Why is 18650 cylindrical lithium-ion battery selected as the battery of electric vehicle?
First of all, a brief understanding of 18650 cylindrical lithium-ion cell is made
18650 actually refers to the shape specification of the battery. It is a standard battery model determined by Sony Company in Japan in order to save cost in that year. 18 represents the diameter of 18mm, 65 represents the length of 65mm, and 0 represents the cylindrical battery. 18650 batteries used to refer to NiMH batteries and lithium-ion batteries. Because NiMH batteries are now less used, they now refer to lithium-ion batteries. The nominal voltage of 18650 lithium-ion battery is generally 3.6V or 3.7V, and the minimum discharge termination voltage is generally 2.5-2.75v. The common capacity is 1200-3300mah.
About consistency
The laminated battery is thin in thickness, large in surface, good in heat sharing and heat dissipation, so Nissan LEAF boldly adopts the passive thermal management system (in fact, it is not managed!), which is carried away by the natural convection of air.
From left to right, leaf consists of a single battery, four single batteries, two battery modules in parallel and two battery modules in series, and then 48 battery modules in series, as shown in the following figure:
As you can see, there is no thermal management system such as fans, coolant pipes, etc. on the battery pack. Maybe this is no move, no win.
On the other hand, the size of the 18650 battery is relatively small, and the temperature difference inside the single battery will not be too large during normal charging and discharging. However, the temperature difference of more than 6000 single cells should also be kept within the range of 5 ° C, which is a very difficult thing. But Tesla did. How? What are the advantages of Tesla's battery management system (BMS) over other electric vehicles?
As shown in the figure above, these pipes are the flow channels of the coolant. The flow channels are evenly distributed in the middle of the battery module, so that each cell can contact the water pipe well, so that the heat taken away by each cell during cooling is almost the same, and the temperature difference can be effectively controlled within a very small range.
In summary, the complexity of Tesla's thermal management system is greatly increased due to the use of small capacity 18650 batteries. In other words, if considering the heat dissipation capacity, the 18650 battery with small capacity is not the best choice.
On energy density
When it comes to energy density, it is necessary to distinguish the energy density of single cell from that of battery pack.
In terms of the energy density of the single battery, 18650 battery is higher than the laminated lithium-ion battery. The energy density of the 33ah lithium-ion battery used by Nissan leaf is 157wh / kg, that of the laminated battery used by gmvolt is about 150wh / kg, and that of the 18650 battery used by Roadster is about 211wh / kg.
However, the 18650 battery management system is more complex, so the extra weight will make the energy density of the battery far lower than that of the monomer. The weight of roadster's battery pack is 450kg, and the energy density is 118wh / kg, while the weight of leaf's battery pack is 225kg, and the energy density is 107wh / kg. At the battery level, the energy density of the two is already equal.
About security
The advantages of lithium-ion batteries are mentioned above, but they also have some disadvantages. Because the laminated lithium-ion battery is usually encapsulated by aluminum-plastic film, which is thin in thickness and poor in mechanical strength, aluminum-plastic film is prone to damage and lead to safety accidents in extreme cases such as vehicle collision. This also explains why Nissan has to add an aluminum shell to the battery module composed of four monomers.
18650 battery is generally steel shell, with better safety; moreover, as mentioned above, with the continuous improvement of 18650 battery production technology level, the safety is also constantly improving.
Tesla also devoted a lot of efforts to deal with the possible safety accidents of these 18650 batteries. If the temperature of a single battery is too high and other abnormal conditions occur, according to the severity of the abnormal conditions, the battery or its module will be powered off to prevent the spread of the accident. Because of the small capacity of the single unit, as long as there is no spread, the severity of the accident will be low.
About cost
18650 lithium-ion battery has the characteristics of large capacity, long life and high safety performance. Because of its small size, light weight and convenient use, it is favored by consumers. With the deepening of research on 18650 battery technology, the consistency and safety of the battery have reached a very high level. As the earliest lithium-ion battery, 18650 battery is also the most mature and stable battery combination in the world, which still occupies the leading position. China produces about several billion 18650 batteries every year, far more than other materials.
Tesla uses 18650 batteries, which can be produced on the production line of Matsushita and other manufacturers in Japan. Under the increasingly fierce competition of 18650 batteries used in consumer electronic products, Panasonic and other manufacturers cooperate with Tesla to upgrade products and use the original production line improved to produce power batteries. Industrial production has a scale effect. When the scale of production products reaches one order of magnitude, the cost will be greatly reduced. A new energy vehicle needs thousands of 18650 batteries, so the purchase cost of the monomer can be controlled.
Summary
It can be said that the 18650 battery is the best choice at this stage as the power source of new energy electric vehicles. With the maturity of battery technology, more and more new batteries will be used in new energy electric vehicles.
Longtaich Battery Packs Spot Welder www.longtaich.com sales@longtaich.com
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