1. Basic working principle of lithium-ion batteries:
Lithium-ion batteries store and release energy through the movement of lithium ions between the positive and negative electrodes. During the charging process, the external power source forces the lithium ions to move from the positive electrode to the negative electrode and store them in the negative electrode material. During the discharge process, the lithium ions are released from the negative electrode and move to the positive electrode, releasing energy for the device to use.
2. Voltage change: At the beginning of charging, due to the existence of internal resistance and polarization effect of the battery, the voltage will be relatively low. As charging proceeds, the chemical reaction inside the battery gradually tends to equilibrium, and the voltage gradually increases until the charging cut-off voltage is reached. Taking the ternary lithium battery as an example, its charging process can be divided into four stages: trickle charging (low-voltage pre-charging), constant current charging, constant voltage charging, and charging termination. In the trickle charging stage, if the battery voltage is lower than 3V, pre-charging will be performed first. At this time, the charging current is 1/10 of the set current, and the voltage rises slowly. When the battery voltage rises above the trickle charging threshold, it enters the constant current charging stage. At this time, the charging current is increased, and the battery voltage gradually increases with the constant current charging process. Generally, this voltage set for a single battery is 3.0~4.2V. When the battery voltage rises to 4.2V, the constant current charging ends and the constant voltage charging stage begins. At this time, the charging voltage is kept at 4.2V and the charging current gradually decreases. When the current drops to 1/10 of the set charging current, the charging ends. During the discharge process, the voltage change trend is opposite to that of the charging process. At the beginning of discharge, the voltage is high. As lithium ions are consumed and the internal resistance of the battery is affected, the voltage gradually decreases until it reaches the discharge cut-off voltage. During discharge, the voltage curve of a lithium-ion battery can be divided into three stages. In the initial stage, the terminal voltage drops rapidly. The greater the discharge rate, the faster the voltage drops. Then, the battery voltage enters a slowly changing stage, which is called the platform area of the battery. The smaller the discharge rate, the longer the platform area lasts, the higher the platform voltage, and the slower the voltage drops. Finally, when the battery is nearly discharged, the battery load voltage begins to drop sharply until it reaches the discharge cut-off voltage.
3. Reasons for voltage fluctuations:
3.1. Internal resistance of the battery: The battery will show a certain internal resistance during the charging and discharging process, which will cause the voltage to decrease.
3.2. Polarization effect: During the charging and discharging process, the positive and negative electrodes of the battery will polarize, that is, the charge distribution on the electrode surface is uneven, resulting in voltage changes.
3.3. Chemical reaction kinetics: The chemical reaction rate inside the battery will also affect the change in voltage. The faster the reaction rate, the faster the voltage changes; conversely, the slower.
During the charging and discharging process of lithium-ion batteries, due to the influence of factors such as the internal resistance of the battery, polarization effect, and chemical reaction kinetics, its voltage will change. This change is part of the normal operation of the battery and is also one of the important indicators for evaluating battery performance and health status.
4. What is the impact of voltage changes on performance of lithium-ion batteries?
4.1. The relationship between voltage and capacity: Generally speaking, the capacity of a battery is proportional to its voltage. This means that as the voltage increases, the capacity of the battery will increase accordingly, allowing it to store more energy. Therefore, high-voltage lithium-ion batteries usually have a longer service life.
4.2. The relationship between voltage and discharge curve: The discharge curve is a curve showing the voltage change over time during the use of a lithium-ion battery. Different voltages affect the shape and slope of the discharge curve. Generally speaking, the discharge curve of a lithium battery shows a steady downward trend, but the shape and rate of decline of the curve will vary with different voltages. This is very important for the normal use of electronic devices and the accurate display of battery power.
4.3. The relationship between voltage and charging and discharging speed: Lithium-ion batteries with higher voltages are able to charge and discharge faster. This means that if we use high-voltage lithium batteries, we can fully charge electronic devices faster and be able to use them for longer. However, excessive voltage may also cause battery overheating and damage, so a trade-off needs to be made during design and use.
4.4. The relationship between voltage and safety: Overcharging and overdischarging of lithium-ion batteries will affect their safety performance and even cause serious accidents such as fire. The voltage limit of the battery is one of the important factors that need to be considered when designing a charging control circuit to ensure the safe use of the battery. When the battery voltage exceeds the normal range, the battery’s protection mechanism, such as power-off or short-circuit protection, may be triggered to prevent battery damage or safety accidents.
4.5. Other effects of voltage changes: Lithium-ion batteries that are in a low voltage state for a long time may accelerate the process of harmful chemical reactions inside the battery, such as the decomposition of the electrolyte and the shedding of active substances, which will lead to irreversible decay of the battery capacity. Low voltage may also cause microscopic changes in the internal structure of the battery, such as uneven expansion and contraction of electrode materials, further aggravating battery aging and performance degradation. The voltage change of lithium-ion batteries has a significant impact on their performance. Therefore, when selecting and using lithium-ion batteries, it is necessary to fully consider the voltage factor and select the appropriate battery type and specifications according to actual needs and application scenarios. At the same time, it is also necessary to pay attention to the voltage status of the battery during use to ensure the safety and stability of the battery.
