Why Does Tesla Have a Crush on Cylindrical Batteries?
Tesla will use 20700 instead of 18650 as the power battery. From 18650 to 20700, the size is changed, and Tesla still insists on using cylindrical batteries.
Why does Tesla have a crush on cylindrical batteries? What are the advantages of cylindrical batteries that make Tesla a “technical man”? The editor will analyze the "current lover" 18650.
Regarding the technical route of using 18650 batteries as new energy electric vehicle batteries, everyone may first turn their attention to Tesla. Tesla tested many types of batteries during battery development, but finally targeted 18650 batteries. What are the advantages and disadvantages of 18650 cells, let's analyze the true colors of 18650.
First, make a brief understanding of 18650 cylindrical lithium ion cells
The 18650 we often say actually refers to the outline specifications of the battery. It is a standard battery model set by Japan's Sony Corporation in order to save costs. 18 indicates a diameter of 18mm, 65 indicates a length of 65mm, and 0 indicates a cylindrical shape. battery. 18650 battery originally refers to nickel-metal hydride batteries and lithium-ion batteries. Because nickel-metal hydride is now less used, it now refers to lithium-ion batteries. The nominal voltage of a single 18650 lithium-ion battery is generally 3.6V or 3.7V; the minimum discharge termination voltage is generally 2.5 to 2.75V. Common capacity is 1200 ~ 3300mAh.
18650 battery is the earliest, most mature and most stable lithium-ion battery, widely used in electronic products. Over the years, Japanese manufacturers have accumulated a great deal of experience in the production process of 18650 batteries, so that the consistency and safety of the 18650 batteries produced have reached a very high level.
In contrast, stacked lithium-ion batteries are far from mature. Common square batteries, soft-pack batteries, and even the size, size, and position of the tabs are not uniform. The production process possessed by battery manufacturers cannot meet the conditions. Most of them are controlled by humans, and the consistency of the battery cannot reach the level of 18650 battery. If the consistency of the battery fails to meet the requirements, the management of a large number of battery strings and parallel-connected battery packs will not allow the performance of each battery to play better, and 18650 cells can solve this problem.
In summary, the battery capacity of the 18650 battery is small and the number of required cells will be large (7104 Model S), but the consistency is good; the capacity of the stacked battery can be made larger (20 Ah to 60 Ah ), The number of monomers can be reduced, but the consistency is poor. In contrast, it is difficult to invest a lot of manpower and material resources to improve the production process of stacked batteries with battery suppliers at this stage. Therefore, when developing Roadster and Model S, Tesla's only option was to buy batteries from the market and develop battery systems on their own. The development of a battery system that manages more than 6000 cells with good consistency and the development of a battery system with more than 200 cells that have poor consistency should be less technically difficult. Even with the increase in the number of single cells, it is easier to manage them if their performance is reliable.
Compared with another very successful pure electric vehicle, Nissan's LEAF, it uses a stacked lithium-ion battery. This is because Nissan has cooperated with NEC for many years and has accumulated a lot in battery technology. It should have considerable skills in quality control. LEAF batteries come from AESC, a joint venture between Nissan and NEC.
It is very interesting to compare the integration of automobile manufacturers with battery manufacturers in the development of electric vehicles in the United States, Japan, and middle regions.
About energy density
When it comes to energy density, it is necessary to distinguish between the energy density of a single cell and the energy density of a battery pack.
In terms of the energy density of the single battery, the 18650 battery is higher than the stacked lithium ion battery. The energy density of the 33 Ah lithium-ion battery used in the Nissan LEAF I found here is 157 Wh / kg, and the energy density of the stacked battery used by GM Volt is about 150 Wh / kg; It is 211 Wh / kg. However, the management system of the 18650 battery is more complicated, so the additional weight will make the energy density of the battery pack much lower than the energy density of the single cell. Roadster's battery pack weighs 450 kg and its energy density is 118 Wh / kg, while the LEAF battery pack weighs 225 kg and has an energy density of 107 Wh / kg. At the battery pack level, the energy density of the two is already comparable.
The various advantages of the stacked lithium-ion battery mentioned earlier, but it also has some disadvantages. Because laminated lithium-ion batteries are generally encapsulated with aluminum-plastic films, the thickness of aluminum-plastic films is thin, and the mechanical strength is poor. In extreme cases such as automobile collisions, aluminum-plastic films are prone to damage and safety accidents. This also explains why Nissan added an aluminum case to the outside of a four-cell battery module.
18650 battery is generally a steel case with better safety; and as mentioned earlier, with the continuous improvement of the production technology level of 18650 battery, the safety is also constantly improved.
Tesla has also devoted a lot of effort in dealing with the possible safety accidents of these 18650 batteries. If an abnormal situation such as excessive temperature occurs in a single battery, depending on the severity of the abnormal situation, the battery or its module will be powered off to prevent the accident from spreading. As the monomer capacity is small, the severity of the accident will be low as long as it does not spread.
18650 lithium-ion battery has the characteristics of large capacity, long life and high safety performance. It is also popular among consumers because of its small size, light weight and easy use. With the continuous deepening of people's research on 18650 battery technology, the consistency and safety of batteries have reached a very high level. As the earliest lithium-ion battery, the 18650 battery is currently the world's most mature and stable battery combination, and it still occupies a leading position. In China, billions of 18650 batteries are produced each year, and this data far exceeds batteries of other materials.
Tesla uses 18650 batteries, which can be produced on the previous production lines of manufacturers such as Panasonic. With the increasingly fierce competition of 18650 batteries used in consumer electronics, manufacturers such as Panasonic and Tesla cooperate to upgrade their products and improve the original production line for the production of power batteries. Industrial production has a scale effect. When the scale of the produced product reaches an order of magnitude, the cost will be greatly reduced. A new energy vehicle requires thousands of 18650 batteries, so the purchase cost of a single unit can be controlled.
It can be said that using cylindrical batteries such as 18650 and 20700 as the source of power for new energy electric vehicles can be said to be the optimal choice at this stage. However, with the maturity of battery technology, more new types of batteries will be applied to new energy electric vehicles in the future.