18650 Battery

30-second summary

18650 Battery

A 18650 battery is a 3.7V lithium-ion battery that is used in many laptop computer batteries, cordless power tools, certain electric cars, electric kick scooters, most e-bikes (bicycles driven or supported by electric motors), portable powerbanks, electronic cigarettes, and LED flashlights.

There are several specific advantages to lithium-ion batteries. The most important advantages are their high cell voltage, high energy density, and no memory effect.

Lithium-ion batteries are used in many laptop computer batteries, cordless power tools, certain electric cars, electric kick scooters, most e-bikes, portable power banks, and LED flashlights.

The overall reaction during discharge is:

C6Li + CoO2 ⇄ C6 + LiCoO2

18650 battery - size, voltage, capacity

18650 Battery

A lithium-ion battery, also known as the Li-ion battery, is a type of secondary (rechargeable) battery composed of cells in which lithium ions move from the anode through an electrolyte to the cathode during discharge and back when charging. 

The cathode is made of a composite material (an intercalated lithium compound) and defines the name of the Li-ion battery cell. The anode is usually made out of porous lithiated graphite. The electrolyte can be liquid, polymer, or solid. The separator is porous to enable the transport of lithium ions and prevents the cell from short-circuiting and thermal runaway.

Chemistry, performance, cost, and safety characteristics vary across types of lithium-ion batteries. Handheld electronics mostly use lithium polymer batteries (with a polymer gel as electrolyte), a lithium cobalt oxide (LiCoO2) cathode material, and a graphite anode, which offer high energy density.

Li-ion batteries, in general, have a high energy density, no memory effect, and low self-discharge. On the other hand, they are expensive and must use monitoring electronics to ensure over-charge and deep-discharge protection. One of the most common types of cells is 18650 battery, which is used in many laptop computer batteries, cordless power tools, certain electric cars, electric kick scooters, most e-bikes, portable power banks, and LED flashlights. The nominal voltage is 3.7 V.

Note that non-rechargeable primary lithium batteries (like lithium button cells CR2032 3V) must be distinguished from secondary lithium-ion or lithium-polymer, which are rechargeable batteries. Primary lithium batteries contain metallic lithium, which lithium-ion batteries do not.

Dimensions and Weight of 18650 Battery

18650 batteries (18 mm x 65.0 mm) have a nominal diameter of 18.0 millimeters. The overall height is 65.0 millimeters. Its weight is around 45 grams.

It has a typical capacity of about 3700 mAh. As with all similar batteries, the 18650 battery’s capacity depends on the current drain and actual cutoff voltage of the powered device.

Cells with a cylindrical shape are made in a characteristic “jelly roll” manner, which means it is a single long “sandwich” of the positive electrode, separator, negative electrode, and separator rolled into a single spool. One advantage of cylindrical cells compared to cells with stacked electrodes is the faster production speed. One disadvantage of cylindrical cells can be a large radial temperature gradient inside the cells developing at high discharge currents.

Lithium-ion vs Lithium polymer batteries

A lithium-ion polymer (LiPo) battery (also known as Li-pol, lithium-poly, and other names) is a type of Li-ion battery with a polymer electrolyte instead of a liquid electrolyte. All LiPo batteries use a high-conductivity gel polymer as the electrolyte. Lithium polymer cells have evolved from lithium-ion and lithium-metal batteries. The primary difference between lithium-ion and Li-pol is that instead of using a liquid lithium-salt electrolyte (such as LiPF6) held in an organic solvent, the battery uses a solid polymer electrolyte (SPE) such as polyethylene oxide (PEO), polyacrylonitrile (PAN), polymethyl methacrylate (PMMA) or polyvinylidene fluoride (PVdF). LiPos provide higher specific energies than other lithium batteries, often used in systems where weight is an important factor, such as mobile devices, drones, and some electric vehicles.

Composition of 18650 Lithium-ion Batteries

lithium-ion battery - composition

A typical lithium-ion cell contains:

