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18650 is a lithium-ion battery, where 18 means a diameter of 18mm, 65 means a length of 65mm, and 0 means a cylindrical battery, that is, they get their name from their size. As for scale, it is larger than an AA battery. 18650 battery is a rechargeable battery, has voltage of 3.7V and has capacity between 1800mAh and 3500mAh. You may also know 26650 battery and 21700 battery, what are they? and what is the difference between them?

1 18650 Battery Basic
1.1 Characteristic
① Large capacity: The capacity of a lithium battery is at least 1200mah or more, or even 3600mah, while the average battery cell is only about 500mah.
② High energy storage efficiency and good stability: It can still maintain full performance output under 70°, and there is generally a protection circuit inside to prevent the battery from burning out.
③ No memory effect: It is not necessary to discharge all the remaining power before charging, and it can be charged and discharged at any time, which is convenient to use.
④ High charge and discharge cycle life: The number of cycles of lithium batteries is tens of thousands and the high temperature resistance is very good.
⑤ Environmental protection, no toxic substances: Non-toxic, harmless, non-polluting, certified by RoHS quality.

Fig 1. 18650 Battery

1.2 Protective Function
① Overcharge protection: When the lithium battery is overcharged, the internal temperature rise of the battery will continue to rise, and a detection system for the battery voltage is added. When the battery overcharge voltage reaches a certain value or time period, the overcharge function will work and stop automatically to protect the battery.
② Over-discharge protection: It means that the battery is always in an overloaded output state. Generally, there is discharge protection. At this time, the battery will be in a standby mode.
③ Overcurrent protection: The overcurrent protection value can be adjusted, some are a few amperes, and the setting is selected according to the actual situation.
④ Short-circuit protection: When the battery is short-circuited, the overcurrent protects the battery from burning.
In addition to these four protection functions, some also have functions such as temperature and balance. Generally, the battery has a built-in PCM protection system with multiple protection functions.

1.3 Basic Parameters
Standard Voltage: 7.4V
Rated Capacity: 2200mAh
Continuous Working Current: 1-3A
Overcurrent Protection Value: 2-5A(adjustable)
Affordable Equipment Power: ≤22V
Overcharge Protection Voltage: 4.25±0.025V/Cell
Discharge Protection Voltage: 2.50±0.05V/Cell
Charging Mode: Constant-current and Constant-voltage
Maximum Charging Voltage: 8.45V-8.55V
Recharging Current: 0.2℃-0.5℃
Charging Temperature: 0~45℃, 45~85%RH
Discharge Temperature: -20~55℃, 46~85%RH
Storage Temperature and Humidity Range
Short term: more than one month: -20℃~+55℃, 45~85%RH
Medium term: more than three months: -20℃~+45℃, 45~85%RH
Long term: within one year: -5℃~+20℃, 45~85%RH
Brightness: Reference Sample
Length: Reference Sample
Thickness: Reference Sample
Weight: <120g

