Maintenance method of electric vehicle power battery for replacing battery core

Lithium Batteries are the best choice for electric vehicle power sources due to their high energy density, high bulk density, high operating voltage, no memory effect, low self-discharge and no pollution. In order to meet the power drive requirements of electric vehicles, it is generally necessary to use Lithium Battery cells in series to increase the voltage level.

The series battery pack has the problem that the cycle life cannot reach the design life. On the one hand, in the practical application of the series battery pack, the decay rate of the individual cells is significantly faster than other cells, thereby affecting the performance of the battery pack in which the cell is located. On the other hand, the charge/discharge characteristics of the cells are inconsistent, and the difference in charge/discharge characteristics of the cells may cause an imbalance in the charge/discharge, thereby causing damage to the overcharged or overdischarged battery, affecting the entire battery pack. The service life.

The operational data shows that after the traditional charging method, the lithium battery pack (80V 60Ah Lithium Iron Phosphate Battery pack) is charged/discharged for nearly 500 cycles, the actual usable capacity is reduced to about 80%, and as the number of times of charging increases, The actual available capacity will further decline.

Obviously, how to maintain the available capacity and extend the service life of the power battery pack is an important issue that needs to be solved urgently. In addition, some battery packs have a significantly lower cycle life during use than normal. The following analysis shows that the available capacity of an Electric Vehicle Battery pack has suddenly dropped. It is pointed out that "short-board batteries" is one of the reasons for the decline in available capacity. It is proposed to use cell replacement methods to improve the consistency level of series cells.

The test results show that the method can improve the measured cycle life of a lithium iron phosphate battery pack (80V 60Ah) with an abnormal performance from 158 times to 629 times, which improves the actual usable capacity and cycle life of the group of power battery packs.

1. Analysis of cycle life test of a power battery pack

1.1 Power battery cycle life test program

The power battery pack is mainly composed of 24 lithium iron phosphate battery cells, a battery management system, and an electrical connector. The parameter of each lithium iron phosphate battery is 3.2V60Ah.

The cycle life test of the power battery pack is carried out using a cyclic charge and discharge tester. The test methods and steps are as follows:

(1) After standing at 20 ° C for 10 min, charge with a current of 87.6 V at 30 A constant current, then charge at a constant voltage until the current is 3 A, the charging stops; the protection voltage is 3.7 V, and the charging stops immediately when the cell voltage exceeds 3.7 V;

(2) standing for 10 min, 5A current constant current discharge to 2.8V;

(3) Repeat steps (1)-(2), cycle to about 80% of the initial capacity, and calculate the cycle life.

The recorded process data includes: time, terminal voltage, current, temperature, charge and discharge capacity, differential pressure, and voltage of each of the 24 cells.

1.2 Power battery cycle life test results

The cycle life of the battery is an important indicator for evaluating the performance of the battery. It refers to charging and discharging the battery once for one cycle. According to certain test standards, when the battery capacity (generally the discharge capacity) is reduced to a certain value (generally The total number of charge/discharge cycles experienced by the battery before being specified as 80% of the initial capacity.

According to the above test scheme, the initial capacity of the battery pack is 60 Ah, and when the battery pack is less than 48 Ah during the cycle, the test is stopped, and the number of statistical cycles is 159.

The cycle life test curve is shown in Figure 1. After 159 cycles of electric vehicle power battery cycle, the discharge capacity has dropped to 47.9Ah, the first drop below 48Ah (80% of initial capacity), so the cycle life of this power battery pack is 158 times, and the cycle life is much lower than average. The number of times determines that this battery pack is an abnormal battery pack.

2. Analysis of short-board effect of cycle life of power battery pack

2.1 Short board effect

The short board effect is also called the barrel principle. No matter how high a bucket is, the height of its water depends on the shortest piece of wood. To increase the capacity of the barrel, you should try to raise the shortest piece of wood. Height, this is the most effective and only way. This principle vividly describes how to determine the strength of an whole and how to improve its overall strength. It has been widely used in enterprise management, human resources and economic management.

In the practical application of the series battery pack, due to the inconsistency of the battery cells, in order to ensure that the cells are not overcharged or overdischarged, a uniform charging and discharging protection voltage is set for each of the cells. Therefore, when a certain cell voltage reaches the protection voltage, the power battery pack stops charging or discharging.

2.2 Research on cycle life decay of power battery pack

During the cycle charging/discharging of the power battery pack, the cells that reach the upper limit of the charging protection in the previous 159 charging process are counted, and the statistical results are shown in FIG. 2 .

