Analysis of the current application status of lithium-ion batteries
The rapidly developing automobile industry has brought convenience to people, while its exhaust has become the main culprit of urban air pollution. On the other hand, as non renewable energy sources, the consumption of oil and natural gas is gradually increasing and will eventually dry up in the near future. Energy conservation and environmental protection have become very prominent social issues.
Electric vehicles (EVS) and hybrid electric vehicles (HEVS) are developed to reduce the energy consumption and pollution caused by fuel vehicles. The advanced power batteries currently used in electric vehicles mainly include nickel hydrogen power batteries and lithium-ion power batteries. Lithium ion batteries have high specific energy, long lifespan, and high specific power. With the reduction of cost, they will gradually replace the current nickel hydrogen batteries as the main energy storage system for electric vehicles. The economic viability of HEVs and EVs in market competition is a crucial aspect. Real time and accurate estimation of battery state of charge (SOC) has a significant impact on the economic viability of hybrid vehicles.
The positive electrode materials of lithium-ion power batteries mainly include lithium manganese oxide, lithium iron phosphate, and nickel cobalt lithium manganese oxide ternary materials. Compared with the other two positive electrode materials, ternary materials have higher platform voltage, specific capacity, and compaction density, which determine the energy density of batteries using this positive electrode material. If the safety performance of ternary lithium-ion batteries can be improved to a certain extent, they will have great application space in electric vehicles and power energy storage systems.
2 Test Objects and Testing Platforms
Test object: cylindrical 18650 ternary lithium-ion power battery. Obtain its basic characteristics. The basic parameters of the single cell battery are: nominal capacity 20Ah, nominal voltage 37V, and mass 41g.
The battery cell and module testing platform can simulate vehicle operating conditions, obtain experimental data during module charging and discharging, and verify the effectiveness of module structure design and BMS design.
The testing platform is divided into 5 parts: battery charging and discharging equipment, constant temperature chamber, battery data acquisition card, battery data recording and analysis and control computer, and battery module to be tested. The computer obtains and records battery information (voltage, temperature, current) through data interaction with the data acquisition card. The data analysis software embedded in the battery model analyzes the battery status and corresponding charging methods, and controls the output of the charging and discharging motor accordingly. The constant temperature box provides temperature field conditions for battery testing, and the charging and discharging motor provides power source and load for the battery.
3 SOC and Open Circuit Voltage Testing
3.1 Single Battery Testing
Battery open circuit voltage (Open circuit voltage, OCV can be used for calibration of battery state of charge estimation, which plays an important role in improving the accuracy of SOC estimation. Nine cylindrical 18650 power lithium-ion single cell batteries were selected for SOC and OCV testing. The testing process is: the lithium-ion battery is fully charged and placed stationary until the battery is stable; discharge at a constant current to ensure that the SOC corresponding to each discharge is 005, and the interval between two discharges is 1 hour to ensure that the battery reaches a stable state. At the same time Record the open circuit voltage of the battery pack. This results in the open circuit voltage of 21 SOC states with SOC ranging from 1, 0.95, 0.90, 0.85, 0.80,…, 0.10, 0.05, and 0.
It can be seen from the experiment that the relationship between the 18650 monomer SOC and OCV is approximately linear with good consistency. Therefore, the open circuit voltage can be used to estimate the state of charge of the battery. Especially after the electric vehicle has been parked for a period of time, the open circuit voltage has a good effect on estimating the initial state of charge of the power battery.
3.2 Battery Pack Testing
18650 cells are selected to form two groups through 5 parallel 10 strings for testing the relationship between the state of charge and open circuit voltage. The testing process is consistent with that of single battery testing. Due to the good consistency of the monomers and the selection of combinations, the battery pack can also maintain good consistency.
4 Capacity of Single Battery
Due to the fact that electric vehicles require the power battery to provide a relatively high specific power during hill climbing, starting, and acceleration, that is, sufficient current discharge to provide sufficient power. This article conducted discharge tests on lithium-ion batteries at 0IC, 0.5C, and 1C rates
From the experimental results, it can be seen that as the discharge current of the battery increases, the discharge capacity of the battery decreases. This is because as the discharge current increases, the concentration polarization inside the battery increases, and the voltage drop caused by the inherent internal resistance of the battery also increases, resulting in a corresponding decrease in the discharge capacity of the battery.
5 Battery Capacity
Due to the significant changes in the working conditions of automobiles, this article conducted discharge tests on lithium-ion batteries at temperatures of -30, -20, -10, 0, 10, 30, 45, and 55 ℃ at 0.5 ℃,. From the experimental results, it can be seen that as the working temperature of the battery increases, the discharge capacity significantly increases. This is due to the slow diffusion rate of lithium ions within the battery at low temperatures, which enhances the battery’s activity as the temperature increases. However, as the insulation time increases, the overall temperature of the battery increases, causing ion activity to gradually become disordered, leading to an increase in internal resistance and a decrease in discharge capacity changes.
This article investigates the relationship between the state of charge and open circuit voltage, discharge rate and capacity, capacity and temperature of ternary lithium batteries. The law of charge state and open circuit voltage, the law of monomer capacity under different discharge rates, and the law of capacity and temperature are obtained.
1) The consistency of the tested cylindrical 18650 lithium battery cells is good, and the state of charge is approximately linearly related to the open circuit voltage. The open circuit voltage can be used to estimate the state of charge. Using open-circuit voltage to estimate the state of charge during the shutdown time of the power battery, and using other estimation methods during charging and discharging can improve the accuracy of SOC estimation.
2) As the discharge current increases, the concentration polarization inside the battery increases, and the voltage drop caused by the inherent internal resistance of the lithium battery also increases. Therefore, as the discharge rate increases, the discharge capacity of the battery decreases.
3) At low temperatures, the diffusion rate of lithium ions within the battery is slower, and as the temperature increases, the battery activity increases and the discharge capacity increases.