1.4 Merits and Drawbacks
1) Large capacity
The capacity of 18650 battery is generally between 1200mah ~3600mah, and the general battery capacity is only about 800mah. If combined into a 18650 battery pack, it can easily break through 5000mah.
2) Long Life
The 18650 battery has a long service life, and the cycle life can reach more than 500 times during normal use, which is more than twice that of ordinary batteries.
3) High Safety Performance
The 18650 battery has high safety performance. In order to prevent the short circuit of the battery, the positive and negative electrodes of the 18650 batteries are separated. Therefore, the possibility of short-circuiting has been reduced to the extreme. A protection board can be added to avoid overcharging and overdischarging of the battery, which can also prolong the service life of the battery.
4) High Voltage
The voltage of 18650 lithium battery is generally 3.6V, 3.8V and 4.2V, which is much higher than the 1.2V voltage of nickel-cadmium and nickel-metal hydride batteries.
5) No Memory Effect
It is not necessary to empty the remaining power before charging, which is convenient to use.
6) Small Internal Resistance
The internal resistance of the polymer battery is smaller than that of the general liquid battery, and the internal resistance of the domestic polymer battery can even be below 35mΩ, which greatly reduces the self-consumption of the battery and prolongs the standby time of the mobile phone. This polymer lithium battery that supports large discharge current is an ideal choice for remote control models, and has become the most promising product to replace nickel-metal hydride batteries.
7) It can be combined in series or in parallel to form a 18650 lithium battery pack.
8) Wide Range of Use
18650 batteries can be employed in Notebook computers, walkie-talkies, portable DVDs, instrumentation, audio equipment, model aircraft, toys, video cameras, digital cameras and other electronic equipment.
1) The biggest disadvantage of the 18650 battery is that its size has been fixed, and it is not very well positioned when it is installed in some notebooks or some products. Of course, this can also be said to be an advantage, which is compared to other polymer lithium batteries, etc. This is a disadvantage in terms of the customizable and changeable size of lithium batteries. Compared with some products with specified battery specifications, it has become an advantage.
2) The production of 18650 batteries requires a protection circuit to prevent the battery from being overcharged and causing discharge. Of course, this is necessary for lithium batteries, which is also a common drawback of lithium batteries, because the materials used in lithium batteries are basically lithium cobalt oxide materials, and lithium batteries made of lithium cobalt oxide materials cannot be discharged at large currents, and their safety is poor.
3) The production conditions of 18650 batteries are high, compared with general battery production, they have high requirements for production conditions, which undoubtedly increases the production cost.

2 26650 Battery
2.1 Intro Info
The 26650 battery is a cylindrical lithium battery with a diameter of 26mm and a length of 65mm. It is used in power tools, lighting, wind and solar energy storage, electric vehicles, toys, instrumentation, ups backup power supply, communication equipment, medical equipment and military lights.

Fig 2. 26650 Battery

2.2 Basic Parameters
Cycle performance: 2000 times (1C charge/1C discharge, capacity retention rate ≥80%, 100% DOD)
Maximum continuous discharge current: 9.6A
Pulse discharge current: 15A, 5s
Operating temperature: Charge: 0°C ~ 55°C, discharge: -20°C ~ 60°C
Storage temperature: -20°C ~ 45°C
Battery weight: 86g (approx.)
Nickel-cobalt-manganese ternary lithium-ion 26650 single-cell nominal voltage is generally: 3.6V
Nominal capacity: 4500mAh (capacity range 4500-4650mAh)
AC internal resistance: ≤30mΩ (plus PTC type)
Charging conditions: Cut-off voltage 4.2±0.05V, cut-off current 0.01C. (Note: Charge with 0.5C constant current to 4.2V, and charge with constant voltage until the current drops to 0.01C and cut off)
Discharge cut-off voltage: 2.75V
Cycle performance: 500 times (1C charge/1C discharge, capacity retention rate ≥70%, 100% DOD)
Maximum continuous discharge current: 13A
Pulse discharge current: 15A, 5s
Operating temperature: Charge: 0°C ~ 55°C, discharge: -20°C ~ 60°C
Storage temperature: -20°C ~ 45°C
Battery weight: 92g (approx.)

2.3 18650 Battery vs 26650 Battery
1) Different Rated Capacity
The rated capacity of IFR26650 is 3000mAh, and the rated capacity of IFR18650 is 1100~1400mAh.
2) Different Diameters
The diameter of the IFR26650 is 26mm, and the diameter of the IFR18650 is 18mm.
3) Different Reference Quality
The production test quality of IFR26650 is 94 grams, and the IFR18650 is 45 grams.
18650 lithium batteries are used in lighting, industrial supporting lithium battery packs, power tool batteries, electric bicycle batteries, power lithium battery packs, etc., while 26650 batteries are used in integrated solar street light lithium battery packs, energy storage stations, solar energy storage batteries and so on.
The 26650 battery will gradually replace the 18650 battery in the application of power batteries. And with the large-scale use of lithium batteries, it will inevitably be a trend that larger-capacity 26650 batteries replace the trendy 18650 lithium batteries in the 3C era.