In the cycle life test of the power battery pack, the number of times the second, fifth, ten, 17, and 23 of the 24 batteries reached the upper limit of the charging protection was 7, 10, 240, 11, and 8 times, respectively. The number of times the charge protection upper limit voltage is reached is 86.16% of the total number of times.

According to the "short board effect" analysis, the 10th section of the 24 batteries is the main reason for the shortening of the cycle life of the electric vehicle power battery pack, and such a battery core is called a "short board battery core". By replacing the "short board cells" to improve cell consistency, it is expected to increase the available capacity of the power battery pack.

3. Cell replacement technology

The cell replacement technology mainly solves the problem that the battery pack is replaced with the original battery pack when the battery pack is replaced, and the matching quality directly determines the service life of the battery pack after replacement.

3.1 Battery Matching System

The workflow of the battery matching system is as follows:

(1) The battery cell used for replacement (can be a new battery core or an old battery core), through the battery performance test system, obtains the open circuit voltage, capacity, charge/discharge curve and other information of the battery, and enters the electricity. In the core model library, all the battery information to be replaced is stored in the model library;

(2) The battery pack to be matched passes the battery pack performance detection system, and obtains the open circuit voltage, capacity, charge/discharge curve and other information of all the cells in the battery pack, and accurately picks out the “short board battery core” that needs to be replaced. According to the results of the overall evaluation of the actual performance and capacity of other cells in the battery, the battery demand information that needs to be matched is given;

(3) Through the comparison of the open circuit voltage, capacity and charge/discharge curve of the cell, in the cell model library, find the cell with better matching, and the replacement process is completed after the test passes.

3.2 Battery pack performance test

The battery pack performance detection system mainly has two functions: by inquiring about the open circuit voltage, capacity, charge/discharge curve and other information of all the cells in the battery pack, accurately picking out the "short board batteries" that need to be replaced; verifying the reassembly Whether the rear battery pack is well matched.

The portable device consisting of the detection system is mainly composed of a program-controlled charger, a program-controlled load, a wireless acquisition system, and a total controller. The principle of the battery pack performance testing device is shown in Figure 3. The system obtains cell information by communicating with the BMS (Battery Management System) in the battery pack.

Taking the front and rear power battery packs as an example, the battery pack performance detection system is turned on, and the voltage and differential pressure (the highest voltage of the cell voltage at the time of discharge cutoff) are obtained when the battery cells of the battery pack reach the discharge cutoff voltage point (off-voltage 2.8V). The lowest value difference), charge/discharge capacity, etc. are detected. See Table 1 for the voltage information of all the cells when the battery pack is discharged before replacement.

The test data shows that the charging capacity of the 10th cell is 51.54Ah, the discharge capacity is 52.34Ah, the initial discharge capacity is 60Ah, the charge/discharge value is lower than the whole group, especially when discharging, and the discharge value is slightly lower than 0.05V. , indicating that the charging and discharging capacity are insufficient. At this time, the battery capacity of the whole group is 87.23% of the initial capacity, and the discharge voltage of the first cell reaches the discharge cut-off voltage, and the pressure difference is 0.344V, which does not meet the pressure difference not exceeding 0.22. According to the provisions of V, the 10th battery is a short-board battery, and the battery pack needs maintenance.

After replacing the 10th cell, the voltage information of all the cells in the battery pack discharge is shown in Table 2. After replacement, the battery pack has a capacity of 57.58 Ah and a pressure difference of 0.213V. Based on the above research, the replacement index is proposed, that is, the battery capacity decreases to 90%~92% of the initial capacity, and when the pressure difference reaches 0.22V, the battery should be maintained. The reference value is mainly based on the capacity value and the pressure difference is supplemented.

4. Cycle life test after battery replacement

After the certificate reorganization is successfully matched, the test is continued according to the test plan, and the result is shown in FIG. 4 .

Before the cell replacement, during the cycle test of a certain battery pack, the capacity decay rate was faster, and it was cycled to 277 times, and the capacity was reduced to 52.34 Ah (about 87% of the initial capacity). After adopting the cell replacement measure, the discharge capacity is immediately improved. When the number of cycles is 1062 times and the discharge capacity is 47.99Ah (about 80% of the initial capacity), the cycle life of the battery pack is 1061 times, indicating that the battery cell The replacement method can effectively extend the life of the battery pack.

5 Conclusion

Cell replacement technology improves cell consistency, maintains battery pack usability and extends cycle life. Especially in the case where there is a "short-plate cell" in a part of the battery pack, the usable capacity is suddenly reduced and the life is shortened, and the maintenance effect is more remarkable. With the promotion of electric vehicles, the maintenance technology of electric vehicle battery packs based on battery replacement has good economy and broad application prospects.