Cylindrical Lithium Battery: 21700 VS 18650
Wound electrodes have the advantage of faster production compared to stacked electrodes. Since its market introduction in 1994, the 18650 format became a popular standard for Li-ion cells across the consumer industry. Increases in diameter by 3 mm and in height by 5 mm lead to the new 21700 format. Similarly for both cell formats is the wound jelly roll which has approximately the form of an Archimedean spiral.
There are many sizes of lithium-ion cylindrical batteries, and the number of sizes continues to grow. 21700 batteries and 18650 batteries are two of the common types of lithium batteries. Both of these batteries are named by the size of their own.
Obviously, you can find out from the name that these two types of batteries are actually different in size. But they are suitable for the same devices in most cases.
Many people will be torn in the choice of these two batteries in the end which battery is better? Hopefully this article will help you solve this problem.
What is a 21700 battery?
21700 batteries are rechargeable lithium-ion cells that are used to power various devices like flashlights and vaping accessories. Similar to other lithium-ion cells, the 21700 battery is named after its dimensions to identify size and compatibility quickly. The 21700 batteries are high-capacity lithium-ion cylindrical battery with 26mm diameter and 65mm length. There are two types of 21700 batteries: The protected 21700 and the unprotected 21700.
What is the difference between the protected 21700 and the unprotected 21700?
The protected type of 21700 batteries have a button on its top while the unprotected type are flat on top. The most important is the protected type of 21700 batteries are safer than the unprotected because its electronic circuit protects the battery when electrical risks such as short circuits happens. But the unprotected 21700 batteries are cheaper and they only can be used in controlled environments because they are more likely to overcharging, over-discharging and have short circuits.
Specification:
Weight: about 65g
Voltage: 3.6V
Standard capacity: 4800mAh
Charging voltage: 4.2V
Discharge cut-off voltage: 2.5V
PROs of 21700 batteries:
1.）The energy density of the 21700 type battery is higher than 18650 type battery.
2.）The cost will be reduced.
3.）The 21700 batteries are lighter in weight.
The capacity supported by 21700 battery are more than 4000mAh, even some 5000mAh have emerged. Therefore, the larger capacity could help to increasingly prolong the lives of modern devices and to upgrade user’s using experiences.
CONs of 21700 batteries:
1.）The larger capacity of the 21700 battery needs longer charging times.
2.）technology is unstable and mature. The 21700 battery production and processing equipment, technology and so on can not be ready in a day, so now this battery can not be mass production. And not as fast as the development trend of 18650 technology.
What are 21700 batteries used for?
21700 batteries are usually used in flashlights, scooters, LED lights, miner’s lamps, lighting products, power banks, mobile power supplies, backup power supplies, computers, mobile devices, electric cars, bicycles, communications, medical, solar energy, etc.
21700 for power tools:
Like the proposed 4680 cells, the packs of 21700 cells designed for power tools use improved packaging to deliver increased performance. For example, a standard 18V battery using 18650 cells can produce up to 800 W of power output. The newer packs based on 21700 cells can produce up to 1,440 W, an 80% increase. As noted above, the 21700 inherently has about 50% greater capacity and energy density than the 18650 for discharge rates up to about 3.75C.Want More Details: Download our battery design ebook. Lithium Battery Design Design Ebook Download(2M, 20 pages, PDF)
18650 battery
18650 It has a disadvantage in terms of energy density and cost, but it performs better in terms of standardization, automation, technological maturity and ladder utilization.
Cylindrical batteries, especially 18650, due to their own structural characteristics and the standardization of their models, the level of automation in the production of cylindrical batteries is the highest among the three main battery forms. This makes it possible to have a high degree of consistency, and the yield is improved accordingly. Data show that the yield rate of major foreign manufacturers such as Samsung and Panasonic can reach 98%, while Chinese manufacturers can also exceed 90%.
PROS of 18650 battery
1) As mentioned earlier, the monomer consistency is better；
2) The monomer itself has good mechanical properties. Compared with square and soft-packed batteries, a closed cylinder can obtain the highest bending strength at approximately the same size.；
3) The technology is mature and the cost is low, but at the same time, the space for cost optimization has been almost consumed.；
4) The energy of the monomer is small, and the form is easy to control in the event of an accident, but this is also becoming the reason for its replacement (what about the popular saying, those who make you will also destroy you, and those who can’t kill you will also make you strong. The same is true for things)
CONS of 18650 battery
1) In the context of electric vehicles, the number of cylindrical monomers in the battery system is very large, which makes the complexity of the battery system greatly increased, regardless of the mechanism or the management system, compared with the other two types of batteries, the system-level cost of cylindrical batteries is high.
2) Under the conditions of uneven temperature and environment, the probability of alienation of the characteristics of a large number of batteries increases. Of course, the reason why Tesla chose 18650 at the beginning of the design is believed to be just a helpless choice, because 10 years ago, only cylindrical batteries could be produced in large quantities. Qualified power batteries. On the contrary, the safety and thermal management needs of the battery are the driving force for the development of its powerful electronic control system.
3) The room for energy density to rise is already very small. According to the news in 2016, Chaowei has achieved a monomer capacity of 4050mAh, and the specific energy of the battery is 306Wh/kg.
Many wonder what the most significant difference between a 21700 battery and a 18650 battery.
What is the difference between a 21700 battery and a 18650 battery?
From a physical level, it is simple;
1.）the size is the difference. 21700 batteries are slightly larger than 18650 batteries.
