Previously, Musk announced at the Tesla Battery Day event that Model Y will be the first to use the latest 4680 batteries to replace the 21700 batteries that have been in service for less than a year. Moreover, most of Tesla's future models are likely to use cells of this specification.
So, why did Musk rush to change battery cell specifications for Tesla products, and why did the 21700 battery cell "fall out of favor" in such a short period of time and become an outcast in Musk's eyes? It all starts with battery thermal management and cost.
Spontaneous combustion, spontaneous combustion, or spontaneous combustion
There is no shortage of spontaneous combustion topics in Tesla news. Whether domestic or foreign, whether on the road or in an underground garage, Tesla seems to All are destabilizing factors. Among them, the Tesla Model S produced in 2012-14 is the most "famous".
In fact, Tesla has recently been involved in lawsuits due to spontaneous combustion. According to data, Tesla already knew that there was a problem with battery cooling as early as 2012 when Model S was mass-produced, and it might even be possible. This led to a fire, and today, 8 years later, the problem still has not been solved! This evidence comes from Tesla’s internal emails, which caused a huge sensation in North America after the emails were exposed. Shortly after the emails were exposed, Tesla’s stock price fell sharply. Two months after this incident, Musk announced that Tesla would use 4680 cells.
According to the email, Tesla has commissioned three companies to test and investigate the battery cooling configuration, and the results all showed that there was a problem with the end connection accessories of the Model S cooling system. Tesla's battery cooling system is achieved by adding external coolant pipes to the battery. However, the end joints of the pipes use low-strength aluminum materials, which are easily worn and cracked, causing coolant leakage, causing the car battery to short-circuit and catch fire, or cause a fire in the car battery. Flammable residue is left inside the battery.
Public opinion pointed out that Tesla deliberately concealed battery pack safety issues and forced the immature Model S to be launched on the market, which ultimately led to spontaneous combustion incidents in many places around the world.
Tesla has not officially expressed its position on this result, but many Model S users have already started to complain. They believe that the products they purchased have objective defects. Although there will not be 100% problems, who will And can you guarantee that bad luck will not happen to you? The 18650 battery cell equipped with Model S has a large number of cells, which results in poor temperature control performance and a greater possibility of series combustion. This is the main reason for many car owners to worry.
Battery packs composed of 18650 cells are small in size. In order to ensure the compactness of the battery pack, the gaps between the single cells are also relatively small, which makes it difficult to arrange thermal conductive materials. , and also increases the cost of the BWS "battery pack management system". After all, it is troublesome to balance the voltage of so many cells.
After 21700 battery cells were adopted by Tesla, this shortcoming was effectively improved. Not only were the number of battery cells smaller at the same capacity, but the gaps between them were also larger. Thermal conductive materials It can be better arranged between cells and balance the heat of the entire battery pack. However, the 21700 battery cells also have some flaws: 21700 does not reduce the size of the battery pack, nor can it implement a flat-panel cooling solution. The reason is that the 21700 cells are still a bit short, so Tesla's battery packs still use a two-layer stacking method. The layout is still flawed in terms of thermal management. In addition, due to the limitation of single unit capacity, the charging power of 21700 cannot achieve further breakthroughs, which is one of the factors restricting the energy supply rate of electric vehicles.
Therefore, considering all aspects of cost and performance considerations, Tesla used a higher 4680 battery cell to achieve a single-layer layout, making it possible to dissipate heat from a single-layer flat panel.
The principle of flat plate heat dissipation of the battery pack is to directly fix the copper anode material of the lithium battery on the bottom plate. Since copper is a good thermal conductive material, the heat of each cell can be directed to the bottom plate. , and the bottom plate plays a role in balancing the temperature and cooling of the battery core.
The single-layer flat-panel layout can significantly reduce the use of thermal conductive materials in the battery pack, which is very attractive in terms of cost.
The volume and structure of the 4680 battery pack are more compact than before
The performance of the vehicle has been improved
After the cooling method changes, Tesla can pack the batteries more tightly , the size of the battery pack can be significantly reduced. This is an advantage because the mass is concentrated more in the center of the vehicle, which reduces the polar moment of inertia and improves the vehicle's handling. Although the center of gravity of an electric vehicle is very low because the battery is placed on the floor, this does not mean that it is beneficial to handling. The ideal center of gravity distribution of a vehicle must be to concentrate the weight in the middle of the vehicle, and an overly large battery The package allows the weight to be distributed throughout the entire vehicle bottom, which is actually not conducive to the dynamic transformation of the vehicle chassis. This time Tesla shrinks the battery pack, which can effectively improve this shortcoming of electric vehicles.
In addition, because the single battery has become larger, the positive and negative electrodes can withstand a greater voltage difference, and the charging power of the battery is much higher than before. The advantage is that the charging time is shorter and the temperature during charging is lower. The rise is lower.
The initial internal resistance of the new 4680 tipless battery is 3 milliohms at 10?SOC, and gradually decreases to 2 milliohms at 80?SOC. The resistance of a 2170 battery is 23/20/20 milliohms. We can see that the resistance of the battery is reduced to about 10. In comparison, the technology marked in Tesla's patent application can achieve a reduction of 5-20 times.
Comparison chart of charging power and time of 21700 and 4680 battery cells
Che’s comment
Through technical analysis, it can be seen that Tesla frequently changes battery cells Specifications are the changes it has to make to improve defects in the product. In comparison, the Tesla products you buy now no longer seem perfect.
In addition, Tesla’s battery innovations mainly come from innovations in peripheral non-core structures, such as increasing the size of the battery cells and relatively reducing the proportion of the casing. The real improvement in energy density must and can only be achieved through material innovation. Innovations in production processes can reduce costs but have little impact on energy density. Innovation in battery positive and negative electrode materials will bring about a leap in performance of the lithium battery electrochemical system, but this requires a long period of scientific research accumulation.
This article comes from the author of Autohome Chejiahao and does not represent the views and positions of Autohome.