Battery cell charging method and charging system

Abstract

A battery cell charging method according to an embodiment of the present invention is for a plurality of battery cells connected in series. The battery cell charging method includes the steps of charging the plurality of battery cells with a first constant current, and cutting off the first constant current for battery cells that have already reached a set voltage; and, when the first constant current for the plurality of battery cells is cut off, charging the plurality of battery cells with a second constant current lower than the first constant current, and cutting off the second constant current for battery cells that have again reached the set voltage.

Description

[Technical Field]

[0001] [CROSS-REFERENCE TO RELATED APPLICATIONS] This application claims the benefit of priority based on Korean Patent Application No. 10-2022-0018477 filed February 11, 2022 and Korean Patent Application No. 10-2022-0133592 filed October 17, 2022, and all contents disclosed in the documents of said Korean patent applications are incorporated herein by reference.

[0002] The present invention relates to a method and system for charging battery cells, and more particularly to a method and system for charging a plurality of battery cells connected in series. [Background technology]

[0003] As the popularity of electric vehicles and other devices increases along with technological developments and increased demand for mobile devices, the demand for secondary batteries as an energy source is rapidly increasing, with lithium secondary batteries, which have high capacity and energy density, being in particularly high demand.

[0004] Generally, a secondary battery is manufactured by placing an electrode assembly, which includes a negative electrode, a positive electrode, and a separator, in a cylindrical or rectangular metal can or a pouch-shaped case made of an aluminum laminate sheet, and then injecting an electrolyte into the electrode assembly. The secondary battery thus manufactured must be activated by performing predetermined charging and discharging cycles before it can function as a battery. This process is called the formation process or activation process.

[0005] The charger / discharger used in this activation process charges and discharges the battery cells according to a predetermined recipe (e.g., electrical electrodes for charging and discharging). The activation process is very important because the quality and sorting of each battery cell is determined based on the results of the charge / discharge operating characteristics.

[0006] Conventional chargers charge battery cells using a constant current-constant voltage (CC-CV) recipe. Charging using this CC-CV recipe (hereinafter referred to as "CC-CV charging") is a method in which a constant current is initially applied to a battery cell to quickly charge the battery cell to a set voltage, and then a constant voltage is applied while the current is continuously reduced so that the battery cell maintains the set voltage.

[0007] When charging each battery cell individually, CC-CV charging has the advantage of high charging accuracy because it can accurately charge the battery cells to a set voltage.

[0008] However, in the process of mass production of secondary batteries, it is preferable that a charger / discharger simultaneously charge and discharge a large number of battery cells in order to increase productivity. In particular, in order to reduce the cost of the charger / discharger and the area of ​​the charge / discharge chamber, it is necessary to introduce a series-type charger / discharger that can simultaneously charge and discharge a plurality of battery cells connected in series.

[0009] When such a serial charger / discharger performs CC-CV charging, there are problems in that the time required to fully charge multiple battery cells increases, and the voltage distribution of the fully charged multiple battery cells becomes very large, resulting in reduced charging accuracy. [Prior art documents] [Patent documents]

[0010] [Patent Document 1] KR10-2014-0117923A(2014.10.08) [Patent Document 2] KR10-2014-0134925A(2014.11.25) Summary of the Invention [Problem to be solved by the invention]

[0011] SUMMARY OF THE INVENTION An object of the present invention is to provide a battery cell charging method and charging system that can shorten charging time and improve charging accuracy. [Means for solving the problem]

[0012] A battery cell charging method according to an embodiment of the present invention is for a plurality of battery cells connected in series, and may include the steps of charging the plurality of battery cells with a first constant current, and cutting off the first constant current for battery cells that have reached a predetermined voltage; and, when the first constant current for the plurality of battery cells is cut off, charging the plurality of battery cells with a second constant current lower than the first constant current, and cutting off the second constant current for battery cells that have reached the predetermined voltage again.

[0013] The time for charging the plurality of battery cells with the first constant current may be longer than the time for charging with the second constant current.

[0014] The time for charging the plurality of battery cells with the first constant current may be 90% or more of the time required to fully charge the plurality of battery cells.

[0015] The drop from the first constant current to the second constant current may be stepwise.

