What is a 18650 Lithium Battery?

Ⅰ Introduction

18650 lithium battery (Li-ion, Lithium Ion Battery): 18650 lithium battery has the advantages of light weight, large capacity, no memory effect, etc., so it has been widely used. Many digital devices use lithium ion batteries as power sources, despite its price is relatively expensive. Lithium battery has a high energy density, its capacity is 1.5 to 2 times that of a nickel-hydrogen battery of the same weight, and it has a very low self-discharge rate. In addition, lithium-ion batteries have almost no "memory effect" and do not contain toxic substances. In addition, please note that lithium batteries are generally marked with 4.2V lithium battery or 4.2V lithium secondary battery, 4.2V lithium ion rechargeable battery.

Lithium battery standard 3.7V or 4.2V is the same. It's just that the manufacturer's label is different. 3.7V refers to the platform voltage (ie, typical voltage) discharged during battery use, while 4.2V refers to the voltage when the battery is fully charged. The voltage common rechargeable 18650 lithium batteries is 3.6 or 3.7v. When fully charged is 4.2v, which has little to do with the power (capacity). The mainstream capacity of 18650 batteries is from 1800mAh to 2600mAh, (18650 power battery capacity Mostly 2200~2600mAh).

18650 Lithium Battery

It is generally believed that when the no-load voltage of the lithium battery is lower than 3.0V, it is considered to be exhausted (the specific value depends on the threshold of the battery protection board, such as low as 2.8V and 3.2V). Most lithium batteries cannot be discharged with no-load voltage below 3.2V, otherwise over-discharge will damage the battery. Generally, lithium batteries on the market are basically used with a protection board, so over-discharge will also cause the protection board to fail to detect the battery, thus unable to charge the battery.

4.2V is the highest limit voltage for battery charging. It is considered that the battery is fully charged when the no-load voltage of the lithium battery is charged to 4.2V. During the battery charging process, the battery voltage gradually rises from 3.7V to 4.2V, and the lithium battery cannot be charged. Charge the no-load voltage above 4.2V, otherwise, it will damage the battery. This is the special feature of lithium batteries.

Ⅱ Uses of 18650 Lithium battery

The 18650 battery life theory is 1000 cycles of charging. Because of the large capacity per unit density, most of them are used in laptop batteries. In addition, because the 18650 has very good stability at work, it is widely used in various electronic fields: often used in high-end glare flashlights and portable power supply, wireless data transmitter, electric heating warm clothes, shoes, portable instruments, portable lighting equipment, portable printers, industrial instruments, medical instruments, etc.

Ⅲ Principle of charge and discharge

The working principle of a lithium-ion battery refers to its charging and discharging principle. When the battery is charged, lithium ions are generated on the positive electrode of the battery, and the generated lithium ions move to the negative electrode through the electrolyte. The carbon as the negative electrode has a layered structure. It has many micropores. The lithium ions that reach the negative electrode are embedded in the micropores of the carbon layer. The more lithium ions are inserted, the higher the charging capacity.

18650 Lithium Battery

In the same way, when the battery is discharged (that is, the process when we use the battery), the lithium ions embedded in the carbon layer of the negative electrode are released and move back to the positive electrode. The more lithium ions that return to the positive electrode, the higher the discharge capacity. What we usually call battery capacity refers to the discharge capacity.

It is not difficult to see that during the charging and discharging process of lithium-ion batteries, lithium ions are in a state of movement from positive → negative → positive. If we compare the lithium-ion battery to a rocking chair, the two ends of the rocking chair are the two poles of the battery, and the lithium ion is like an excellent athlete running back and forth on both ends of the rocking chair.

Charging and discharging process

Lithium battery charging control is divided into two stages. The first stage is constant current charging. When the battery voltage is lower than 4.2V, the charger will charge with a constant current. The second stage is the constant voltage charging stage. When the battery voltage reaches 4.2V, due to the characteristics of lithium batteries, if the voltage is higher, it will be damaged. The charger will fix the voltage at 4.2V and the charging current will gradually decrease. When it is reduced to a certain value (generally 1/10 of the set current), the charging circuit is cut off, the charging completion indicator light is on, and the charging is completed.

