18650 battery pack 3.7v

　　Effect of fast charging strategy selection on 18650 battery life

　　With the gradual increase in the popularity of electric vehicles, more and more electric vehicles have entered the daily life of ordinary consumers. The service life of power batteries is often one of the focuses that people worry about, which is also the cause of second-hand new energy. An important reason for the low value retention rate of automobiles, so the cycle life is also a very important assessment index for power batteries.

　　With the gradual increase in the popularity of electric vehicles, more and more electric vehicles have entered the daily life of ordinary consumers. The service life of power batteries is often one of the focuses that people worry about, which is also the cause of second-hand new energy. An important reason for the low value retention rate of automobiles, so the cycle life is also a very important assessment index for power batteries. Generally speaking, the cycle life of lithium-ion batteries and the choice of positive and negative electrode materials and electrolyte have the greatest impact on the cycle life, followed by the use strategy of lithium-ion batteries, such as charging and discharging systems, operating temperature, etc., will affect the cycle life of lithium-ion batteries have a noticeable impact. Peter Keil (first author, corresponding author) and Andreas Jossen of the Technical University of Munich, Germany, analyzed the impact of charging strategies on the cycle of different types of power 18650 batteries. First of all, let's analyze several common charging strategies 1. Constant current and constant voltage (CCCV) constant current and constant voltage charging is the most common and common charging method. Charging, when the set voltage is reached, the control voltage remains unchanged and the charging current is continuously reduced until the current reaches the set value or the time reaches the set value (as shown in Figure a below). The charging time of this charging strategy is mainly affected by the constant current charging current Icc, and the charging capacity is mainly affected by the charging cut-off voltage Vch and the constant voltage charging cut-off current Iend, and related studies have also shown that high charging cut-off voltage Vch and large A high charging current Icc will lead to a significant deterioration in the life of the Li-ion battery. 2. The multi-step constant current charging method (MSCC) no longer uses constant voltage charging in this charging strategy, but adopts a multi-step constant current charging strategy with decreasing charging current, for example, after using I1 constant current charging to the cut-off voltage , continue to use a smaller current I2 to charge to the cut-off voltage, and so on until the current decreases to the final cut-off current (as shown in Figure b below). 3. Pulse charging method (PC) This charging method can be found in some related literature reports. In the charging process of this charging strategy, a series of short-term adjustments to the charging current or even the charging direction (discharging) may be used. Time pulse structure, there are two common pulse charging strategies, one is to replace only the constant voltage charging part of CCCV charging with pulse charging (as shown in Figure c below), and the other is to replace the entire process with pulse charging (As shown in Figure d below). 4. Accelerated charging (BC) The so-called accelerated charging mode is to add a high-current CC or constant power charging process on the basis of the CCCV charging mode, so as to achieve the purpose of reducing the charging time (as shown in Figure e below). There are studies It shows that the accelerated charging mode can effectively improve the charging efficiency of the lithium-ion battery without significantly affecting the cycle life of the lithium-ion battery.

　　In the experiment, the author chose three power type 18650 lithium-ion batteries from Sanyo, Sony and A123. The rated capacity of the battery is about 1A. The information of the three batteries is shown in the table below. The discharge curve of the battery under low current. Four charging systems were used in the experiment, CCCV (constant current constant voltage), CCPC (constant current pulse), PC (pulse) and BC (accelerated charging).

　　Since the CCCV charging strategy is the most common charging method, the CCCV charging method is also used as a reference in this test. Table 2 shows the strategy used for CCCV charging, mainly examining different charging currents (1A, 3A and 5A). and the effect of charging cut-off voltage. Table 3 shows the CCPC (constant current pulse) charging strategy, with 3A as the charging current. Table 4 shows the PC (pulse) charging strategy, in which the high current is 5A, the low current is 1A, and the duration of the high and low current is the same, mainly testing the impact of different frequencies on the battery. Table 5 shows the BC (accelerated charging) charging strategy, which mainly tests the impact of accelerated charging in different SoC ranges on battery performance.

　　1. Constant current and constant voltage charging (CCCV) 1.1 Influence of charging current It is completed in the process of constant current charging, and the constant voltage charging capacity only accounts for a very small part, while the time for constant voltage charging of battery B accounts for a large proportion. The charging current has the greatest impact on the charging time of the battery. For example, if we increase the charging current from 1A to 3A, the charging time can be shortened by 50%. If we continue to increase the charging current to 5A, only the charging time of batteries A and C is shortened by 1/ 3. As for battery B, the constant voltage charging time is longer, so the charging time after increasing the charging current to 5A is not significantly shortened compared with 3A.

