What are the cylindrical lithium batteries?
Compared with soft packs and square lithium batteries, cylindrical lithium batteries are the earliest commercialized, most automated, and currently the lowest cost lithium battery pack. At present, there are mainly 18650 cylindrical lithium batteries and 21700 cylindrical lithium batteries in the market. Other common cylindrical lithium battery specifications are 14650, 17490, 18650, 21700, 26650 and so on.
Cylindrical lithium batteries are divided into different systems of lithium iron phosphate, lithium sulphate, lithium manganate, sulphate manganese mixed, and ternary materials. The outer shell is divided into steel shell and polymer. Two types of batteries with different material systems have different advantages. At present, the cylinders are mainly steel-shell cylindrical lithium iron phosphate batteries, which are characterized by high capacity, high output voltage, good charge and discharge cycle performance, stable output voltage, large current discharge, stable electrochemical performance, and use Safe, wide operating temperature range, and environmentally friendly.
Cylindrical lithium battery pack structure: The monomer is mainly composed of positive electrode, negative electrode, diaphragm, positive and negative electrode collector, safety valve, overcurrent protection device, insulator and shell. For the shell, there were many early steel shells, but currently aluminum shells are the main ones.
Single overcurrent protection device, the design of each manufacturer is not the same, according to the different safety requirements, the price will be different, and it can be customized. General safety devices mainly include PTC positive temperature coefficient resistors and fuse devices.
PTC, when there is excessive current, resistance heating, and temperature accumulation will promote the increase of PTC resistance. When the temperature exceeds a threshold, it will increase suddenly, which is equivalent to isolating the faulty cell from the overall circuit. Open it to avoid further thermal runaway.
The fuse device, in principle, is a fuse. When encountering excessive current, the fuse will blow and the circuit will be disconnected.
The difference between the two protection devices is that the former is recoverable, while the latter’s protection is one-time. Once a failure occurs, the system must consider replacing the defective battery cell to work normally.
Characteristics of cylindrical lithium batteries
Cylindrical lithium battery pack cells, especially 18650, due to their own structural characteristics, and due to the standardization of their models, the automation level of cylindrical cell production is 3 The highest among the main battery cell types. This makes a high degree of consistency possible, and the yield rate is increased accordingly.
Advantages of cylindrical lithium battery
1) As mentioned above, the monomer consistency is better;
2) The monomer itself has good mechanical properties, which is comparable to the square Compared with the soft pack battery, the closed cylinder can get the highest bending strength under the approximate size;
3) The technology is mature and the cost is low, but at the same time, the space for cost optimization has been almost consumed. ;
4) The single energy is small, and the form is easy to control in the event of an accident, but this is also becoming the reason for it to be replaced (how to say the popular saying, what makes you successful will also destroy you, If you can’t kill you, it will make you stronger. The same is true.)
5) Cylindrical lithium battery packs are grouped
Round batteries are relatively easier to use, so PACK The solution is simple and the heat dissipation effect is good. The heat dissipation problem should be solved when the square battery packs.
Disadvantages of cylindrical lithium batteries:
1) In the context of electric vehicles, the number of cylindrical monomers in the battery system is very large, which greatly increases the complexity of the battery system. Regardless of institution or management system, compared to the other two types of batteries, the cost of cylindrical batteries at the system level is relatively high;
2) Under the conditions of uneven temperature environment, the probability of a large number of battery cell characteristics being alienated increases. Of course, The reason why Tesla chose 18650 at the beginning of the design, I believe it was just a helpless choice, because 10 years ago, the only qualified power batteries that could be mass-produced were cylindrical batteries. The safety and thermal management requirements of the battery are instead the driving force for the research and development of its powerful electronic control system.
3) There is little room for the increase in energy density. In the 2016 news, Chaowei achieved a single capacity of 4050mAh and a cell specific energy of 306Wh/kg. Since then, no higher records have been seen. In a given space, there are only dead materials, which is recognized as a difficult road.
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