Why 18650 cylindrical lithium-ion batteries are used as the batteries of electric vehicle?

First, a brief understanding of 18650 cylindrical lithium-ion batteries

18650 actually refer to the shape specification of the battery. It is a standard battery model set by Japan SONY Company in order to save cost. Among them, 18 means diameter of 18mm, 65 means length of 65mm, and 0 means cylindrical battery. The 18650 battery originally referred to the nickel-hydrogen battery and the lithium-ion battery. Since the nickel-hydrogen battery is now less used, it is now referred to as a lithium-ion battery. The nominal voltage of a single section of 18650 lithium-ion battery is typically 3.6V or 3.7V; the minimum discharge termination voltage is typically 2.5 to 2.75V. The common capacity is 1200 ~ 3300mAh.

About consistency

The 18650 battery is the earliest, most mature and stable lithium-ion battery widely used in electronic products. Over the years, Japanese manufacturers have accumulated a lot of experience in the production process of 18650 batteries, which makes the consistency and safety of the produced 18650 batteries reach a very high level.

In contrast, stacked lithium-ion batteries are far from mature. Commonly used are square batteries, soft pack batteries, and even the size, size, and position of the tabs are not uniform. The production process of battery manufacturers cannot meet the conditions. Most of them are dominated by humans, and the consistency of the battery does not reach the level of 18650 batteries. If the consistency of the battery does not meet the requirements, the management of a large number of battery strings and battery packs formed in parallel will not allow the performance of each battery to be better, and the 18650 battery can solve this problem.

In summary, the 18650 battery has a small monomer capacity and requires a large number of monomers (Model 104 has 7104), but the consistency is very good; the capacity of the stacked battery can be made larger (20Ah to 60Ah), single the number of volumes can be reduced, but the consistency is poor. In contrast, it is difficult at this stage to invest a lot of manpower and resources with battery suppliers to improve the production process of stacked batteries.

Therefore, when developing Roadster and Models, Tesla's only option was to purchase batteries from the market and develop battery systems. Develop a battery system that manages more than 6,000 cells with good consistency and develops a battery system with a poor consistency of more than 200 sections. In contrast, the technical difficulty of the former should be lower. Even if the number of single cells increases, if the performance of these batteries is reliable, management is still easier.

Compared to another very successful pure electric car, Nissan's LEAF 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 and has considerable skill in quality control. LEAF's battery comes from AESC, a joint venture between Nissan and NEC.

It is very interesting to compare the combination of battery manufacturers and battery manufacturers in the development of electric vehicles in the United States, Japan and China.

About heat dissipation

The laminated battery has a thin thickness and a large surface, and the heat and heat dissipation are good. Therefore, the Nissan LEAF boldly adopts a passive thermal management system (in fact, it is not managed!), and the heat is taken away by the natural convection of the air.

From left to right, the LEAF consists of a single cell, a battery module consisting of two cells and two strings, and a battery pack consisting of 48 battery modules connected in series as shown below:

It can be seen that there are no thermal management systems such as fans and coolant pipes on the battery pack. Probably this is no trick to win.

Conversely, when watching Tesla, the 18650 battery is relatively small, and the temperature difference inside the single cell 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 a range of no more than 5 ° C, which is a very difficult thing. However, Tesla did it. How to do it? What are the advantages of Tesla's Battery Management System (BMS) compared to other electric vehicles?

As shown in the above figure, 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 be in good contact with the water pipe, so that the heat that each cell carries away during cooling is almost similarly, the temperature difference can be effectively controlled within a small range.

In summary, the complexity of Tesla's thermal management system has increased significantly due to the small capacity of the 18650 battery. That is to say, if you consider the heat dissipation capability, using a small capacity 18650 battery is not the best choice.

About energy density

When it comes to energy density, it is necessary to distinguish the energy density of a single cell from the energy density of a battery pack.

In terms of the energy density of a single cell, the 18650 battery is higher than the stacked lithium ion battery. The energy density of the 33Ah lithium-ion battery used by Nissan LEAF is 157Wh/kg, the energy density of the stacked battery used by GM Volt is about 150Wh/kg, and the energy density of the 18650 battery used by Roadster is about 211Wh/kg.

However, the management system of the 18650 battery is more complicated, and thus the additional weight will make the energy density of the battery pack much lower than the energy density of the monomer. The Roadster has a battery pack weight of 450 kg and an energy density of 118Wh/kg, while the LEAF battery pack weighs 225 kg and has an energy density of 107Wh/kg. At the battery level, the energy density of the two is already comparable.

About security

The various advantages of the stacked lithium ion battery have been mentioned above, but it also has some disadvantages. Since the stacked lithium ion battery is generally packaged in an aluminum plastic film, the thickness of the aluminum plastic film is thin and the mechanical strength is poor. In the extreme case of collision of the automobile, the aluminum plastic film is liable to be damaged and cause a safety accident. This explains why Nissan adds an aluminum shell to the outside of the four-cell battery module.

The 18650 battery is generally a steel case, which is safer. Moreover, as the production level of the 18650 battery is continuously improved, the safety is also increasing.

Tesla also put a lot of effort into dealing with the possible safety incidents of these 18650 batteries. If a single battery has abnormal conditions such as excessive temperature, the battery or its module will be powered off to prevent the accident from spreading according to the severity of the abnormal situation. Due to the small monomer capacity, the severity of the accident will be low as long as it does not spread.

About cost

The 18650 lithium-ion battery has the characteristics of large capacity, long life and high safety performance. It is also favored by consumers because of its small size, light weight and convenient use. 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, the 18650 battery is also the most mature and stable battery combination in the world, and still holds the leading position. China produces 18650 batteries per year for billions of knots, far exceeding the batteries of other materials.

Tesla uses 18650 batteries, which can be produced by the previous production lines of Panasonic and other manufacturers. In the case of increasingly fierce competition for 18650 batteries used in consumer electronics, Panasonic and other manufacturers have cooperated with Tesla to upgrade their products and improve the original production lines for the production of power batteries. Industrial production has a scale effect, and when the scale of the product is up to an order of magnitude, the cost is greatly reduced. A new energy vehicle requires thousands of 18,650 batteries, so the cost of purchasing individual units is controllable.

Summary

It can be said that the use of 18650 batteries as the source of power for new energy electric vehicles is the best choice at this stage. As battery technology matures, more new types of batteries will be used in new energy electric vehicles.

The page contains the contents of the machine translation.