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All Things You Need To Know About 18650

What’s the 18650 battery cell?

The 18650 battery is probably the most versatile among Lithium ion batteries, powering applications from torchlights to Tesla cars. Having been launched in the 1990s, it is a veteran among Lithium ion batteries. By international battery nomenclature, the number 18650 represents its shape and dimensions. Thus, the18650 is a cylindrical battery of 18 mm diameter and 65 mm length.

Who started this battery model?

The first company to start commercial production of the 18650 battery was Panasonic. From their corporate history webpage, you can see that the 18650  battery was developed in 1994, in response to market demand for smaller and lighter rechargeable batteries. Compared to disposable batteries, these Lithium ion batteries delivered superior performance due to high voltage, higher energy density and capacity. The 18650 batteries proved to be a hit and many other companies started manufacturing this model. Over the years, Panasonic launched newer versions of the 18650, with improved capacities and energy densities. Competitors like Samsung and LG Chem as well as newcomers like Enpower and NanoGraf joined the fray, introducing their versions of the 18650 battery. Figure 1 compares the energy densities of 18650 batteries made by various manufacturers.

Figure 1 Energy Density of 18650 Batteries Made By Various Manufacturers

18650 vs AA/21700/26650/18500/14500

Prior to the 18650 model, the battery of choice for small electrical and electronic devices was the AA size battery. Conceived before the first world war, this size is currently available in a variety of disposable and rechargeable models, including Nickel and Lithium ion types. Disposable AA batteries based on Zinc Carbon or Alkaline chemistry are in high demand for use in flashlights, cameras, clocks, calculators, toys, and a host of other daily use gadgets.

During the 1990s, the Lithium ion battery manufacturers adopted numeric nomenclature for battery sizes. Many models that were larger as well as smaller than the 18650 were launched, to capitalize on the boom in the personal electronics, power tools and the electric vehicle markets. The various Lithium ion batteries which emerged at that time included 21700/26650/18500/14500. Several of these sizes have been constantly upgraded over the years. The Panasonic 18650  alone comes in a wide range of models. Due to overlap in their market segments, It is interesting to compare the 18650 with the other Lithium ion batteries, as shown in the following table:

 

Name Chemistry Types Dimensions

(Dia x length)

Nominal Voltage Capacity Cycle Life
 AA

 

Disposable: Zinc Carbon, Alkaline 14.5 mm x 15 mm 1.5 400 to 2850 mAh Not applicable
Rechargeable:

Nickel Cadmium, Nickel Metal Hydride, Nickel Zinc, Lithium ion.

 

14.5 mm x 15 mm 1.2 to 3.6 600 to 2850 mAh 500 to 1000 cycles
18650

 

Lithium Ion 18 mm x 65 mm 3.6 2000 to 3600 mAh 300 to 500
21700

 

Lithium Ion 21 mm x 70 mm 3.6 3000 to 5000 mAh 500 to 2000
26650

 

Lithium Ion 26 mm x 65 mm 3.6 5000 mAh 1500
18500 Lithium Ion 18 mm x 50 mm 3.6 1500 to 2200 mAh 300
14500

 

Lithium Ion 14 mm x 50 mm 3.6 600 to 1600 mAh 300 to 500

 

18650 Common Applications

The high capacity and energy density of the 18650 batteries relative to their size, makes them suitable for compact devices as well a portable storage and transportation power packs. Hence the applications of the 18650 batteries are spread across mobility, communications, computing, power storage, motorized gadgets and toys. A representative list is as follows:

  • Electric Cars
  • Electric Bicycles
  • Battery driven power tools
  • Drones
  • Mobile Phones, Tablets, and Laptops
  • Electric Toys
  • MP3/MP4 Players and headphones.
  • Power Banks.
  • Toys and household gadgets.

18650 Battery Manufacturing Process

The conventional manufacturing process for Lithium Ion batteries can be divided in to 3 stages, namely:

  • Electrode preparation
  • Cell assembly
  • Battery electrochemistry activation

Each of these stages occurs in several steps which are illustrated in Figure 2  and described below:

Slurry Mixing

This is the first step of electrode preparation. Here, the active ingredients for each electrode, along with additives, binders and suitable solvents are mixed to form uniform slurries. In the case of the 18650 batteries, the Cathode ingredients are Lithium Oxide, Nickel, Manganese and Cobalt, dissolved in an organic solvent. The Anode coating material is Graphite dissolved in aqueous medium. Typically, planetary mixers are used to prepare uniform slurries for large scale production

Coating

Each slurry is then pumped to a coating machine, where Anode and Cathode current collectors are coated with their respective active ingredients. The Cathode collector is an Aluminium strip whereas the Anode collector is a Copper strip. Both sides of each strip are coated. Typically, the Aluminium strip is 20 microns thick, while the copper foil is 10 microns thick. The typical Cathode and Anod thicknesses after double side coating are 125 microns and 126 microns respectively. The coating thicknesses depend on the electrode mixture composition.

Drying

After coating, the electrodes strips are wet, so they need to be dried. The wet strips are fed by a roller system into a dryer. Here the solvents are evaporated and conveyed to solvent recovery section. Note that this drying operation is to evaporate the solvents The main vacuum drying operation comes later.

Solvent Recovery

The expensive and toxic organic solvent is recovered from the dryer vapours by condensation followed by distillation. After this step, the dry, coated electrode strips are sent for calendaring.

Calendering

Calendering is a compaction process in which the coated strips are pressed between rollers at a prescribed temperature and pressure. This is done to ensure uniform pore distribution and increase the adhesion strength between the electrode coating materials and the metallic current collectors.

Slitting

After Calendering, the electrode strips are cut to the prescribed cell width in a slitting machine.  Conventional slitting machines are equipped with adjustable blades or chisels for this purpose.