[0016] The battery cell charging method may further include charging the plurality of battery cells with a third constant current lower than the second constant current when the second constant current is cut off for the plurality of battery cells, and cutting off the third constant current for the battery cells that have reached the set voltage.

[0017] In the charging with the first constant current, the first constant current may be cut off for one battery cell among the plurality of battery cells earlier than the other battery cells, and in the charging with the second constant current, the second constant current may be cut off for the one battery cell later than the other battery cells.

[0018] A voltage distribution among the plurality of battery cells immediately after the second constant current is cut off may be smaller than a voltage distribution among the plurality of battery cells immediately after the first constant current is cut off.

[0019] A battery cell charging system according to an embodiment of the present invention is for a plurality of battery cells connected in series, and may include: a charging circuit that charges the plurality of battery cells with a constant current; a voltage sensor that measures the voltage of the battery cells; a relay switch that bypasses the constant current so that the constant current is cut off to the battery cells when the voltage of the battery cells measured via the voltage sensor reaches a preset voltage; and a controller that controls the charging circuit to reduce the constant current to recharge the plurality of battery cells when the constant current is cut off to the plurality of battery cells.

[0020] The charging circuit can decrease the constant current in a stepwise manner.

[0021] The charging circuit may decrease the constant current at least three times until the plurality of battery cells are fully charged.

[0022] The constant current may be cut off at different times for at least some of the battery cells.

[0023] The voltage distribution among the battery cells may decrease with each constant current drop in the charging circuit. [Effects of the Invention]

[0024] According to a preferred embodiment of the present invention, the time required to fully charge a plurality of battery cells can be reduced, and the voltage distribution among the fully charged battery cells can be made uniform.

[0025] Furthermore, according to a preferred embodiment of the present invention, a correlation may be high between a capacity measured by discharging a plurality of charged battery cells and an actual capacity measured by individually charging and discharging each battery cell, thereby improving the reliability of determining and sorting the quality of each battery cell.

[0026] In addition, the present invention can include other effects that can be easily predicted by a person skilled in the art from the configuration of the preferred embodiment of the present invention.

[0027] The drawings attached to this specification illustrate preferred embodiments of the present invention and, together with the detailed description of the invention described below, serve to further understand the technical concept of the present invention. Therefore, the present invention should not be interpreted as being limited solely to the matters depicted in these drawings. [Brief explanation of the drawings]

[0028] [Figure 1] 1 is a schematic circuit diagram of a battery cell charging system according to one embodiment of the present invention; [Figure 2] 4 is a flowchart of a battery cell charging method according to another embodiment of the present invention. [Figure 3] 3 is a current characteristic diagram of a plurality of battery cells charged by the battery cell charging method shown in FIG. 2; [Figure 4] 3 is a voltage characteristic diagram of a plurality of battery cells charged by the battery cell charging method shown in FIG. 2; [Figure 5] 10 is a graph showing voltage distributions of a plurality of battery cells charged by battery cell charging methods according to experimental examples and comparative examples of the present invention; [Figure 6]10 is a graph showing a correlation between the measured capacity and the actual capacity of a battery cell charged by a battery cell charging method according to an experimental example of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0029]

[0033] The present invention will now be described in detail with reference to the accompanying drawings, in order to enable those skilled in the art to easily carry out the present invention. However, the present invention may be embodied in various different forms, and is not limited to the following embodiments.

[0030] In order to clearly explain the present invention, detailed descriptions of parts that are not relevant to the explanation or related known technologies that may unnecessarily obscure the gist of the present invention will be omitted, and when adding reference symbols to components in each figure in this specification, the same or similar reference symbols will be used for the same or similar components throughout the specification.

[0031] Furthermore, the terms and words used in this specification and claims should not be interpreted in a limited way to their ordinary or dictionary meanings, but should be interpreted as meanings and concepts that are consistent with the technical idea of ​​the present invention, in accordance with the principle that an inventor can appropriately define the concepts of terms in order to best explain his or her invention.

[0032] FIG. 1 is a schematic circuit diagram of a battery cell charging system according to one embodiment of the present invention.

[0033] A battery cell charging system (hereinafter referred to as "charging system") according to an embodiment of the present invention can charge a plurality of battery cells 10 connected in series. The charging system may be configured to be included in a series-type charger / discharger, but is not limited thereto.

