Detailed explanation of the advantages and disadvantages of 18650 lithium batteries

18650 refers to the external specifications of the battery. It is a standard battery model set by SONY in order to save costs. 18 means that the diameter is 18mm, 65 means that the length is 65mm, and 0 means that it is cylindrical. 18650 battery originally refers to nickel-hydrogen battery and lithium battery, because nickel-hydrogen battery is used less now, so now it refers to lithium battery. The nominal voltage of a single cell of 18650 lithium battery is generally 3.6V or 3.7V; the minimum discharge termination voltage is generally 2.5 ~ 2.75V. The common capacity is 1200～3300mAh. 1. About consistency 18650 battery is the most mature and stable lithium battery, widely used in electronic products. Over the years, Japanese manufacturers have accumulated a lot of relevant experience in the 18650 battery processing technology, making the consistency and safety of the 18650 battery output have reached a very high level. In contrast, stacked lithium batteries are far from mature. The common ones are square batteries, soft-pack batteries, and even the size, size, and position of the tabs are not uniform. The processing technology possessed by battery manufacturers can not meet the conditions, most of which are controlled by humans, and the consistency of the battery is not up to the level of 18650 battery. If the consistency of the battery fails to meet the requirements, the management of a large number of battery packs formed in series and parallel will not allow the performance of each battery to better play, and 18650 batteries can handle this problem. In summary, the cell capacity of the 18650 battery is small, and the number of cells required will be a lot (ModelS has 7104), but the consistency is very good; the capacity of the stacked battery can be made larger (20Ah~60Ah), single The number of bodies can be reduced, but the consistency is poor. In contrast, at this stage, it is difficult to invest a lot of manpower and material resources to improve the processing technology of laminated batteries together with battery suppliers. Therefore, when developing Roadster and ModelS, Tesla's only option is to buy batteries from the market and develop its own battery system. In contrast, the technical difficulty of developing a battery system that manages more than 6000 cells with good consistency is less than that of developing a battery system with more than 200 cells with poor consistency. Even if the number of single cells increases, it is easier to manage if the performance of these cells is reliable. WeChat public account: Shenzhen LED Chamber of Commerce compares another more successful pure electric vehicle, Nissan's LEAF, which uses laminated lithium batteries. This is because Nissan has been cooperating with NEC for many years and has accumulated a lot of battery technology and has considerable skill in quality control. LEAF’s batteries come from AESC, a joint venture between Nissan and NEC. 2. Regarding heat dissipation capacity The laminated battery has thin thickness, large surface, good heat distribution and heat dissipation capabilities, so Nissan LEAF boldly adopts a passive thermal management system (in fact, it does not manage), and the heat is carried by the natural convection of the air. go. LEAF goes from a single cell to a battery module composed of four single cells in two parallel and two strings, and then to a battery pack composed of 48 battery modules in series, without any fan, coolant pipes and other thermal management systems. Looking at Tesla on the other hand, the 18650 battery is relatively small, and the temperature difference inside the single battery during normal charging and discharging will not be too large. However, the temperature difference of more than 6000 single cells should also be kept within the range of not more than 5°C, which is a very difficult thing. But how did Tesla do it? What are the advantages of Tesla's battery management system (BMS) compared to other electric vehicles? As shown in the figure above, these pipes are the flow channels of the cooling liquid, and 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 taken away by each cell during cooling is almost Similarly, the temperature difference can be effectively controlled within a small range. In summary, due to the use of small-capacity 18650 batteries, the complexity of Tesla's thermal management system is greatly increased. In other words, if you only consider the heat dissipation capacity, using a small-capacity 18650 battery is not the best choice. 3. About energy density When talking about energy density, we must first distinguish between the energy density of a single battery 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 laminated lithium battery. The energy density of the 33Ah lithium battery used by Nissan LEAF is 157Wh/kg, the energy density of the laminated battery used by GMVolt is about 150Wh/kg, and the energy density of the 18650 battery used by Roadster is about 211Wh/kg. However, the 18650 battery management system is more complicated, and the additional weight will make the energy density of the battery pack much lower than the energy density of a single cell. The weight of the Roadster battery pack is 450kg and the energy density is 118Wh/kg, while the weight of the LEAF battery pack is 225kg and the energy density is 107Wh/kg. In terms of battery packs, the energy density of the two is comparable. Disclaimer: Some pictures and content of articles published on this site are from the Internet. If there is any infringement, please contact to delete. 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