3 21700 Battery
3.1 Info about 21700
The 21700 battery is a cylindrical battery with a diameter of 21mm and a height of 70.0mm. Its charge density is currently the highest energy density and lowest cost battery in the world, and it is cost-effective.

Fig 3. 21700 Battery

3.2 Basic Parameters
The positive electrode is converted to nickel, the performance is not affected, the consistency is good, and it can be directly used as a battery pack.
*Rechargeable Li-ion Cell
*Size: Diameter 21mm, Length 70mm
*Weight: about 65g
*Rated voltage: 3.6V
*Standard capacity: 4800mAh
*Internal resistance: about 13 milliohms
*Charging voltage: 4.2V
*Discharge cut-off voltage: 2.5V
*Discharge current: 10A (15-20A can be discharged instantaneously).
*Applications: flashlights, scooters, LED lights, miner's lamps, lighting products, power banks, mobile power supplies, backup power supplies, computers, mobile devices, cars, bicycles, communications, medical, energy storage, solar energy, etc.

3.3 21700 Battery Advantages
1) The energy density of the 21700 type battery is higher than that of the well-known 18650 type battery. The number of single cells in use can be greatly reduced, and the cost will be reduced after grouping. The capacity of a 18650 battery is about 2600-3600 mAh, while a 21700 battery supports more than 4000 mAh, even 5000mAh has appeared on the market. And the larger capacity is increasingly beneficial to extend the battery life of modern devices.
2) The single volume of the rechargeable battery is increased by 35%. Taking the Tesla 21700 rechargeable battery as an example, the energy of a single battery can be increased by 34.8ah, an increase of 35%.
3) The net weight of the system software is estimated to be reduced by 10%. The total capacity is more than 21,700. With the increase of single volume and the increase of single energy ratio, the total number of batteries required under the same kinetic energy can be reduced by about 1/3, and the total number of metal components and electrical components selected for the battery pack can reduce the difficulty of managing information systems coefficient. After converting SDI (Samsung Digital Interface) to the new 21700 rechargeable battery, it was found that the system software reduced the net weight by 10% over the existing battery.

3.4 18650 Battery vs 21700 Battery
The 18650 rechargeable battery has high reliability and stability, and the performance index of the 21700 battery is much higher than that of the 18650 battery. In addition, compared with other battery models, the raw materials, processing technology and technical steps of the 21700 rechargeable battery are more advanced than the 18650 rechargeable battery level. Therefore, the 18650 and 21700 production lines are the best match.

4 Technical Specifications Comparison
4.1 18650 Battery Spec
Nominal Voltage: 3.6V
Nominal Capacity: 2,850 mAh
Minimum Discharge Voltage: 3V
Maximum Discharge current: 1C
Charging Voltage: 4.2V (maximum)
Charging current: 0.5C
Charging Time: 3 hours (approx)
Charging Method: CC and CV
Cell Weight: 48g (approx)
Cell Dimension: 18.4mm (dia) and 65mm (height)

4.2 26650 Battery Spec
Voltage: 3.2V
Technologie: Lithium Iron Phosphate
Dimension: 26.2 (Ø) x 65.6 (H) mm
Weight: 80g
Standard capacity: 2300mAh – 0.5C (current value of 2300mA at 1C°)
Max. charge voltage: 3.65 ± 0.05 V
Inner resistance: ≤15mΩ
Max. discharge voltage: 2.0V
Cycle characteristic: 1500 (C/5) – 300 (10C)
Working temperature: 0 ~ 55°C Discharge: -20°C ~ 60°C

4.3 21700 Battery Spec
Voltage: 3.7V
Capacity: 3500- 5600mAh
Operating voltage: 2.5- 4.2V
Cutoff voltage: 2 – 2.5V
Weight: 55gms to 75gms
Charge density (Energy per cell): 10.5- 13.7Wh
Charge discharge cycle: 500 to 2000
Continuous discharge current: 20- 35 amps
Optimum /Minimum charging time: 2.5 hrs to 3.5 hrs
Charging voltage: 4.2V- 5V


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