[0034] The charging system may include a charging circuit 100 that charges a plurality of battery cells 10 with a constant current, an interrupting circuit 20 that interrupts the current flowing to the battery cells 10 based on the voltage of the battery cells 10, and a controller 200 that controls the charging circuit 100 and the interrupting circuit 20.

[0035] The charging circuit 100 may include a power supply or may be connected to an external power supply. The charging circuit 100 may be configured to supply a constant current to the plurality of battery cells 10. The charging circuit 100 may be configured to vary the magnitude of the constant current supplied to the plurality of battery cells 10.

[0036] There is no limitation on the configuration of the charging circuit 100. Since such a charging circuit 100 is a well-known technology, those skilled in the art can easily configure the charging circuit 100 using a number of electrical elements.

[0037] For example, the charging circuit 100 may include a plurality of resistors connected in parallel and having different magnitudes, and a plurality of switches connected in series with the plurality of resistors. In this case, the magnitude of the current flowing from a power source having a constant voltage can be varied by changing the combination of the plurality of resistors determined by turning on and off the plurality of switches.

[0038] A plurality of interruption circuits 20 may be provided corresponding to the plurality of battery cells 10. The number of battery cells 10 and the number of interruption circuits 20 are not limited.

[0039] For example, the plurality of battery cells 10 may include a first battery cell 10a, a second battery cell 10b, a third battery cell 10c, and a fourth battery cell 10d connected in series. The plurality of interruption circuits 20 may include a first interruption circuit 20a that interrupts the current flowing to the first battery cell 10a based on the voltage of the first battery cell 10a, a second interruption circuit 20b that interrupts the current flowing to the second battery cell 10b based on the voltage of the second battery cell 10b, a third interruption circuit 20c that interrupts the current flowing to the third battery cell 10c based on the voltage of the third battery cell 10c, and a fourth interruption circuit 20d that interrupts the current flowing to the fourth battery cell 10d based on the voltage of the fourth battery cell 10d.

[0040] Each interrupting circuit 20 can interrupt the current based on the voltage of each battery cell 10. Each interrupting circuit 20 may be connected in parallel with each battery cell 10.

[0041] Each interrupting circuit 20 may include a voltage sensor 21 and a relay switch 22. The voltage sensor 21 and the relay switch 22 may be connected in parallel to the battery cell, respectively.

[0042] The voltage sensor 21 can measure the voltage of the battery cell 10 .

[0043] The relay switch 22 can bypass the constant current to the battery cell 10 so that the constant current is cut off when the voltage of the battery cell 10 measured via the voltage sensor 21 reaches a preset voltage.

[0044] The controller 200 may include at least one processor. The controller 200 may control the charging circuit 100 to adjust the magnitude of the constant current provided by the charging circuit 100.

[0045] The controller 200 can control the on / off of the relay switch 22 based on the measured voltage of each voltage sensor 21. More specifically, when the voltage of the battery cell 10 measured via the voltage sensor 21 reaches a preset voltage, the controller 200 can close the relay switch 22 to bypass the constant current to the battery cell 10 so that the constant current is cut off.

[0046] Even if the plurality of battery cells 10 have the same capacity in design, slight differences in capacity may occur during the process of going through multiple processes for actually manufacturing the battery cells 10. Therefore, at least some of the plurality of battery cells 10 may reach a set voltage at different times. That is, the plurality of battery cells 10 may have the constant current cut off at different times.

[0047] A voltage drop may occur in the battery cell 10 where the constant current is interrupted. Therefore, voltage distribution may occur between the battery cells where the constant current is interrupted at different times. In order to increase the accuracy of charging, it is preferable to minimize such voltage distribution.

[0048] To this end, the controller 200 may control the charging circuit 100 to reduce the constant current and recharge the battery cells 10 when the constant current is cut off from the battery cells 10. The reduction in the constant current supplied from the charging circuit 100 may be performed in a stepwise manner.

[0049] More specifically, when all of the battery cells 10 reach the preset voltage and the constant current is cut off, the controller 200 reduces the constant current supplied by the charging circuit 100 and opens all of the relay switches 22 so that the reduced constant current flows to all of the battery cells 10.

