2.2 Cyclic performance testing of samples

At an ambient temperature of (25 ± 2) ℃, the battery module samples were subjected to a 0.5 C charge and discharge cycle test using a constant current constant voltage charging system (CC-CV) and a constant current discharge system. The battery pack was subjected to cycle testing on a multi-channel testing machine.

Test method for samples A and B: First, charge at a constant current of 1.5 A to 4.2 V (one of the monomers has a voltage greater than 4.2 V), then charge at a constant voltage of 4.2 V until the current is less than 2.12 A, and let it rest for 5 minutes. Discharge at a constant current of 1.5 A again until the termination voltage is 2.8V (the voltage of one cell is less than 2.8V), and charge and discharge in this cycle for 200 times.

Test method for sample C: First, charge at a constant current of 1.5 A to 16.769 V, and then charge at a constant voltage of 17.769 V until the current is less than 3.47 A and remains stationary for 5 minutes. Discharge at a constant current of 1.5 A again until the termination voltage is 2.5V (the voltage of one cell is less than 2.5V), and charge and discharge in this cycle for 200 times.

Test method for sample D: First, charge at a constant current of 1.5 A to 14.616 V, then charge at a constant voltage of 14.616 V until the current is less than 4.3 A, and let it rest for 5 minutes. Discharge at a constant current of 1.5 A again until the termination voltage is 2.5 V (the voltage of one cell is less than 2.5 V), and charge and discharge in this cycle for 200 times.

2.3 Vibration resistance test of samples

At an ambient temperature of (25 ± 2) ℃, the vibration resistance performance of the battery module sample is tested. The vibration resistance test of the battery is conducted on a vibration test bench, and the testing process is as follows:

(1) At an ambient temperature of (20 ± 5) ℃, the battery module is discharged at a constant current of 1.5 A to the discharge cut-off condition specified by the manufacturer. At an ambient temperature of (20+5) ℃, the battery module is charged at 1.5 A until the terminal voltage reaches the charging cut-off condition specified by the manufacturer, and the charging is stopped.

(2) Fasten the battery module to the vibration test bench and conduct a linear sweep frequency vibration test according to the following conditions: discharge current: 1 A; Vibration direction: single vibration up and down; Vibration frequency: 10~55Hz; Maximum acceleration: 30m/s; Sweep cycle: 10 times; Vibration time: 2 hours.

During the vibration test, it is not allowed to experience sharp changes in discharge current, abnormal voltage, deformation of the battery shell, electrolyte overflow, etc. The connection should be reliable and the structure should be intact. Loosening of the installation is not allowed.

(3) SBM (AC Internal Resistance Tester) tests internal resistance.

(4) Repeat steps (1), (2), and (3) a total of 4 times.

3 Results and Discussion

3.1 Cyclic Life Test Results and Discussion

The charging and discharging voltages of samples A, B, and C are 2.8~4.1V, while the charging and discharging voltages of sample D are 2.5~3.65 V. Four samples exhibit good cycling performance.

The initial discharge capacity of sample A is 40.562 Ah. After 100 cycles, the discharge capacity is 39.759 Ah with a capacity retention rate of 98.02%, indicating good cycling performance. After 200 cycles, the discharge capacity is still as high as 39.309 Ah with a capacity retention rate of 96.91%, indicating good cycling performance.

The initial discharge capacity of sample B is 68.838 Ah. After 100 cycles, the discharge capacity is 68.402 Ah with a capacity retention rate of 99.37%, indicating good cycling performance. After 200 cycles, the discharge capacity is still as high as 67.789 Ah with a capacity retention rate of 98.48%, indicating good cycling performance.

The initial discharge capacity of sample C is 2.013 Ah. After 100 cycles, the discharge capacity is 1.946 Ah with a capacity retention rate of 96.67%, indicating good cycling performance. After 200 cycles, the discharge capacity is still as high as 1.862 Ah with a capacity retention rate of 92.50%, indicating good cycling performance.

The initial discharge capacity of sample D is 82.601Ah. After 100 cycles, the discharge capacity still reaches 81.575 Ah with a capacity retention rate of 98.76%, indicating good cycling performance. After 200 cycles, the discharge capacity still reaches 80.716 Ah with a capacity retention rate of 97.72%, indicating good cycling performance.

Through comparison, it can be seen that after 200 cycles, sample B has the highest capacity retention rate of 98.48%. Next are sample D and sample A. Sample C has the lowest capacity retention rate, only reaching 92.50%.

3.2 Vibration resistance test results and discussion

The initial internal resistances of the four samples were 2.324, 1.53, 66, and 1.9 mΩ, respectively.

As the number of vibrations increases, the internal resistance of sample A first decreases and then gradually increases, while the internal resistance of sample B gradually increases. The change in internal resistance of sample C is not very obvious, while the internal resistance of sample D first decreases and then gradually increases.

It can be seen that after the charging and discharging cycle, the internal resistance of the battery will slightly decrease. This is because the vibration not only causes the connection part of the battery to become loose, but also increases the polarization resistance of the battery. After the fifth vibration, the connection reliability of sample D is the worst, followed by sample A, and the connection reliability of samples B and C is better. Samples B and C are small capacity cylindrical cells. Although the grouping method is more complex, if the welding process is reliable, the connection reliability will also be guaranteed.

4 Conclusion

Through theoretical analysis of small capacity cylindrical lithium-ion power batteries, it was determined that they have good characteristics. Four samples of lithium-ion power battery modules were selected and their cyclic performance and vibration resistance were tested. The test results show that after 200 charging and discharging cycles, the discharge capacity of sample B is 67.789 Ah, and the capacity retention rate is 98.48%. The connection reliability between sample B and C is good. Small capacity cylindrical lithium-ion power batteries have good cycling performance and high connection reliability, and can be applied to pure electric logistics vehicles.