Excessive charging and discharging of lithium-ion batteries can cause permanent damage to the positive and negative electrodes. The excessive discharge causes the negative carbon sheet structure to collapse, and the collapse will cause the lithium ions to be unable to be inserted during the charging process; overcharging causes too many lithium ions to be inserted into the negative carbon structure, causing some of the lithium ions to be unable to be released anymore.

Some chargers are implemented using cheap solutions, and the control accuracy is not good enough, which can easily cause abnormal battery charging and even damage the battery. When choosing a charger, try to choose a big brand of 18650 lithium-ion battery charger. The brand-guaranteed 18650 lithium-ion battery charger has four protections: short-circuit protection, over-current protection, over-voltage protection, battery reverse connection protection, etc.

Overcharge protection: When the charger overcharges the lithium-ion battery, in order to prevent the internal pressure from rising due to temperature rise, the charging state must be terminated. For this reason, the protection device needs to monitor the battery voltage, and when it reaches the battery overcharge voltage, it activates the overcharge protection function and stops charging.

Over-discharge protection: To prevent the over-discharge of the lithium-ion battery, when the voltage of the lithium-ion battery is lower than its over-discharge voltage detection point, the over-discharge protection is activated, the discharge is stopped, and the battery is kept in a low quiescent current standby mode.

Over-current and short-circuit protection: When the discharge current of the lithium-ion battery is too large or a short-circuit condition occurs, the protection device will activate the over-current protection function.

Ⅳ General configuration

The nominal voltage of a single cell is generally: 3.6V or 3.7V

The charging voltage is generally: 4.20V (4.2V-4.3V for lithium cobalt oxide)

The minimum discharge termination voltage is generally: 2.75V, lower than this voltage will easily lead to a serious drop in battery capacity or even scrap

Maximum charge termination voltage: 4.20V

Diameter: 18±0.2mm

Height: 65±2.0mm

Capacity: above 1000mAh, the conventional capacity is 2200mAh-3200mAh, the 18650 battery capacity is the highest is LG, which can achieve 3600mAh, but the price is not low.

Ⅴ Advantages and Disadvantages

Advantages

1. Large capacity. The capacity of 18650 lithium battery is generally between 1200mah and 3600mah, while the general battery capacity is only about 800mah. If combined into a 18650 lithium battery pack, the 18650 lithium battery pack can easily exceed 5000mah.

2. Long life. The 18650 lithium battery has a long life, and the cycle life can reach more than 500 times in normal use, which is more than twice that of ordinary batteries.

3. High safety performance. The 18650  lithium battery has high safety performance. In order to prevent the battery from short-circuiting, the positive and negative electrodes of the 18650  lithium battery are separated. Therefore, the short-circuit phenomenon may have been reduced to the extreme. A protection board can be installed to avoid overcharge and over-discharge of the battery, which can also extend the service life of the battery.

4. High voltage. The voltage of the 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 polymer batteries is smaller than that of ordinary liquid batteries. The internal resistance of polymer batteries can even be below 35mΩ, which greatly reduces the self-consumption of the battery and prolongs the standby time of the mobile phone. This kind of polymer lithium battery that supports a large discharge current is an ideal choice for remote control models, and it has become the most promising product to replace  Ni-MH batteries.

7. It can be combined in series or parallel to form a 18650 lithium battery pack.

8. Wide range of use: Notebook computers, walkie-talkies, portable DVDs, instrumentation, audio equipment, model airplanes, toys, camcorders, digital cameras, and other electronic equipment.

Disadvantages

The biggest disadvantage of the 18650 lithium battery is that its size has been fixed, and it is not well-positioned when installed in some notebooks or some products. Of course, this disadvantage can also be said to be an advantage. This is compared with other lithium batteries such as polymer lithium batteries. This is a disadvantage in terms of customizable and transformable size. Compared to some products with specified battery specifications, it has become an advantage.

The production of 18650 lithium 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 disadvantage of lithium batteries, because the materials used in lithium batteries are basically lithium cobalt oxide materials, and lithium cobalt oxide materials cannot discharge at high currents.

The 18650 lithium battery requires high production conditions. Compared with the general battery production, the 18650 lithium battery requires high production conditions, which undoubtedly increases the production cost.