　　1.2 Influence of charge cut-off voltage The charge cut-off voltage will affect the electrode potential of the positive and negative electrodes, so it will have a significant impact on the cycle life of the lithium-ion battery. The figure below shows the change of the capacity of three batteries A, B and C with the charge cut-off voltage. It can be seen that the charge cut-off voltage of batteries A and B has a significant impact on the capacity of the battery. Every time the charge cut-off voltage decreases by 100mV, the battery capacity will decrease. It will decline by 10-20%, and because battery C uses LFP positive electrode, the capacity of the battery above 3.4V has almost nothing to do with the charging cut-off voltage of the battery. The following is the cycle performance of the battery under different cut-off voltages. Overall, the cycle performance of the battery is decreasing as the charge cut-off voltage increases. For battery A, the impact of the charge cut-off voltage is relatively small. The cut-off voltage is reduced from 4.2V to 4.1V, and the life is only increased by about 100 times. To significantly increase the life of A battery, it needs to be reduced to 4.0V, but this will reduce the capacity of the battery by 30%, and increase the charging voltage of the battery to 4.25. V will only slightly reduce the cycle life of the battery. Battery B is much more sensitive to the charging voltage. After reducing the charging voltage from 4.1V to 4.0V, the capacity retention rate of the battery increases by 5% after 800 cycles, but further reducing the charging cut-off voltage to 3.9V will affect the cycle life of the battery. There is no significant effect, but if the charging cut-off voltage is increased to 4.15V, the decay speed of battery B will be significantly accelerated. Battery C is the most sensitive to the charge cut-off voltage. If the charge cut-off voltage is increased from 3.6V to 3.65V, the cycle life of the battery will be significantly reduced, but reducing the charge cut-off voltage has no significant impact on the cycle life of the battery.

　　2. Pulse charging (CCPC/PC) 2.1 Constant current pulse (CCPC) The constant current pulse charging system is similar to the constant current and constant voltage charging system, except that the constant voltage charging stage is replaced by pulse charging. Compared with common charging time, CCPC The constant current and constant voltage charging will actually take a little longer, so CCPC has no advantage in charging time. The figure below shows the cycle data of the three batteries using the CCPC and PC charging strategies. It can be seen that when the CCPC charging strategy is used for the three batteries, the cycle life of the battery is actually the same as that of the CCCV charging strategy using the same charging current.

　　2.2 Pulse charging (PC) is different from the previous CCPC charging strategy. The PC charging strategy adopts a pulse method throughout the charging process. Because the polarization is relatively large during the pulse charging process, the pulse charging will make the charging of the battery The capacity has been reduced. For example, under the pulse charging strategy of battery A, the battery can only be charged to 80% of the capacity, and battery C can be charged to 95% of the capacity. In terms of charging time, the PC charging strategy is almost the same as the CCCV charging system with the same current. From the cycle data in the above figure, it can be seen that the cycle life of the battery with the PC charging strategy is almost the same as that of the CCCV charging strategy with the same current. 3. Accelerated charging (BC) The accelerated charging strategy is to use a larger charging current (5A) for charging in a lower SoC range, and then switch to a small current (1A) CCCV charging method for charging after reaching the cut-off voltage, so charging The time is relatively shortened. After testing, the charging time of battery A using the BC charging strategy is 57 minutes, and that of battery C is 48 minutes, which is between the CCCV charging time of 1A and 3A current. The figure below shows the cycle performance of batteries A and C using the BC charging strategy. From the figure, it can be seen that for battery A, if accelerated charging (5A) is used from 0%, the battery’s decay speed will increase significantly, which is different from 5A. The current CCCV charging strategy is close. However, if the charging is accelerated from 10% SoC and 20%, the cycle life will be greatly improved, which is close to the CCCV charging strategy with 3A current. For battery C, if the accelerated charging SoC is 0% and 10%, the battery's decline rate will be accelerated significantly, and if the accelerated charging is started at 20% SoC, it will have less impact on the cycle life of the battery. This also shows that accelerated charging at lower SoCs results in a significant impact on the cycle life of the battery.

　　The work of Peter Keil shows that the charging current has the greatest impact on the battery cycle life. When batteries A and C are charged at a high current of 5A, the cycle life of the lithium-ion battery will be severely shortened, but the cycle life of battery B seems to be affected by the charging current. has little effect. The second is the charge cut-off voltage of the battery. A high charge cut-off will seriously shorten the service life of the lithium-ion battery, especially for batteries B and C, increasing the charge cut-off voltage by 50mV will seriously affect the cycle life of the battery. From the charging strategy point of view, the pulse charging strategy has no significant impact on the cycle life of the battery (compared to CCCV), but the pulse charging does not reduce the charging time, especially the CCPC charging strategy will also cause the charging time of the battery to be extended. . The accelerated charging (BC) strategy can reduce a certain charging time, but the accelerated charging strategy needs to carefully select the SoC range for accelerated charging. Try not to use accelerated charging in the range of too low SoC, so as not to affect the cycle life of the battery.