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Uncover the true face of the 18650 battery: consistency and energy density

Author: Source: Datetime: 2016-11-23 18:04:25

Tesla will use 20700 instead of 18650 as a power battery, from 18650 to 20700, the change is the size, the same is still stick with the use of a cylinder battery.


For the 18650 batteries with a new energy electric car battery technology roadmap, we first look at tesla. Tesla in the battery development, testing a lot of types of batteries, but the final goal is locked in the 18650 battery, what are the advantages and disadvantages of 18650 of the electric core, let us analyze the true face of 18650.


First of all, the 18650 cylindrical lithium-ion batteries to do a brief understanding


We often say that the daily 18650 is the standard form of the battery, is a standard battery of Japanese SONY company in order to save cost and set, where 18 represents a diameter of 18mm, 65 65mm length, 0 for cylindrical battery. 18650 battery of the original refers to nickel metal hydride batteries and lithium ion batteries, due to the nickel hydrogen is now less used, so now more than the lithium ion battery. 18650 lithium ion battery single nominal voltage is generally: 3.7V or 3.6V; minimum discharge termination voltage is generally: 2.5 ~ 2.75V. Common capacity is 1200 ~ 3300mAh. It also can be used in solar power generator systems.


On consistency


18650 battery is the earliest, most mature, the most stable lithium ion battery, widely used in electronic products. Over the years, the Japanese manufacturers in the 18650 battery production process has accumulated a lot of experience, so that the output of the 18650 battery consistency, security has reached a very high level.


In contrast, laminated lithium ion battery is far from mature, there is a common square battery, soft cell, even size, size, lug position are not unified, production of battery manufacturers have cannot meet the conditions, most of the artificial control, the consistency of the battery is not up to 18650 batteries standard. If the consistency of the battery does not meet the requirements, a large number of battery string, parallel formation of the battery pack management will not allow the performance of each battery to play better, while the 18650 core can solve this problem.

In summary, the monomer battery capacity of 18650 small, the number of monomers required a lot (Model 7104 S), but very good consistency; solar backup generator also use the 18650 battery, stacked battery capacity can be made larger (20 Ah to 60 Ah), monomer quantity can be reduced, but the poor consistency. In contrast, it is difficult to put in a lot of manpower and material resources to improve the production process of stacked battery. Therefore, in the development of Roadster and S Model, Tesla's only option is to buy batteries from the market, to develop their own battery system. Development of a set of more than 6 thousand sections of the consistency of a good consistency of the battery system and the development of a set of more than and 200 very poor consistency of the battery system, in contrast, the former technical difficulty should be lower. Even if the number of single cell increases, but if the performance of these batteries is reliable, it is easy to manage.


Compared to another very successful pure electric vehicles, Nissan LEAF, which uses a laminated lithium ion battery. This is because the Nissan and NEC cooperation for many years, in battery technology accumulation is very strong, there should be a considerable skill in quality control. LEAF's battery comes from AESC, Nissan's joint venture with NEC.


In contrast, the medium in different parts of the automobile manufacturers in the development of the electric car and battery maker of hezonglianheng is a very interesting thing.

Stacked cell thickness of thin, large surface, both heat, heat capacity is good, so Nissan LEAF very bold use of passive thermal management system (in fact, does not manage! The natural convection of the air will take the heat away.


From left to right, LEAF from monomer, to four single cell battery two and two series consisting of the battery module, for example these batteries always with BMS system, it can use in solar powered portable generator, and then to the 48 battery modules in series with the composition of the battery as shown below:

battery pack

It can be seen that there is no fan on the battery, the cooling liquid pipe and other thermal management system. Probably this is no trick wins.


In turn, look at the Tesla, the 18650 battery head is relatively small, in the normal charge and discharge of the single battery internal temperature difference will not be too large. However, the temperature of the more than 6 thousand single cell should also be kept in the range of not more than 5 degrees C, this is a very difficult thing.


As above, these pipes are channel cooling fluid flow, evenly distributed in the middle of each battery module, can make only batteries can work well with water contact, so each electric core in cooling, heat away is almost the same, it can effectively control the temperature in a very small range.


In summary, due to the adoption of a small capacity of 18650 batteries, the complexity of the Tesla thermal management system is greatly increased by the. That is, if the heat capacity from the consideration, the use of small capacity of 18650 batteries is not the best choice.

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