Why would a high performance new energy electric car choose 18650 lithium battery as a power source? This is the question of many users, in order to obtain a higher capacity, it is necessary to use more single cells in parallel to achieve, this does not seem to be safe, as Tesla such a high-performance electric car why will any choose this model of battery as an early design models, the next we will analyze the reasons for this;
About performance
18650 battery is an early, mature and stable lithium-ion battery, widely used in electronic products. Over the years, Japanese manufacturers have accumulated a lot of relevant experience in the production process of 18650 batteries, making the output of 18650 battery consistency and safety have reached a very high level.
In contrast, the laminated lithium-ion battery is far from mature, the common square battery, soft pack battery, even the size, size, pole lug position, etc. are not uniform, the battery manufacturers have the production process can not meet the conditions, most of the human control, the battery consistency is not up to the 18650 battery level. If the consistency of the battery does not meet the requirements, the management of the battery pack formed by a large number of batteries in series and parallel will not allow each battery to perform better, while 18650 cells can solve this problem.
To sum up, 18650 cells have a small capacity and the number of cells required will be large (ModelS has 7104 cells), but the consistency is very good; the capacity of laminated cells can be made larger (20Ah to 60Ah) and the number of cells can be reduced, but the consistency is poor. In contrast, at this stage, it is difficult to invest a lot of human and material resources to improve the production process of cascaded batteries together with battery suppliers. Therefore, when developing the Roadster and ModelS, Tesla's only choice was to go to the market to buy batteries and develop its own battery system. Developing a battery system that manages more than 6,000 individual cells with good consistency versus developing a battery system with more than 200 cells with poor consistency should be less technically challenging than the former. Even if the number of individual cells increases, it is still easier to manage them if their performance is reliable.
Compare this to another very successful pure electric vehicle, Nissan's LEAF, which uses a laminated lithium-ion battery. This is because Nissan has been working with NEC for many years and has accumulated a lot of battery technology and has considerable merit in quality control. the battery for the LEAF comes from AESC, a joint venture between Nissan and NEC.
Lithium-ion battery processing custom
It is very interesting to compare the combination of different regional automakers in the U.S., Japan and China in developing electric vehicles with battery manufacturers.
About heat dissipation capability
The thin thickness and large surface of the laminated battery has a good ability to even out and dissipate heat, so the Nissan LEAF is bold enough to use a passive thermal management system (actually, it's not managed!) , the heat is carried away by the natural convection of air.
From left to right, the LEAF goes from a single cell, to a battery module consisting of four single cells in two parallel and two series, to a battery pack consisting of 48 battery modules in series as shown below.
As you can see, the battery pack does not have any fans, coolant pipes and other thermal management systems. Probably this is no trick is better than a trick.
On the contrary, looking at Tesla, the 18650 battery is relatively small, and the temperature difference inside the single cell during normal charging and discharging is not too big. However, the temperature difference of more than 6000 single cells should be kept within the range of not more than 5C, which is a very difficult thing. But Tesla has done it. What is the advantage of Tesla's Battery Management System (BMS) over other electric vehicles?
These pipes are coolant flow channels, which are evenly distributed in the middle of the battery module, allowing each cell to be in good contact with the water pipes, so that each cell takes away almost the same amount of heat when cooling, and the temperature difference can be effectively controlled within a small range.
To sum up, the complexity of Tesla's thermal management system is greatly increased due to the use of small capacity 18650 cells. In other words, if we consider the heat dissipation capability, using small capacity 18650 battery is not the best choice.
About energy density
When it comes to energy density, it is important to distinguish between the energy density of a single cell and the energy density of a battery pack.
In terms of energy density of a single cell, 18650 cells are higher than laminated lithium-ion batteries. The energy density of the 33Ah lithium-ion battery used in the Nissan LEAF is 157Wh/kg, the energy density of the laminated battery used in the GMVolt is about 150Wh/kg, while the energy density of the 18650 battery used in the Roadster is about 211Wh/kg. The energy density of the Roadster battery pack is 450kg with an energy density of 118Wh/kg, while the LEAF battery pack weighs 225kg with an energy density of 107Wh/kg. at the pack level, the energy density of the two is already comparable.
About safety
The various advantages of laminated lithium-ion batteries were mentioned earlier, but it also has some disadvantages. As laminated lithium-ion batteries are generally packaged with aluminum-plastic film, which is thin and has poor mechanical strength, it is easy for the film to break under extreme conditions such as car collisions, resulting in safety accidents. This also explains why Nissan has to add an aluminum case outside the battery module composed of 4 single cells.
18650 batteries are generally steel cases, which have better safety; and as mentioned earlier, with the continuous improvement of 18650 battery production process, safety is also improving.
Tesla has also put a lot of effort in dealing with the possible safety accidents of these 18650 batteries. If a single cell has an abnormal condition such as over temperature, depending on the severity of the abnormality, the cell or the module it is in will be disconnected to prevent the accident from spreading. Due to the small capacity of a single cell, the severity of the accident will be low as long as it does not spread.
Related Costs
18650 lithium-ion batteries have high capacity, long life, high safety performance, and are popular among consumers because of their small size, light weight, and ease of use. With the deepening of research on 18650 battery technology, the consistency and safety of the battery have reached a very high level. As an early lithium-ion battery, 18650 battery is also a mature and stable battery combination in the world, and still occupies a priority position. China produces about billions of 18650 batteries every year, far more than the batteries of other materials.
Tesla uses 18650 batteries, which can be produced using the previous production lines of Japanese manufacturers such as Panasonic. In the increasingly competitive situation of 18650 batteries used in consumer electronics, Panasonic and other manufacturers work with Tesla to upgrade their products and improve the original production line for the production of power lithium batteries. Industrial production has a scale effect, when the scale of production products reach an order of magnitude, the cost will be greatly reduced. A new energy vehicle to thousands of 18650 batteries, so the purchase cost of a single unit can be controlled.
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