14 Knowledge About Lithium-Ion Batteries
1. How is the safety feature of lithium ion achieved?
⑴ The diaphragm is automatically shut down at 135℃, using the internationally advanced Celgars2300PE-PP-PE three-layer composite membrane. When the battery temperature reaches 120℃, the membrane pores on both sides of the PE composite membrane are closed, the battery internal resistance increases, and the internal temperature rise of the battery slows down. When the battery temperature reaches 135℃, the PP membrane pores are closed, the internal circuit of the battery is short-circuited, and the battery no longer heats up, ensuring the safety and reliability of the battery.
⑵ Adding additives to the electrolyte When the battery is overcharged and the battery voltage is higher than 4.2V, the electrolyte additives polymerize with other substances in the electrolyte, the internal resistance of the battery increases significantly, a large area of short circuit is formed inside the battery, and the battery no longer heats up.
⑶ Composite structure of battery cover The battery cover adopts a notched explosion-proof ball structure. When the battery heats up, part of the gas generated during the activation process inside the battery expands, the internal pressure of the battery increases, and the pressure reaches a certain level. The notch ruptures and deflates.
⑷ Various environmental abuse tests Various abuse tests are carried out, such as external short circuit, overcharge, puncture, impact, burning, etc., to examine the safety performance of the battery. At the same time, the battery is subjected to temperature shock tests and mechanical performance tests such as vibration, drop, and impact to examine the performance of the battery in the actual use environment.
2. Why does the constant voltage charging current gradually decrease?
Because when the constant current process ends, the electrochemical polarization inside the battery remains at the same level throughout the constant current. During the constant voltage process, under the action of the constant electric field, the internal Li+ concentration polarization is gradually eliminated, and the migration number and speed of the ions are manifested as a gradual decrease in current.
3. What is the capacity of the battery?
The capacity of the battery is divided into rated capacity and actual capacity.
The rated capacity of the battery refers to the minimum amount of electricity that should be discharged under certain discharge conditions specified or guaranteed when the battery is designed and manufactured. Li-ion specifies that the battery is charged for 3 hours at room temperature, constant current (1C) and constant voltage (4.2V) controlled charging conditions. The actual capacity of the battery refers to the actual amount of electricity released by the battery under certain discharge conditions, which is mainly affected by the discharge rate and temperature (so strictly speaking, the battery capacity should indicate the charging and discharging conditions). Common units of capacity are: mAh, 1Ah=1000mAh).
4. What is the internal resistance of the battery?
It refers to the resistance encountered by the current flowing through the battery when the battery is working. It consists of two parts: ohmic internal resistance and polarization internal resistance. A large internal resistance of the battery will cause the battery discharge working voltage to decrease and the discharge time to shorten. The size of the internal resistance is mainly affected by factors such as the battery material, manufacturing process, and battery structure. It is an important parameter for measuring battery performance.
Note: Generally, the internal resistance in the charged state is used as the standard. The internal resistance of the battery needs to be measured with a special internal resistance tester, and cannot be measured with the ohm range of a multimeter.
5. What is open circuit voltage?
The open circuit voltage of a lithium battery refers to the potential difference between the positive and negative electrodes of the battery when the battery is not connected to any load. The open circuit voltage is about 4.1-4.2V after full charge, and about 3.0V after discharge. The battery's state of charge can be judged by the open circuit voltage of the battery. What is the working voltage? The discharge working voltage is about 3.6V.
6. Internal air pressure of the battery?
The internal air pressure of the battery is caused by the gas generated by the sealed battery during the charging and discharging process, which is mainly affected by factors such as battery materials, manufacturing processes, and battery structure. The main reason for its generation is that the gas generated by the decomposition of water and organic solutions inside the battery accumulates in the battery. Continuous overcharging at a high rate will cause the battery temperature to rise and the internal pressure to increase. In severe cases, it will have a destructive effect on the performance and appearance of the battery, such as leakage, bulging bottom, increased internal resistance of the battery, and shortened discharge time and cycle life. Any form of overcharging of Li-ion will cause serious damage to the battery performance or even explosion. Li-ion needs to adopt constant current and constant voltage charging method during charging to avoid overcharging the battery.
7. Why do batteries have to be stored for a period of time before they can be packaged and shipped?
The storage performance of the battery is an important parameter to measure the stability of the battery's comprehensive performance. After a certain period of storage, the battery's capacity and internal resistance are allowed to change to a certain extent. After a period of storage, the electrochemical properties of the internal components can be stabilized, and the self-discharge performance of the battery can be understood to ensure the quality of the battery.
8. Why is formation necessary?
After the battery is manufactured, the internal positive and negative electrode materials are activated through a certain charging and discharging method to improve the battery's charging and discharging performance and self-discharging, storage and other comprehensive performance. The process is called formation. The battery powder can only show its true performance after formation.
9. What is capacity grading?
During the manufacturing process of the battery, due to process reasons, the actual capacity of the battery cannot be completely consistent. Through a certain charging and discharging system, the process of testing and classifying the battery by capacity is called capacity grading.
10. What is voltage drop?
The battery is charged to more than 80% according to the quality standard, and its no-load voltage is measured. The 5W/2W battery is used as a load to connect the positive and negative ends of the battery. The switch is used as the disconnection of the battery, and the devices of the path are connected in series. After turning on the switch, the voltage drops by no more than 0.4V in 5 seconds, which is qualified. It is mainly used to test the battery load performance.
11. What is static resistance? What is dynamic resistance?
⑴ Static resistance is the internal resistance of the battery during discharge.
⑵ Dynamic resistance is the internal resistance of the battery during charging.
12. What welding methods are used for positive and negative electrodes?
⑴ Ultrasonic spot welding machine is used for the positive electrode.
⑵ Pulse spot welding machine is used for the negative electrode.
13. The purpose and position of adhesive tape on battery core?
⑴ The position of the battery core adhesive tape does not deform after the battery core is wound. The bottom adhesive tape prevents the bottom of the positive electrode sheet in the battery core from contacting the battery shell and causing a short circuit.
⑵ The side adhesive tape prevents the battery core from deforming after winding. The bottom adhesive tape prevents the bottom of the positive electrode sheet in the battery core from contacting the battery shell and causing a short circuit.
14. What are the humidity requirements for the drying room? Why should the battery be weighed before filling?
⑴ Relative humidity is below 6%.
⑵ Weigh to calculate the exact amount of liquid to be injected.
