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In recent years, China’s new energy vehicle industry has shown explosive growth, which has attracted much attention to the power battery systems used in new energy vehicles. As one of the key components of electric vehicles, the service life of the power battery system directly affects the overall use of the vehicle. In the past, research on the cycle life of power batteries was often limited to individual battery cells or modules, and there were few reports on research on power battery systems.

Due to the short board effect, the performance of battery systems is usually determined by the worst individual cells inside, so the inconsistency of individual cells can lead to a significant decrease in the performance of the battery system, especially the lifespan of the battery system will be greatly affected. Therefore, attempting to identify the decay pattern of battery system life, establishing a life assessment method and life model for power battery systems, providing a basis for establishing fast life testing and evaluation methods for power batteries, is of great significance for the rational use of battery systems in the entire vehicle.

**1 Test Object and Equipment**

Research object: The experiment adopts a 310.8 V, 37 Ah high-energy ternary power battery system for hybrid vehicles as the research object. The power battery system is composed of 7 modules in series, and each power battery module is composed of 12 power battery cells in series. The combination form of the entire power battery system is 1 parallel connection and 84 series connection.

Test equipment: The power battery system uses a power battery simulator to conduct cycle life and power internal resistance tests. Use a water cooler to cool the battery system in the cycle, and conduct a lithium-ion power battery system cycle life test bench. The power battery unit is subjected to cycle life testing using a power battery simulator and environmental chamber, and an electrochemical workstation is used for AC impedance testing.

**2 Test methods**

## 2.1 Single cell cycle test method for power batteries

In order to ensure the comparability of experimental results, battery monomers with good consistency were selected from the same batch of samples, and comparative experiments were conducted at different discharge depths (DOD range) at different temperatures. The cycling test method for power battery monomers is as follows

(1) 100% charge and discharge depth (100% DOD): The battery cell cycle test is conducted at room temperature and 40 ℃, respectively. The battery is charged at 1 C constant current until the cell voltage reaches 4.24 V, then switched to constant voltage charging until the current is less than or equal to 1.85 A, and stopped charging. Let it stand for 30 min, and discharge at 1 C constant current until the cell voltage reaches 3.00 V. Let it stand for 30 min, and repeat the above steps for the cycle test; Perform capacity calibration and AC impedance testing every 100 cycles.

(2) 80% charge and discharge depth (80% DOD): The battery cell cycle test is conducted at room temperature and 40 ℃ environment, respectively. 1 C constant current is used to charge the cell to a voltage of 4.24 V, and it is allowed to stand for 30 minutes. 1 C constant current is used to discharge the cell to a voltage of 3.00 V, and it is allowed to stand for 30 minutes. The above steps are repeated for the cycle test, and capacity calibration and AC impedance test are conducted every 100 cycles.

## 2.2 Cycle test method for power battery system

(1) 100% charge discharge depth (100% DOD). In order to avoid the impact of inconsistent temperature inside the rabbit battery system on its cycle life, the experiment was conducted at room temperature (25 ± 5) ℃, with a coolant temperature of 25 ℃ and a flow rate of 8 L/min. Charge with 1 C until the total voltage reaches 352.8 V, then switch to constant voltage charging until the current is less than or equal to 1.85 A and stop charging (CC-CV), let it stand for 30 minutes: discharge with 1 C constant current until the individual voltage reaches 3.00 V, let it stand for 30 minutes; A total of 170 cycles were conducted.

(2) 80% charge and discharge depth (80% DOD): The battery system cycling test is conducted at room temperature, with a coolant temperature of 25 ℃ and a flow rate of 8 L/min. Charge with 1 C constant current until the total voltage reaches 348.6 V and let it stand for 30 min, then discharge with 1 C constant current until the total voltage reaches 290.8 V and let it stand for 30 min; A total of 2500 cycles were conducted. Perform a capacity calibration every 200 or 100 cycles, and conduct a DC resistance (DCR) test at a fixed SOC specific charging and discharging current.

Capacity calibration involves conducting 3 100% DOD charging and discharging tests on the battery system; The DCR test first requires the battery system 1 C to be charged to a total voltage of 311.56 V (CC-CV, cut-off current of 1.85 A), left to stand for 30 minutes, then 20 A charging and 20 A discharging for 10 seconds each, 120 A charging and 120 A discharging for 10 seconds each, 1 C discharging to a single cut-off voltage of 3.00 V, and then calculating the DC resistance values under each pulse current

# 3 Analysis of single cell cycle test data

## 3.1 Single cell discharge capacity and number of cycles

Power battery cells were subjected to 500 cycle life tests at room temperature (25 ± 5) ℃ with 80% DOD and 100% DOD: 100% DOD charging and discharging were performed every 200 or 100 cycles to calibrate the capacity.

The initial discharge capacity of the battery cycle life is 38.00 Ah. After 200 cycles, the capacity retention rate is 100.63%, which is greater than the capacity retention rate of 99.46% after 170 cycles of 100% DOD in the battery system;

After 500 cycles, the discharge capacity is 37.57 Ah and the capacity retention rate is 98.87%.

The initial discharge capacity of 80%DOD cycle life is 38.73 Ah.

After 200 cycles, the capacity is 38.36 Ah and the capacity retention rate is 99.04%.

After 500 cycles, the discharge capacity is 36.66 Ah and the capacity retention rate is 94.66%. After 400 cycles of 80% DOD in the battery system, the capacity retention rate is 96.72%, and after 600 cycles, the capacity retention rate is 91.76%

Capacity voltage curve of power battery cells at room temperature. It can be seen that the discharge voltage platform of the NCM ternary system battery is between 4.15 – 3.30 V, and the charging voltage platform is between 3.50 – 4.20 V. The capacity voltage curves of 80% DOD after 0-500 cycles show a significant decrease in discharge capacity after every 200 or 100 cycles at this charging and discharging depth. After 0-500 cycles, the capacity voltage curve of 100% DOD showed no significant decrease in discharge capacity.

The power battery cell was subjected to 500 cycle life tests at 80% DOD and 100% DOD in an environment of (40 ± 5) ℃. The initial discharge capacity of the 80% DOD cycle life of the battery cell is 40.19 Ah, and the discharge capacity retention rate after 200 cycles is 94.65%; After 500 cycles, the discharge capacity is Ah and the capacity retention rate is 91.22% DOD. The initial discharge capacity is after the cycle life, and the capacity retention rate is 95.82%;

After 500 cycles, at room temperature and 40 ° C, the cyclic discharge capacity retention rate is greater than 80% DOD cyclic discharge capacity retention rate (full discharge capacity after cycle end/initial full discharge capacity)

The rate of capacity degradation during cycling at 40 ℃ is greater than that at room temperature, indicating that high temperature will accelerate battery capacity degradation and reduce battery cycling life.

At 40 ° C, the discharge capacity of a single power battery decreases rapidly between 0-300 cycles at 80% DOD, and rapidly between 100-200 cycles at 100% DOD

## 3.2 Single Cell AC impedance

AC impedance spectra of power battery cells before and after 80% DOD cycle life at room temperature and 40 ℃. The battery impedance of lithium-ion batteries includes the impedance of the electrolyte, the charge and mass transfer impedance at the electrode electrolyte interface, and the diffusion impedance of lithium ions near the electrode and its interface.

The impedance spectrum of the electrode consists of a semicircle in the high-frequency region and a diagonal line in the low-frequency region. The impedance of the electrolyte significantly increased before and after 500 cycles at room temperature and 40 ℃ for a single 80% DOD battery, from 0.9 m Ω and 1.0 m Ω before cycling to 2.0 m Ω and 2.4 m Ω after cycling, respectively.