Figure 2 Typical Manufacturing Process For 18650 Lithium Ion Battery
Vacuum drying

The Conventional drying process is done under vacuum at 60°C–150°C heating for over twelve hours with the option of inert gas supply. Removal of moisture ensures that side reactions and corrosion are minimized. Hence the moisture levels of the electrodes are verified after drying. With this step, the electrodes are ready for the next stage which is Cell assembly.

Winding

The cylindrical multilayer formation of the 18650 battery is accomplished in this step. The Anode and Cathode strips along with the intervening separator strip are wound in a special machine which maintain precise winding tensions and foil alignments. If the winding tension is small, it will affect the internal resistance and shell entry rate. Too much tension is easy to causes short circuit or electrode fracture risk.

Figure 3 The 18650 Battery After Winding And Tab Welding

Welding

In this step, metallic strips called Tabs are welded to the Anode and cathode rolls. These are the junctions where electricity flows between the external circuit and the battery electrodes. This is a challenging step, since damage due to welding can damage the cell. Resistance welding is widely practiced. Ultrasonic welding, which is much costlier, is also used by some manufacturers. Figure 3 depicts a typical 18650 cell, after winding and tab welding.

Enclosing

After winding and tab welding, each cylindrical roll is placed its steel casing. The Casing is filled with electrolytes and sealed. The electrolyte injection process is done under a vacuum in an inert gas atmosphere. The Oxygen concentration must be less than 10 ppm to prevent electrode oxidation. The casing is then sealed by laser welding, to complete the cell fabrication.

Formation/Aging

Though the cells have been assembled, they are not yet ready to be shipped. This is because their electrodes must be activated. They must undergo multiple low rate charge and discharge cycles, to form protective films on the electrode surfaces. The film, called an SEI layer is formed by electrolyte decomposition and deposition on the electrode surface. This step is called the formation step. Some gas is generated which has to be removed. After completing the formation cycles, the cells are stored on aging shelves for complete electrolyte wetting and SEI stabilization. After aging and final degassing, the cells are ready to be packed and shipped. Quality checks are performed before packing, to ensure that the products meet the required specifications.

How To Test The Quality Of The 18650?

Routine quality tests to verify the nominal battery specifications provided on the battery data sheets are described below. Specific applications such as electric vehicles will need many more stringent tests. Randomized samples are selected for testing from each batch.

  • Visual Check:  Absence of defects such as deep scratch, crack, rust, discoloration or leakage.
  • Dimension Check: Accuracy of diameter and length to be checked.
  • Battery Energy (Power) test: This is measured by standard charge and discharge tests. The standard charge involves charging at constant current of 0.5C and also by charging at constant voltage of 4.2V with tapering current, finally stopping at 50 mA. The standard discharge is involves discharging at a constant current of 0.2C, till a voltage of 2.50V is reached.
  • High Drain Charge/Discharge test: The charging rate in this case is at constant current of 0.5C till voltage of 4.20V, and end current of 50mA. Cells are then discharged at constant current of 0.5C till a low voltage of 2.50V. Cells are rested for 10 minutes after charge and 20 minutes after discharge.
  • Cycle Life: Cells are charged and discharged as per the high drain test, for 500 cycles. A cycle is defined as one charge and one discharge. The 501stdischarge power should be greater than 70 percent of the initial battery power.
  • Storage test:Cells are charged at standard rate and stored in a temperature-controlled environment at 23ºC ± 2ºC for 30 days. After storage, cells are discharged at the standard rate. The residual power should be greater than 90 percent of initial power.
  • High Temperature test: Cells are charged at standard rate and stored in a temperature-controlled environment at 60ºC for 1 week. After storage, the cells are discharged at standard rate and then cycled for 3 cycles to obtain recovered power. The power recovery should be greater than 80 percent.
  • Drop test: Cells are charged at standard rate and are dropped onto a wooden floor from 1.0 meter height. A total of 3 drops are performed, comprising 2 drops from each cell terminal and 1 drop from the side of cell casing. There should not be any leakage or temperature rise.

18650 NMC Battery Safety

Many of us would be using these batteries for our portable electrical and electronic devices or household gadgets and toys. Since Lithium ion batteries can catch fire due to runaway chemical reactions, the following general safety precautions need to be kept in mind.

  • Do not use visibly damaged batteries
  • Check that positive and negative terminals are properly connected in the charger.
  • Ensure that the battery is stored in a dry area away from heat or direct sunlight.
  • Keep the battery away from children or pets. Especially be careful with toys, do not let children play unsupervised with battery operated toys.
  • Use only the recommended charger.
  • Do not mix different specifications or old and new batteries your device.
  • Do not short circuit the battery with a wire or any conducting material.
  • Always buy Protected Lithium-Ion) batteries. These have inbuilt protection against common. dangers, such as overcharge, over discharge, short circuit/over current, and temperature.

How To Store The 18650 Cells

When 18650 batteries have to be stored in bulk or large stationary power supplies, the biggest risk is uncontrolled fire. Typical precautions to mitigate the fire risk are as follows:

  • Perform Frequent visual inspections of the batteries for signs of damage
  • Storage rooms must be dry, cool, well-ventilated and free from high levels of humidity.
  • Do not store flammable or combustible materials nearby.
  • Keep sharp objects and conductive materials away from batteries.
  • Ensuring that staff are fully trained on the emergency procedures
  • Staff should be aware of special precautions in dealing with fires involving Lithium-ion batteries.
  • Proprietary metal battery storage cabinets or fireproof safety bags are available and should be used.
  • The storage area must have suitable smoke detection system which provides adequate warning to other occupants of the building
  • Limit the size of storage areas, and do not store anything else in the same area.
  • Provide a suitable fixed extinguishing agent flooding system.
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