## 2.2 Analysis of Cycle Test Data for Power Battery System

(1) Power battery system 100% charge discharge deep cycle

The power battery system was subjected to 170 cycle life tests at 100% charge and discharge depth (100% DOD), with a coolant temperature of 25 ℃ and a flow rate of 8 L/min during the cycling process at room temperature (25 ± 5) ℃. The relationship curve between charging and discharging capacity and number of cycles shows that the initial discharge capacity is 38.94 Ah, and the discharge capacity after 170 cycles is 38.73 Ah, with a capacity retention rate of 99.46%. Among them, the coulombic efficiency (which is equal to the percentage of discharge capacity and charging capacity) is always greater than 100%; In the first 15 cycles, the discharge capacity showed an upward trend, indicating that the power battery system is in the activation process

(2) 80% deep cycle life of charging and discharging for power battery system

System discharge capacity and number of cycles.

The power battery system is subjected to a 2500 cycle life test at room temperature (25 ± 5) ℃, with a coolant temperature of 25 ℃ and a flow rate of 8L/min during the cycling process, using 80% DOD.

Perform a one-time performance test every 200 or 100 cycles (with a calibration capacity of 200 cycles before 1600 cycles and 100 cycles after 1600 cycles). Perform 3 times of 100% DOD charging and discharging to calibrate the capacity. And conduct DCIR tests under different pulse currents at 50% SOC.

The initial discharge capacity of the battery system is 38.98 Ah. After 2500 cycles, the discharge capacity is only 10.20 Ah. Before 1200 cycles, the capacity decays slowly, with a capacity loss of 5.58 Ah. After this, the capacity rapidly decays, with a capacity loss of 23.2 Ah between 1200 and 2500 cycles, with a capacity loss rate of 59.5%. During the full cycle life, the capacity decay rate is 73.8%. The Coulomb efficiency shows a trend of first increasing and then decreasing. Before 400 cycles, the Coulomb efficiency continuously increases and then gradually decreases. After 1700 cycles, the Coulomb efficiency is less than 100%

The overall pattern of the cycle life of this power battery system is that the capacity decay accelerates with the increase of cycle times. This is different from the trend of battery cell capacity degradation reported in literature, as the battery system is composed of a large number of battery cells, and the inconsistency of battery cells has a significant impact on the capacity of the battery system. At the same time, it also blurs the trend of battery cell capacity change, making it different from the trend of battery cell capacity change.

System cycle life and individual pressure difference

In order to study the effect of battery cell pressure difference on battery system capacity, in 2500 cycle tests, the pressure difference between the highest voltage and the lowest voltage of 84 battery cells in the charging and discharging end battery pack was recorded in each performance test. From the experimental results, it can be seen that the initial discharge terminal voltage difference of the battery system is 0.171 V, and the charging terminal voltage difference is 0.018 V. After 2500 cycles, the discharge terminal voltage difference is 0.550 V, and the charging terminal voltage difference is 0.286 V. From the results, it can be seen that on the one hand, the pressure difference at the discharge end is always greater than that at the charging end throughout the entire cycle life, and shows a gradually expanding trend.

On the other hand, as the number of cycles increases, both the charging end pressure difference and the discharging end pressure difference continue to increase. And the rate of increase is getting faster and faster;

Correspondingly, during the cycling process, the capacity degradation rate of the battery system also becomes faster and faster as the pressure difference of the battery cell increases, especially after 1200 cycles, this corresponding pattern becomes more obvious.

In the early stage of the cycle life test, the pressure difference of the battery system is relatively small, and its capacity degradation is mainly caused by the capacity degradation of the individual battery cells that make up the system. As the number of cycles increases, the voltage of some battery cells decreases, causing the total voltage or cell voltage of the battery system to reach the discharge cut-off condition in advance. In contrast, other cells have not yet reached the discharge cut-off condition, resulting in the incomplete discharge of this part of cell capacity and a decrease in the discharge capacity of the battery system.

Therefore, in the case of a large pressure difference, the discharge capacity of the battery system cannot fully reflect the capacity of the battery system itself. In summary, the trend of capacity change in battery systems is a comprehensive manifestation of the attenuation of battery cell capacity itself and the intensification of inconsistency between battery cells, which is significantly different from the law of cell capacity attenuation.

