium iron phosphate battery. Generally, the capacity of the lithium iron phosphate battery is only about 60 to 70% at 0°C, and only 20 to 40% is left at -20°C. Therefore, in the cold northern winter, electric vehicles must have the function of heating the battery modules, which naturally consumes more electricity.


   is followed by materials. The diffusion capacity of different materials is very different, such as lithium cobalt oxide, lithium manganate, lithium iron phosphate, NCM, NCA, etc., which have very good performance Cathode materials, and the latter two are also the two materials with the best performance and relatively high application popularity. This is also an important reason why today's lithium batteries are named after cathode materials.


   In the field of battery industry, the charge-discharge rate is usually used to describe the relationship between the charging speed and the current. For example, the rate when the battery is fully charged in 1 hour is called 1C. The rate of 30 minutes is called 2C, and so on, over 1C can be called fast charge. Nowadays, the charging rate of lithium-ion batteries can generally be 1C-3C, and the highest can go to 5C, but the discharge rate of starting Noir is naturally still far behind.

Typical lithium-ion battery charging characteristic curve


   In addition to the bottleneck of the maximum charging rate, different SOC (State of Charge, that is, the state of charge, that is, the remaining power) the charging rate that the battery can withstand is also different. Generally speaking, the characteristics of the battery in the charging process are roughly similar to the above picture, and the charging rate will follow a slow-fast-slow rhythm. Generally, when the SOC reaches more than 90%, the internal resistance of the battery will increase significantly, causing the charging rate to slow down. If you pay attention to most of the electric vehicles currently on sale, you will find that they will advertise that they can fill a large battery in a relatively short period of time, such as 1 hour or even 30 minutes, in a specific fast charging state. The proportion of electricity is generally around 80%-90%, which is what it means.


   So if you are an electric car user and want to save the charging time as much as possible, try not to use the power below 10% at will, and it may not be necessary when charging It must be full, more than 90%, or the mileage that can meet your next trip needs is enough.


What is the limitation on the charging speed of    charging equipment?


   In addition to the bottleneck of the battery itself, the peripheral charging equipment also has its own limitations. Simply put, the greater the output power of the charging pile, the shorter the charging time. However, the charging pile cannot increase the charging power infinitely. Let me first talk about the general process of charging an electric car.


   When it comes to car charging, the first thing everyone thinks of is the charging pile. Simply put, the greater the output power of the charging pile and the smaller the battery capacity, the shorter the charging time. This is the same as filling a pool with water. The larger the drain pipe and the smaller the pool, the shorter the time it takes. However, as an electric vehicle user, of course, I hope my battery capacity is also large enough, so it is natural to increase the power of the charging pile. Charging piles for vehicles are generally divided into two types: AC charging piles and DC charging piles. Let's separate the two situations.


   First talk about the universal AC charging pile. It mostly uses 220V AC charging with the same voltage as the household. Generally, the current is only 16A or 32A, and the charging speed is relatively slow. When the battery capacity is about 20kwh, it takes about 6-8 hours to be fully charged.

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