V=π*r²* h
FOR 18650 V=16532.1 cubic mm
FOR 21700 V=24232.95 cubic mm
The extra 5 mm length and 3 mm diameter of the 21700 battery cell can increase its volume by 47%. This also means an increase of 7,700 cubic millimeters of space, which can be used to fill the anode, cathode and electrolyte materials that transmit energy.
2.）Both come in flat top and button top forms. However, 18650 li-ion battery is smaller than 21700 li-ion battery.
From a battery performance level:
1.)21700 li-ion battery has larger capacity
Being that the 21700 battery is slightly larger, it will also provide more energy/power and runtime. Experimental studies by scholars have shown that the 21700 battery has about 5.3 circles of electrodes more than the 18650 battery. Taking into account the change in the height of the 21700 battery, the overall 21700 battery has about 51% more capacity than the 18650 battery.One thing to consider here is the mAh of these batteries. mAh’s on 18650 batteries will range from 2000 up to 3500. On 21700 batteries, you can expect mAh’s to range between 3000 up to 5000 mAh’s. You may see advertising for 18650 batteries that claim to have up to 9000 mAH.
2.)21700 battery has smaller inter resistance than 18650 battery
For cylindrical cells these parameters correlate with each other. This contributes to the lower performance of high energy cells. Electrode thickness has a possitive influence on cell resistance. 21700 battery is thick than 18650 battery.
Going from the 18650 to the 21700 format, the cell resistance decreases noticeably and shows a relatively flatter correlation to anode coating thickness.
The reason is the larger usable coated cathode area in the larger 21700-type cells, especially due to the outer windings of the jelly roll.
3.)21700 battery has lower discahrge rate than 18650 battery
Electrode thickness has a negative impact on the discharge rate capability. As we all known that 21700 battery is thick than 18650 battery.
Cells with thicker electrodes experience higher losses by limited transport, resulting in lower discharge energy and underutilized electrode active material. Thegeneral trend for all tested cell types was found that therate-capability is limited by the temperature on the cell surface due to current flow.
4.)21700 battery has higher energy density than 18650 battery
Change 18650 cell into 21700 cell, battery energy density can be increased by more than 20%. 21700 battery power density is better than 18650 batteries. According to the current disclosure of Tesla, under the existing conditions, the energy density of 21700 battery systems is approximately 20% higher than the 250WH to the original 18650-300 Wh battery system. At the same time, according to the Panasonic lithium battery power test in monomer batteries, the bulk density of its 21,700 battery is significantly higher than that of the 18650 cell monomer.
From price level:
21700 battery pack is cheaper than 18650 battery pack for the relatively same specification.
Compared with 18650 battery system, 21700 battery system cost down by about 9%. Main reasons are as following:
a.Each 21700 single lithium battery contains more than 50% more energy than 18650, so the total number of single lithium batteries in use can be greatly reduced.
b. Since the 21700 lithium battery has more energy, the total number of parts and facilities required to manufacture the same Wh lithium battery is reduced, which is beneficial to reduce costs.
c. Since the 21700 lithium battery has more energy, the total number of parts and facilities required to manufacture the same Wh lithium battery is reduced, which is beneficial to reduce costs.
d. Lithium battery housing, because the diameter of the 21700 lithium battery is larger, it can accommodate more lithium batteries, so the shell required for each Wh, the 21700 lithium battery is 33% less than the 18650 lithium battery, so the shell cost of the 21700 lithium battery is lower than 18650.
e. The number of lithium batteries with the same Wh number is reduced by 33%, so the requirements for the injection and sealing process are also somewhat reduced, and the cost is somewhat reduced.
f. Similarly, with the reduction of the overall number of lithium batteries, the requirements for chemical facilities have also been greatly reduced, increasing the rate and reducing costs.
At the Pack level, 21700 has the same advantages：
1) The first is the selection of 21700 lithium battery rechargeable battery 18650 lithium battery. Under the condition that the battery capacity of the battery pack is the same, the total number of single lithium batteries in the battery pack will be greatly reduced, reducing the complexity of the battery pack structure and the difficulty factor of the pack.
2) The relatively small total number of lithium batteries also reduces the regulatory difficulty factor of BMS.
3) The replacement of the 21700 lithium battery with the 18650 lithium battery will not reduce the porosity in the lithium battery module, so the size of the battery pack is the same as the total number of cooling structures.
different battery pack specifications
18650 21700
Compact 1P battery: 2.0Ah–3.0Ah (36 Wh–54Wh) Compact 1P battery: 3.0Ah–4.0Ah (54wh–72Wh)
Universal 2P battery: 4.0Ah–6.0Ah (72 Wh to 108 Wh) Universal 2P battery: 6.0Ah–8.0Ah (108 Wh–144Wh)
Large capacity 3P battery: 9.0Ah (162Wh) Large-capacity 3P battery: 9Ah–12Ah (162 Wh–216 Wh)
*Note: 1P refers to a battery pack made of 5 batteries in series. 2P refers to a battery pack made of 2 1P in parallel, and 3P refers to a battery pack made of 3 1P in parallel.
In summary
the size has been increased from 18650 to 21700, and the benefits are as follows：
1) When the energy density is appropriately increased, conventional materials can be selected, with stable performance and high cost performance.；
2) The design of the multi-pole headphone structure can be appropriately carried out to reduce the internal resistance；
3) At the same energy density, graphite with fast charging characteristics can be selected to improve fast charging performance.；
4) Appropriately increasing the diameter and height can obtain more effective volume.
5) The capacity of the single battery cell is increased, the proportion of auxiliary components is reduced, and the packaging cost is reduced.Want More Details: Download our battery design ebook.
Lithium Battery Design Design Ebook Download(2M, 20 pages, PDF)
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