Lithium ion batteries have high energy density, their capacity are 1.50 to two times of nickel hydrogen battery at same weight, and they have extremely low self discharge rating. Moreover, lithium ion batteries has nearly no memory effect and does not have toxic materials. Those points of lithium ion batteries make it be used everywhere

Now, mainly used cylindrical lithium ion battery is 18650 and 21700.

Li ion cell of classical 18650 type is straightly compared with 21700 type regarding thermal, electrochemical, geometrical property etc. Inner temperature sensor gives insight of heat activity. Resemblance is found in voltage curve and capacity wilt at 1.0C cycle at 25 degree. The capacity and energy increasing each cell is about 50 percents from 18650 to 21700 for discharging ranging from 0.50C to 3.75C. The result is compared with count of electrode area based on Archimedean spiral's arc length and geometrical datas collected from X-ray CT (computed tomography) measurement. 2 evaluation method for collection number of electrode winding from computed tomography. image is compared. Calculation is made for the windings count as functions of the periodic winding distances --thicknesses of cathode, anode, double separators. on electrode curvature of both types. Influence of intended extensions of the model diameter by few microns -- Observed in commercial cylinder cell, same as bigger models such as 32700 at extra electrode winding and cell capacity is estimated.

18650 type
18650 is nowadays referred to lithium ion battery, a standard lithium ion battery model defined by SONY company in Japan to save cost, which 18 indicates diameter 18mm, 65 indicates length 65mm, 0 indicates cylinder battery. General 18650 battery includes NCM lithium ion and lithium iron phosphate.

When Tesla was developing electric car, they tried all type battery, but eventually, 18650 was selected, Technology way. The reason Tesla is capable to have performance which is not worse than traditional fuel cars, especially motor technology, should benefit from Tesla's high-ranking battery technology. Why did Tesla select 18650?

Mature technique, high consistency
18650 battery had been widely using in digital electric industry. It is most mature and earliest and the most steady lithium ion battery. After years of cumulative skill, Japanese suppliers have cumulative 18650 in consuming products. Advanced techniques are very well applied at the field of electric vehicle. Panasonic company is one of the worldwide biggest battery technology and scaling supplier. Comparing to other suppliers, they have fewest product defect and bigger scale, also it’s easy to choose battery at better consistency.

For contrast, other battery, such as laminated lithium ion battery, is not mature so much. Most product can’t even be unified on dimensions, the production technique of battery supplier can’t achieve condition. generally, the consistency of battery doesn’t approach the level of 18650. If battery consistency fail to achieve requirement, the management of large amount battery string and battery pack that forming in parallel and in series won’t allow the performances of each battery to be better applied, but 18650 battery can solve this problem well.

High safety
18650 lithium batteries have high safety property, non explosive, non combustible, non toxic, non polluting, and pass RoHS trademark certification; resistant from high temperature, the discharge efficiency is 100 percents at 65°C.

18650 are in general packed inside steel shell. In extreme case such as car impact, it could reduce occurrence of safety accident and offer better safety. Moreover,18650 size is small, but cell energy could be controlled at smaller range. Comparing with usage of large battery cell, even if one cell of the battery pack doesn’t work, it can be decreased impact of the failure.

High energy density
18650 lithium batteries capacity  are in general between 600mah to 3500mah, and the common battery capacity is about 2000mah. If combined into a 18650 lithium battery pack, the 18650 lithium battery pack can exceed 10000mah. The capacity is 1.5 to two time of nickel hydrogen battery at same weight, and they have extremely low self discharge rating. Nowadays its energy density could achieve 250Wh/kg, which reach Tesla high cruise range requirement.

Cheap

18650 lithuim batteries have long serving life, the life span could achieve more than 800 times for general usage, which is far more than 2 times of ordinary battery. 18650 products has a high degree of technic maturity, and the structure devise, manufacture technique, manufacture device, deriving 18650 module technique mature as well, all of these decrease the operated and maintain cost.

Currently, 18650 batteries, which are at large used, have a history of development for long time. Even though the technique are relatively mature comparing to other type of batteries, it still face problem for instance fast charge, high temperature production and intricate grading. At this condition, 21700 cylinder ncm batteries come to market.

21700:
21700 are the type of the cylinder batteries, which specially refer to: 21 indicates external diameters is 21 millimeter; 70 indicates height is 70 millimeter, 0 indicates to cylinder batteries.
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