Analysis of 18650 battery performance: consistency and energy density

Takeaway: to use 18650 batteries as a new energy electric car battery technology route, one may turn to tesla. Tesla in battery development, test a lot of kinds of battery, but the final target is the 18650 battery. What are the advantages and disadvantages, after all, 18650 batteries, let's analyze 18650 battery true color.

On security

Tesla will use 20700 instead of 18650 power battery, from 18650 to 20700, the size change, constant is tesla insisted on using cylindrical batteries.

Why tesla for cylindrical battery "love", "cylindrical battery" exactly what are the advantages to tesla never forget the "technical man"? The editor makes up to 18650 to analyze the current "lover".

To use 18650 batteries as a new energy electric car battery technology route, one may turn to tesla. Tesla in battery development, test a lot of kinds of battery, but in the end target cells in 18650, exactly what are the advantages and disadvantages of 18650 batteries, let's analyze 18650 true color.

First of all 18650 cylindrical lithium-ion batteries do know briefly

We often say 18650 daily actually refers to the appearance of the battery specifications, which is Japan's SONY Company in order to save costs and set a standard cell types, including 18 said to 18 mm in diameter, 65 said length is 65 mm, 0 means for cylindrical batteries. 18650 former refers to the nickel metal hydride batteries and Lithium ion batteries, as a result of nickel metal hydride is now used less, so now more refers to the lithium ion battery. 18650 single lithium ion batteries is commonly: nominal voltage of 3.6 V or 3.7 V. Minimum discharge termination voltage is commonly: 2.5 ~ 2.75 V. Common capacity is of 1200 ~ 3300 mah.

About consistency

18650 battery is one of the earliest and most mature, the most stable lithium ion battery, widely used in electronic products. For many years, the Japanese manufacturers in the production process of 18650 batteries on accumulated lots of experience, make the output of 18650 battery consistency, security, have reached a very high level.

Cascading the lithium ion battery, by contrast, is far from mature, common square battery, soft package battery, even the size, size, ear position are not unified, such as battery manufacturers of production technology can’t meet the conditions, most people control is given priority to, the consistency of the battery can’t reach 18650 battery level. If the battery can’t meet the requirements, the consistency of the large number of batteries in series, in parallel to form the management of the battery pack will also can't let to better play to the performance of each cell, and 18650 batteries can solve this problem.

About the heat capacity

The laminated battery is thin, has a large surface, and has a good soaking and cooling capacity, so nissan's LEAF boldly USES a passive thermal management system (which it doesn't!). , heat is carried away by natural convection of the air.

From left to right, the LEAF from monomer to four monomer battery two and two strings of battery module.

Nissan LEAF using NEC cascading batteries

As you can see, there is no any battery thermal management system such as fan, cooling fluid pipeline. This is probably not recruit wins a recruit.

In turn the Tesla, 18650 batteries of the relatively small size, monomer battery internal temperature during normal charge and discharge also too won't big. But, more than 6000 monomer battery temperature should also keep in the range of no more than 5 ° C, which is a very difficult thing. However, Tesla made it. How to do it? Seeing the Tesla battery management system (BMS) what are the advantages compared with other electric cars?

Tesla battery module in the surface system

As shown above, these pipes are cooling fluid flow, flow evenly distributed in the middle of the battery module, can let each batteries can contact with water, so that each cell take away heat is almost the same as when cooling, temperature can effectively control in a small range.

In summary, with small capacity of 18650 batteries, Tesla is greatly increased the complexity of thermal management system. That is to say, if the heat dissipation ability into consideration, the use of small capacity of 18650 battery is not the best choice.

About the energy density

When it comes to energy density, it must distinguish between monomer battery energy density and energy density of battery pack.

Monomer battery energy density, 18650 battery is more than the cascading lithium ion batteries. I'm here to check the nissan LEAF used in 33 ah lithium ion battery energy density is 157 wh/kg, GMVolt cascading batteries used in the energy density of about 150 wh/kg; The Roadster is used in the 18650 battery energy density is about 18650 wh/kg. 18650 battery management system, however, more complex, so the extra weight will make the energy density of the battery pack is far less than the energy density of the monomer. Roadster battery pack weight is 450 kg, the energy density is 118 wh/kg, and the LEAF weight of the battery pack is 225 kg, and the energy density is 107 wh/kg. In the battery level, both the energy density is equal.

Tesla use panasonic 18650 cylindrical batteries

In summary, 18650 monomer battery capacity is small, the required number of monomer will be a lot of ModelS (7104), but it's good consistency; Cascading the capacity of the battery can be done larger ah (20 to 60 ah), monomer amount can reduce, but poor consistency. At this stage, by contrast, together with the battery supplier is hard to get into a lot of manpower material resources to improve the production technology of cascading battery. Therefore, in the development of the Roadster and ModelS, the only option for Tesla from market to buy batteries, battery system developed. Develop a set of consistency of good management of more than 6000 section monomer battery system and development of a set of more than 200 poor battery system consistency, by contrast, the technical difficulties of the former should be more lower. Even if the number of single cells increased, but if the battery performance is reliable, is easier to manage.

Compared with another very successful pure electric vehicles, nissan's LEAF, it USES is cascading lithium ion batteries. This is because the nissan with NEC cooperation for many years, accumulated in battery technology is very deep, there should be a fairly capability in terms of quality control. The LEAF's battery from AESC, nissan's joint venture with NEC.

Compared with the United States and Japan, the medium in different parts of the car manufacturers in the development of electric vehicles with battery maker of the nation is a very interesting thing.

The previously mentioned cascading various advantages of lithium ion battery, but it also has some disadvantages. Due to the cascading lithium ion battery is generally adopts the aluminum-plastic film packaging, and aluminum-plastic film thickness thin, poor mechanical strength, in extreme cases, such as vehicle collision, aluminum-plastic film damage cause safety accident easily. That would explain why nissan in 4 of monomer battery module, and an aluminum shell outside.

18650 battery is commonly steel shell, safety is better; And the front also referred to as 18650 battery production technology level unceasing enhancement, security has been improved.

Tesla in response to these 18650 battery safety accidents that may occur, also spent a lot of effort. If single cell appear the high temperature anomalies, according to the badness of abnormal situation, this battery or its own module will power to prevent the spread of the accident. Spread because of the single capacity is small, as long as it doesn't happen, the severity of the accident will be lower.

About the cost

18650 lithium ion battery has big capacity, long life, safety performance is high, and because of small volume, light weight, easy to use, favored by consumers. With the deepening of the 18650 battery technology research, the consistency, safety of battery has reached the very high level. 18650 as the first lithium ion battery, the battery is currently the world's most mature, the most stable battery combination, still occupy the leading position. About billions of China's annual production of 18650 batteries, battery the data than other materials.

And Tesla USES 18650 batteries, can use Japan panasonic manufacturer before production line for production. Used in consumer electronic products increasingly fierce competition in the 18650 battery case, panasonic and other manufacturers to cooperate with Tesla upgrade the product, after reforming the original production line for the production of power battery. Industrial production has a scale effect, an order of magnitude as the size of the product reaches, the cost would be greatly reduced. A new energy car requires tens of thousands of section 18650 battery, so the monomer of purchasing cost control.

Use the cylindrical battery such as 18650, 20700, as it were, as a new energy source of power for electric vehicles, at this stage can be said to be the best choice. But as the battery technology mature, the future also will be more new type of battery used in new energy electric vehicles.

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