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Life Test and Analysis of Lithium Ion Power Battery System

Life Test and Analysis of Lithium Ion Power Battery System

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.

 Cycle life test results of lithium ion power battery system
Cycle life test results of lithium ion power battery system

Figure 1 Cycle life test results of lithium ion power battery system

Cyclic charge and discharge data of lithium ion power battery system
Cyclic charge and discharge data 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.

The relationship between charge and discharge capacity and cycle charge and discharge times
The relationship between charge and discharge capacity and cycle charge and discharge times

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

The relationship between capacity retention rate and cycle charge and discharge times
The relationship between charge and discharge capacity and cycle charge and discharge times

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.

Second-order RC model of lithium-ion power battery

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Second-order RC model of lithium-ion power battery

Second-order RC model of lithium-ion power battery

What is the second-order RC model of lithium-ion power battery? Next, the second-order RC model of lithium-ion power battery is introduced.

We have learned How does the lithium ion power battery work.The charge and discharge process of lithium-ion power batteries is a very complex electrochemical process, and its performance parameters are affected by many factors such as charge and discharge depth, current intensity, temperature, and vibration. It takes a lot of time and cost to detect the changes of various parameters of the battery through the method of battery charge and discharge test, and there are potential safety hazards such as fire and explosion. By establishing an accurate battery model to simulate the change state of various performance parameters of the battery during the charging and discharging process, not only can safety be improved, but also a lot of test time and cost can be saved. Therefore, this section studies the establishment of a lithium-ion power battery model..

The battery models commonly used at this stage mainly include equivalent circuit models, mechanism models, empirical models based on experimental data, electrochemical models, neural network models, and random models. Among them, the equivalent circuit model can express the relationship between the output characteristics of the battery and the internal parameters, and can be used to predict the state of charge SOC, the state of health SOH and other parameters of the battery. It can well reflect the working state of the battery. It has a simple structure, easy modeling, Convenient parameter identification and other advantages. Since the order of the second-order RC equivalent circuit model is appropriate, the engineering is easier to realize, and the steady-state and transient characteristics of the battery can be taken into account, so this article chooses the second-order RC model as the lithium-ion power battery model, as shown in Figure Shown.

Second-order RC equivalent circuit model of lithium-ion power battery
Figure Second-order RC equivalent circuit model of lithium-ion power battery

Among them, VOC represents the ideal voltage source, representing the open circuit voltage of the lithium-ion power battery; R0 represents the ohmic internal resistance; two RC structures are used to represent the polarization reaction of the battery, where RS, and RL are the polarization internal resistance, CS, and CL is the polarization capacitance;
I(t) represents the current, and Vbat represents the measurable battery terminal voltage. Let τ1=Rs. Cs,τ2=RL. CL, τ1 and τ2 respectively represent the short time constant and long time constant in the dynamic response process of the lithium-ion power battery.

What are the characteristics of lithium ion power battery,You can also learn more about.