What is the life test and analysis of lithium-ion power battery system?
Electric vehicles are most concerned about the life of the entire battery system, not just the life of the single battery. Due to the limitations of test equipment, test time, test cost and other conditions, most of the current research on life estimation of lithium-ion power batteries only focuses on battery cells rather than battery systems. This chapter aims at the life test and analysis of a certain vehicle-mounted lithium-ion power battery system, and provides more reference value data for the life estimation of the power battery system.
1.1 Test object and test bench
The test object is a lithium-ion power battery system for a hybrid electric vehicle. The battery system is a lithium iron phosphate battery with a combined structure of 90 strings, a nominal capacity of 6.5Ah, and a working voltage range of 207~342V. The cycle life test benches for vehicle-mounted lithium-ion battery systems mainly include lithium-ion power battery systems, Dicaron battery test systems, desktop computers, CANcaseXL, etc.
1.2 Test equipment
The main experimental equipment used in the cycle life test of the lithium-ion power battery system includes: Dicaron battery test system, hybrid test bench and HTH1920-40A high and low temperature humidity and heat test box.
The Dicaron battery test system is suitable for performance testing and data analysis of most power battery systems. The test system can load different test conditions for the test objects according to the test requirements and purposes. During the test, the actual vehicle conditions and the actual vehicle environment can be simulated, and the test can be as close as possible to the actual use environment. The hybrid test bench can meet the power system with a power of no more than 200kW, and is suitable for the powertrain test requirements of most models. HTH1920-40A high and low temperature damp heat test chamber can meet the test requirements of -40℃~150℃, the humidity range is adjustable from 25%RH to 98%RH (20℃~150℃), the maximum load is 250kg, which can meet the battery System testing requirements.
1.3 Test methods and results
The cycle life test of the lithium-ion power battery system is carried out at a temperature of (25±2) ℃, and a 1C constant current is used for the cyclic charge and discharge test. The charge and discharge capacity of the battery system is measured after 360 cycles during the test. The test was carried out 2520 cycles in total.
The method of measuring the charge and discharge capacity of the battery pack: First, use 1C constant current to charge the battery system until it reaches the charging cut-off condition specified by the manufacturer (BMS automatic protection). After standing for 30 minutes, use 1C constant current to discharge the battery system to BMS automatic protection, and record the discharge capacity value. After standing for another 30 minutes, use 1C constant current to charge to the BMS automatic protection, record the charging capacity value, and finally stand for another 30 minutes. The charge and discharge capacity test needs to be carried out 3 times to accurately obtain the charge and discharge remaining capacity value of the battery system. The recorded test results are shown in Figure 1.
Figure 1 Cycle life test results of lithium ion power battery system
Figure 2 Data processing of cyclic charge and discharge test of lithium-ion power battery system
Simple processing of the data obtained from the cycle life test of the lithium-ion power battery system, in which the coulomb efficiency is equal to the percentage of the discharge capacity to the charge capacity, and the discharge capacity retention rate is equal to the percentage of the average discharge capacity to the nominal capacity (6.5Ah), the result is obtained as shown in picture 2. According to the data of the average charge and discharge capacity, the relationship curve between the charge and discharge capacity and the number of cycles is drawn, as shown in Figure 3. It can be seen from Figure 3 that with the increase in the number of cycles of charge and discharge, the irreversible reaction and the expansion of inconsistency within the battery cause the charge and discharge capacity of the battery system to continue to decrease. The test data is used to calculate the average discharge capacity attenuation rate of the battery pack. It is 0.365Ah/1000 times. During the test, the coulombic efficiency of the battery pack fluctuates around 97.5%, and the maximum difference is less than 0.6%. The distance between the charging curve and the discharging curve in Figure 3 remains stable, which also reflects that the Coulomb efficiency of the battery pack is basically unchanged.
Figure 3 The relationship between the charging and discharging capacity of the lithium-ion power battery system and the number of cycles of charging and discharging
Figure 4 The relationship between the capacity retention rate of the lithium-ion power battery system and the number of cycles of charge and discharge
The relationship between the capacity retention rate of the lithium-ion power battery system and the number of cycles of charge and discharge is shown in Figure 4. After 2520 cycles, the battery pack capacity retention rate is 89.38%. The nominal capacity of the lithium-ion battery system for the test is 6.5Ah. If the remaining available capacity of the battery system is less than 80% of the nominal capacity as the end of life, the average attenuation rate obtained from the test is 0.365Ah/1000 times for calculation. The cycle life of the battery pack is 4191.9 times.
The above is the life test and analysis of the lithium-ion power battery system.
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