Compared with soft packs and square lithium batteries, the 18650 cylindrical lithium battery is the earliest commercially available, the highest level of automation, and the lowest current cost. Tesla has been holding it for many years, basically maintaining the situation with soft packs and square batteries.
After Tesla announced that Model 3 had adopted the 21700, the cylindrical battery family also had one more star member. This article takes a look at several technical points related to cylindrical batteries. Unless specifically stated below, the cylindrical battery specifically refers to the 18650.
A look at the power battery will reveal that the cylindrical 18560 battery is the most studied battery type with the most technical discussions. The monomer is mainly composed of a positive electrode, a negative electrode, a separator, a positive electrode negative electrode collector, a safety valve, an overcurrent protection device, an insulating member and a housing. Shell, early steel shell more, the current aluminum shell.
Single-unit over-current protection device, each manufacturer's design is not the same, according to the different security requirements, the price to go different, can be customized. The general safety devices mainly include PTC positive temperature coefficient resistors and fuse devices.
PTC, when there is a large current, the resistance heats up, the temperature accumulation promotes the PTC resistance to rise. When the temperature exceeds a threshold, it increases suddenly, which is equivalent to isolating the faulty cell from the overall circuit to avoid further thermal runaway. happened.
The fuse device, in principle, is a fuse. When a large current is encountered, the fuse is blown and the circuit is disconnected.
The difference between the two protection devices is that the former can be restored and the latter's protection is one-time. Once a failure occurs, the system must consider that the replacement battery will work properly.
Cylindrical core features
Cylindrical cells, especially 18650, due to their own structural characteristics, but also due to the standardization of their models, the automation level of cylindrical cell production is the highest among the 3 main cell types. This makes a high degree of consistency possible, and the yield is correspondingly improved. Data shows that the yields of major manufacturers such as Samsung and Panasonic can reach 98%, while Chinese manufacturers can also exceed 90%.
1) As mentioned earlier, monomer consistency is better;
2) The mechanical properties of the monomer itself are good. Compared with the square and soft-packed batteries, the closed cylindrical body can obtain the highest bending strength under approximate dimensions.
3) The technology is mature and the cost is low, but at the same time, the space for cost optimization has also been consumed;
4) The energy of the monomer is small. In the event of an accident, the form is easy to control, but this is also the reason why it is replaced (what would happen if the popular sentence is spoken, and your accomplishment will destroy you, and killing you will certainly make You are strong and you know this.)
Disadvantages, 1) In the context of electric vehicles, the number of cylindrical cells in the battery system is large, which makes the battery system greatly increased in complexity, regardless of the organization or management system, compared to the other two types of batteries, system-level costs Cylindrical battery is high;
2) Under the condition of uneven temperature environment, the probability of alienation of a large number of battery cores rises. Of course, Tesla chose 18650 at the beginning of design. I believe it is only a helpless choice because 10 years ago, it was able to mass-produce. Qualified power battery, only cylindrical battery. The safety and thermal management requirements of the battery, on the contrary, are the driving forces behind its powerful electronic control system.
3) The space for energy density rise has been very small. In 2016, Chaowei had achieved a single cell capacity of 4050 mAh and a specific cell energy of 306 Wh/kg, but no higher record was seen after that. In the established space, only dead materials are recognized as a difficult road.
Tesila's Model 3 Battery System Brings Fire to 21700
The Tesla Model 3 fully enabled 21700 lithium-ion batteries, opening a new phase of cylindrical battery capacity. Tesla Model 3's 21700 battery system has an energy density of around 300 Wh/kg, an increase of more than 20% in energy density of the 18650 battery used in the original Model S, a 35% increase in monomer capacity, and a 9% reduction in system cost.