Charge and discharge of 18650 lithium ion battery

According to industrial standards, for 18650 lithium ion battery battery, the nominal capacity is generally the minimum capacity. This capacity is a batch of batteries that are fully charged with cc/cv0.5c at room temperature of 25 degrees, and left for a period of time ( Usually 12 hours). Discharge to 3.0v with a constant current of 0.2c (there is also a standard of 2.75V, but the effect is not large, and the capacity value released from 3v to 2.75v drops quickly without much capacity), because a batch of batteries must have individual The difference is the capacity value of the battery with the lowest capacity actually released. That is to say, the actual capacity of any battery in this batch should be greater than or equal to the nominal capacity.

1. The charging process of 18650 lithium ion battery

Some chargers are implemented with cheap solutions, and the control accuracy is not good enough, which may easily cause abnormal charging of the battery and even damage the battery. When purchasing a charger, try to choose a big brand 18650 lithium ion battery charger. The quality and after-sales are guaranteed to prolong the service life of the battery. The brand-guaranteed 18650 lithium ion battery charger has four protections: short-circuit protection, over-current protection, over-voltage protection, battery reverse polarity 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 needs to be terminated.

To this end, 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: In order to prevent the over-discharge state 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 terminated, and the battery is kept in a standby mode with low quiescent current. Over-current and short circuit protection: When the discharge current of the lithium-ion battery is too large or a short circuit occurs, the protection device will activate the over-current protection function.

The 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 18650 lithium battery, 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 (usually 1/10 of the set current), the charging circuit is cut off, the charging completion indicator light is on, and the charging is completed.

Overcharging and discharging Li-ion batteries can cause permanent damage to the positive and negative electrodes. Overdischarge leads to the collapse of the negative electrode carbon sheet structure, and the collapse will prevent lithium ions from being inserted during the charging process; overcharge causes too many lithium ions to intercalate into the negative electrode carbon structure, and some of the lithium ions can no longer be released.

2. The charging principle of 18650 lithium ion battery

The working principle of 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, and it has many micropores. The lithium ions reaching the negative electrode are embedded in the micropores of the carbon layer. The more lithium ions embedded, the higher the charging capacity.

In the same way, when the battery is discharged (that is, the process of using the battery), the lithium ions embedded in the carbon layer of the negative electrode come out 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 moving state from positive electrode → negative electrode → positive electrode. 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 at the two ends of the rocking chair. Therefore, the experts gave the lithium-ion battery a lovely name rocking chair battery.

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