The main performance of lithium-ion power battery
1.1 Charging and discharging characteristics of lithium-ion power battery
The charging of lithium-ion power batteries needs to take into account safety, reliability and charging efficiency. Usually, the charging method of constant current charging first, and then converted to constant voltage charging to a certain small current is adopted. The main differences between different types of lithium-ion power battery charging methods are:
(1) Different types of lithium-ion power batteries have different currents in the constant current charging stage. According to the different cathode materials and manufacturing processes used in lithium-ion power batteries, there are some differences in the optimal charging current, and the general charging current is between 0.2C and 0.3C.
(The proportion of the two-stage charging capacity to the total capacity is different. The extension of the constant current charging time helps to shorten the total charging time and facilitate the practical application of electric vehicles.
The voltage of lithium-ion power battery is stable in the middle of the discharge, and the voltage drops rapidly in the latter part of the discharge. Effective control should be carried out at the later stage of discharge to prevent over-discharge. If overdischarge occurs, not only the lattice structure of the positive electrode material will change, but the negative copper current collector will also be oxidized, which will cause irreversible damage to the battery. Related standards stipulate that when a series of power battery packs are discharged, in order to prevent overdischarge of a certain cell, when the voltage of a certain cell reaches the discharge cut-off voltage, a discharge protection circuit should be used to terminate the discharge of the battery pack.
1.2 Discharge capacity of lithium-ion power battery
“GB/Z183331-2001 Lithium-ion battery for electric road vehicles” stipulates that after the lithium-ion battery is fully charged according to the charging method given by the manufacturer, it is allowed to stand for 1~5h at a temperature of (20±5)℃, and discharge at a current of 113A. The capacity is the rated capacity of the battery. The electrode material, charging voltage, and working temperature of lithium-ion power batteries have important effects on the discharge capacity.
1.2.1 The influence of electrode material on discharge capacity
It can be seen from Figure 1-1 that at different discharge temperatures, the discharge capacity of C/LiCoO2 batteries is greater than that of C/LiMn2O4 batteries. Under the condition of 21℃, the discharge capacity of C/LiCoO2 battery is 18.6% higher than that of C/LiMn2O4 battery, and the average discharge voltage of the latter reaches 3.9V. Therefore, when discussing the energy density of lithium-ion power batteries, batteries with different electrode materials should be treated differently.
Figure 1-1 Discharge capacity of lithium-ion power battery under different conditions
1.2.2 The influence of operating temperature on discharge capacity
The 18650-type C/LiMn2O4 battery and C/LiCoO2 battery were subjected to the discharge test under the condition of -20℃~60℃, and the results obtained are shown in Figure 1-1. It can be seen from Figure 1-1 that the operation of lithium-ion batteries under low temperature conditions is significantly affected, and the average discharge voltage and discharge capacity are significantly smaller; when the operating temperature is higher than 20°C, as the temperature increases, the two types of lithium-ion batteries The average working voltage and discharge capacity of the battery no longer change significantly.
1.2.3 The influence of charging voltage on discharge capacity
By increasing the end-of-charge voltage, the discharge capacity and specific energy of the lithium-ion power battery can be increased. Take a certain type of 7Ah square lithium-ion battery as an example. Charge the battery to 4.1V and 4.2V with 1A current at a temperature of 25°C, and then use different powers for constant power discharge. The discharge capacity is shown in Figure 1-2 and the figure. Shown in Figure 1-3. You can see from it
Figure 1-2 The relationship between the end-of-discharge voltage of a certain lithium-ion power battery and the energy released
After increasing the end-of-charge voltage, the battery discharge energy increases, and the battery discharge energy increases by 25% when the charging voltage is 4.2V at 45W discharge power. At the same time, it can be seen that as the discharge power increases, the energy released by the battery decreases.
When the charge termination voltage of lithium-ion power battery is high, it will cause partial decomposition of the positive electrode material, deterioration of electrolyte performance, and oxidation of the diaphragm, which accelerates the aging process of the battery and shortens the service life of the battery. Therefore, the charge termination voltage must be strictly controlled.
Figure 1-3 The relationship between the end-of-charge voltage of lithium-ion power batteries and the energy released
1.2.4 The influence of discharge current on discharge capacity
Take a 35Ah square lithium-ion power battery as a sample, and use 5 different discharge currents to conduct a discharge test at an ambient temperature of 25°C. The results are shown in Figure 1-4. It can be seen from the curve in Figure 2-5 that as the discharge current increases, the discharge capacity of the square lithium-ion power battery decreases. This is because lithium-ion power batteries use organic electrolytes, and the internal resistance of the battery is greater than that of other types of batteries, so the discharge performance of lithium-ion power batteries under high current conditions is poor.
Figure 1-4 The relationship between discharge current and discharge capacity of a certain type of lithium-ion power battery
1.3 Internal resistance of lithium-ion power battery
Internal resistance is an important parameter of lithium-ion power batteries, an important indicator of battery health, and one of the key data in the research of lithium-ion power batteries. Its value has an important influence on the charging and discharging efficiency of the lithium-ion power battery and the thermal characteristics of the battery.
The internal resistance of lithium-ion power batteries is greatly affected by factors such as state of charge (SOC) and temperature. Under the same conditions, the internal resistance of lithium-ion power batteries is larger than that of power batteries with other structures. For example, the single cell internal resistance of a 10Ah valve-regulated lead-acid battery is 2~3mΩ, while the internal resistance of a lithium-ion power battery cell of 8~10Ah is 10mΩ. The large internal resistance causes the specific energy of the battery to drop rapidly at high power output. There are two main reasons for this:
(1) The cathode materials of lithium-ion power batteries mostly use oxides or salts, and their electronic conductivity is worse than that of metals.
(2) Lithium-ion power batteries use organic materials as electrolyte solvents, and the diffusion rate of lithium ions is also affected by the material lattice.
1.3.1 The influence of temperature on internal resistance
Studies have shown that the internal resistance of lithium-ion power batteries remains basically unchanged in the temperature range of 20°C to 50°C, but the internal resistance increases rapidly in a low temperature environment. At 0°C, the internal resistance at room temperature doubles to -10°C. When the internal resistance increases by more than 2 times. Therefore, the heating and heat preservation of the lithium-ion power battery pack should be strengthened when used in a low-temperature environment.
1.3.2 The influence of SOC on internal resistance
Figure 1-5 and Figure 1-6 show the charge and discharge internal resistance of a certain type of lithium manganese battery and lithium iron phosphate battery in different SOC states measured at a room temperature of 25°C. Through analysis, it can be seen that the internal resistance of the battery increases significantly when the SOC is low, and the internal resistance increases rapidly as the SOC decreases when the SOC is less than 40%. The internal resistance of the battery is the smallest and relatively stable when the SOC is less than 40%, and it has a certain degree of stability. The characteristics of the platform are conducive to working as a vehicle power battery. At the same time, it can be seen that the internal resistance of lithium-ion power battery charging and discharging is not much different. The maximum difference between charge and discharge internal resistance of lithium manganate battery is 4.5%, and the maximum difference between charge and discharge internal resistance of lithium iron phosphate battery is about 5%.
Figure 1-5 The relationship between charge and discharge internal resistance and SOC of a certain type of lithium manganate battery
Figure 1-6 The relationship between charge and discharge internal resistance and SOC of a certain type of lithium iron phosphate battery