  • Cathode:  The cathode is the positive or oxidizing electrode that acquires electrons from the external circuit and is reduced during the electrochemical reaction. In the case of lithium batteries, cathode materials are generally constructed from LiCoO2 or LiMn2O4. For the cathode, it is important to hold a large amount of lithium without significant change in structure, have good chemical and electrochemical stability with electrolyte, be a good electrical conductor and diffuser of lithium ions, and be of low cost.
  • Anode: The anode is the negative or reducing electrode that releases electrons to the external circuit and oxidizes during an electrochemical reaction. One of the most common anode materials used today is lithiated graphite, LixC6, which is composed of graphite sheets intercalated with lithium. New materials, such as those based on Silicon and other elemental blends, are being researched. Lithiated graphite has a unit cell with an HCP structure.
  • Separator. A separator is a permeable membrane placed between a battery’s anode and cathode. The main function of a separator is to keep the two electrodes apart to prevent electrical short circuits while also allowing the transport of ionic charge carriers that are needed to close the circuit during the passage of current in an electrochemical cell. Commercially available liquid electrolyte cells use microporous polyolefin materials, such as polyethylene (PE) or polypropylene (PP). Separators in Li-ions have to be electrochemically and chemically stable relative to the electrolyte and electrode materials. Functional separators that use MOF-coated membranes to perform the dual functions of the electrolyte and separator are being developed to support the design of high-performance Li-metal batteries for high-energy systems in electric vehicles and electric aircraft.
  • Electrolyte: The choice of electrolyte in all batteries is critical for performance as well as safety. Most of the electrolytes used in commercial lithium-ion batteries are non-aqueous solutions, in which Lithium hexafluorophosphate (LiPF6) salt dissolved in organic carbonates, in particular, mixtures of ethylene carbonate (EC) with dimethyl carbonate (DMC), propylene carbonate (PC), diethyl carbonate (DEC), and/or ethyl methyl carbonate (EMC). A good electrolyte must have low reactivity with other cell components, high ionic conductivity, low toxicity, a large window of electrochemical voltage stability (0-5V), and be thermally stable.

Characteristics of 18650 Lithium-ion Batteries

To compare and understand the capability of each battery, some important parameters are characteristic of each battery, also within a type of battery. These parameters are a reference when a battery is needed, and specific qualities are required since batteries are used in all types of devices and for infinite purposes.

Cell Voltage

The voltage of electric batteries is created by the potential difference of the materials that compose the positive and negative electrodes in the electrochemical reaction.

Almost all lithium-ion batteries work at 3.8 volts. In order to make current flow from the charger to the battery, there must be a potential difference. Therefore battery chargers or USBs for almost all smartphones provide a voltage of 5V.

Cut-off Voltage

The cut-off voltage is the minimum allowable voltage. It is this voltage that generally defines the “empty” state of the battery.

Li-ion battery has a higher cut-off voltage of around 3.2 V. Its nominal voltage is between 3.6 to 3.8 V; its maximum charging voltage can go to 4– 4.2 V max. The Li‑ion can be discharged to 3V and lower; however, with a discharge to 3.3V (at room temperature), about 92–98% of the capacity is used. Importantly, particularly in the case of lithium-ion batteries used in the vast majority of portable electronics today, a voltage cut-off below 3.2V can lead to chemical instability in the cell, resulting in a reduced battery lifetime.


The coulometric capacity is the total Amp-hours available when the battery is discharged at a certain discharge current from 100% SOC to the cut-off voltage.

Almost all lithium-ion batteries work at 3.8 volts. Lithium-ion 18650 batteries generally have capacity ratings from 2,300 to 3,600 mAh.

C-rate of Battery

C-rate is used to express how fast a battery is discharged or charged relative to its maximum capacity. It has units h−1. A 1C rate means that the discharge current will discharge the entire battery in 1 hour.

Most li-ion batteries can only withstand a maximum temperature of 60°C and are recommended to be charged at a maximum of 45°C under a 0.5C charge rate. C rating for a 18650 battery is usually 1C, meaning we can consume a maximum of 2.85A from the battery.


Batteries gradually self-discharge even if not connected and delivering current. This is due to non-current-producing “side” chemical reactions that occur within the cell even when no load is applied.

Li-ion rechargeable batteries have a self-discharge rate typically stated by manufacturers to be 1.5–2% per month. The rate increases with temperature and state of charge.


Some degradation of rechargeable batteries occurs on each charge-discharge cycle. Degradation usually occurs because electrolyte migrates away from the electrodes or because active material detaches from the electrodes.

Most modern 18650 lithium-ion batteries, which are common for laptop batteries, have a typical cycle life of 300 – 500 (charge, discharge cycles). LFP batteries offer cycle life, which ranges from 2,700 to more than 10,000 cycles depending on conditions.

Depth of Discharge

Depth of discharge is a measure of how much energy has been withdrawn from a battery and is expressed as a percentage of full capacity. For example, a 100 Ah battery from which 40 Ah has been withdrawn has undergone a 40% depth of discharge (DOD).

For lithium-ion batteries, the cycle life of a cell strongly depends on the DOD. The loss of lithium ions and active electrode material is higher for larger DOD cycles. At high DODs, additional degradation mechanisms can occur, resulting in the decomposition and dissolution of cathode material and capacity fading.

Other Types of Batteries

The following list summarizes notable electric battery types composed of one or more electrochemical cells. Four lists are provided in the table. The first list is a battery classification by size and format. Then, the primary (non-rechargeable) and secondary (rechargeable) cell lists are lists of battery chemistry. The third list is a list of battery applications. The final list is a list of different battery voltages.

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