System cycle life and DC resistance

The DCIR test of the battery system involves charging the system to a total voltage of 311.56 V, followed by 20 A charging and 20 A discharging for 10 seconds each, and 120 A charging and discharging for 10 seconds each. The DC resistance values under each pulse current are calculated. DCIR (direct current internal resistance) is a test of the DC internal resistance of a battery, which includes two parts: ohmic resistance and polarization resistance. The measurement of DC internal resistance is a method of considering and measuring both parts of the resistance.

Internal resistance is an important indicator for measuring battery performance. Batteries with low internal resistance have strong high current discharge capacity, while batteries with high internal resistance have the opposite. From the results, it can be seen that as the cycle progresses, DCIR shows a trend of first decreasing, then stabilizing, and then gradually increasing, and the charging and discharging internal resistances show the same changing trend at different currents.

After 1200 cycles, the increase in DCIR internal resistance of the battery system is accelerated, corresponding to the accelerated capacity decay and the accelerated increase in charge discharge terminal pressure difference after 1200 cycles. The internal resistance of 20 A charging and discharging increased from 130.0 mΩ and 120.0 mΩ before the start of the cycle life to 160.0 m Ω and 150.0 mΩ at the end of the cycle life. The internal resistance of 120 A charging and discharging increased from 115.0 mΩ and 113.0 m Ω before the start of the cycle life to 147.5 mΩ and 150.8 mΩ at the end of the cycle life

Due to the total system voltage of 311.56 V, the 20 A charging and discharging power is 6231.2 W, and the 120 A charging and discharging power is 37387.2 W. From the results, it can be concluded that after the end of the cycle life, the power loss rates of the system during charging and discharging at 20 A current are 1.03% and 0.96%, respectively. At 120 A current, the power loss rates during charging and discharging are 5.68% and 5.81%, respectively. The increase in DC internal resistance leads to an increase in power loss in the battery system, and the greater the charging and discharging, the more significant the power loss caused by internal resistance.

In actual use, the DC internal resistance of the power battery system has a voltage divider effect relative to the external load, that is, the larger the internal resistance, the greater the pressure drop caused; At the same time, the increase in internal resistance leads to a corresponding decrease in the external output power of the battery system; The increase in power consumption on internal resistance leads to an increase in heat generation inside the monomer, resulting in an increase in internal temperature.

On the one hand, there is a difference in the increase in internal resistance of each individual cell during the cycling process, and the resulting voltage drop is also inconsistent, resulting in an increase in voltage inconsistency between individual cells; On the other hand, the increase in internal resistance power consumption can lead to an increase in the internal temperature of individual batteries, resulting in a decrease in temperature uniformity within the battery system. The temperature difference will further exacerbate the voltage inconsistency between individual batteries.

Therefore, as the cycle life progresses, the difference in internal resistance between monomers will lead to an increase in voltage inconsistency between monomers. At the same time, an increase in internal resistance will cause an increase in heat generation and a larger temperature difference, further leading to a decrease in voltage consistency between monomers; The coupling effect between internal resistance and temperature will exacerbate the inconsistency between individual voltage, reduce the discharge capacity of the battery system, and shorten its cycle life.

**3 Conclusion**

(1) For battery systems, the internal resistance of battery cells increases, and the pressure difference between cells increases due to the effect of voltage division. At the same time, the increase in internal resistance increases the heat generation inside the battery, and the temperature difference within the battery system will further increase the pressure difference between battery cells.

The coupling effect between changes in internal resistance of individual cells and uneven temperature within the battery system leads to an accelerated increase in individual cell pressure difference, which in turn leads to accelerated capacity degradation of the battery system and affects its cycle life.

(2) The discharge capacity of this ternary system power battery system during the cycling process is independent of the variation of cycling conditions with the number of cycles and follows a power function decay law. This power battery system life model can predict and evaluate the actual service life of the power battery system, and provide a basis for the reasonable use of the battery system.

(3) For power battery monomers, the capacity retention rates of 100% DOD and 80% DOD cycle life at room temperature are both greater than the corresponding capacity retention rates of the battery system. At the same time, the capacity retention rates of power battery monomers after 100% DOD cycle life are greater than those after 80% DOD cycle life at both room temperature and 40 ° C. In addition, the capacity degradation rate of cycle life at 40 ° C is greater than that at room temperature, indicating that the battery will experience rapid capacity reduction at high temperatures, Reduce battery cycle life.