[0050] Thereafter, when the voltage of the battery cell 10 measured via the voltage sensor 21 reaches the set voltage again, the controller 200 closes the relay switch 22 to bypass the constant current dropped to the battery cell 10 so that the constant current dropped to the battery cell 10 is cut off.

[0051] When the constant current dropped to the plurality of battery cells 10 is cut off, the controller 200 can control the charging circuit 100 to further drop the dropped constant current to recharge the plurality of battery cells 10.

[0052] As a series of processes in which the plurality of battery cells 10 reach a set voltage and the constant current is interrupted, and the charging circuit 100 reduces the constant current and resumes charging of the plurality of battery cells 10, is repeated, the voltage distribution among the plurality of battery cells 10 may become smaller. That is, the voltage distribution among the plurality of battery cells 10 may decrease each time the constant current is reduced in the charging circuit 100.

[0053] The series of steps may be repeated a predetermined number of times to complete charging of the plurality of battery cells 10. Preferably, the series of steps may be repeated at least three times. That is, the charging circuit 100 may decrease the constant current at least three times until charging of the plurality of battery cells 10 is completed. This may result in a sufficiently small voltage distribution among the plurality of battery cells 10, thereby improving charging accuracy.

[0054] FIG. 2 is a flowchart of a battery cell charging method according to another embodiment of the present invention, FIG. 3 is a current characteristic diagram of a plurality of battery cells charged by the battery cell charging method shown in FIG. 2, and FIG. 4 is a voltage characteristic diagram of a plurality of battery cells charged by the battery cell charging method shown in FIG. 2.

[0055] Hereinafter, a battery cell charging method performed by the charging system described above will be described as another embodiment of the present invention.

[0056] A battery cell charging method (hereinafter, referred to as a charging method) according to another embodiment of the present invention can charge a plurality of battery cells 10 connected in series.

[0057] The charging method includes a step of charging a plurality of battery cells 10 with a first constant current I1, in which the first constant current I1 is cut off for battery cells 10 that have reached a previously set voltage V1 (hereinafter referred to as the "first charging step"); and a step of charging the plurality of battery cells 10 with a second constant current I2 lower than the first constant current I1 when the first constant current I1 is cut off for battery cells 10 that have reached the set voltage V1 again (hereinafter referred to as the "second charging step").

[0058] The charging method may further include a step (hereinafter, "third charging step") in which, when the second constant current I2 is cut off for the plurality of battery cells 10, the plurality of battery cells 10 are charged with a third constant current I3 lower than the second constant current I2, and the third constant current I3 is cut off for the battery cells 10 that have reached the set voltage V1. Those skilled in the art will readily understand that the charging method may further include a fourth charging step, a fifth charging step, and so on.

[0059] 3 and 4 show an example in which the charging method includes first to fifth charging stages.

[0060] The current characteristic diagram of FIG. 3 shows that the first constant current I1 to the fifth constant current I5 applied to the plurality of battery cells 10 decrease stepwise.

[0061] The voltage characteristics diagram of Fig. 4 shows that the voltages of the four battery cells 10 increase while the constant currents I1 to I5 are applied, and when each battery cell 10 reaches a set voltage V1, the constant currents I1 to I5 are cut off, causing a voltage drop. In the voltage characteristics diagram of Fig. 4, the solid lines indicate the voltage changes of each battery cell 10 while the constant currents I1 to I5 are applied, and the dotted lines indicate the voltage changes of each battery cell 10 while the constant currents I1 to I5 are cut off.

[0062] The first charging stage will be described below. The first charging stage may be a section before time 't1' shown in FIGS.

[0063] The charging circuit 100 may charge the plurality of battery cells 10 with a first constant current I1 (S11). As a result, the voltage of each battery cell 10 may continuously increase and reach a preset voltage V1. However, as described above, since at least some of the plurality of battery cells 10 have different capacities, at least some of the plurality of battery cells 10 may reach the preset voltage V1 at different times.

[0064] The controller 200 determines whether any battery cell 10 has reached the set voltage V1, and can cut off the first constant current I1 for the battery cell 10 that has reached the set voltage V1 (S12) (S13). More specifically, the controller 200 can determine whether each battery cell 10 has reached the set voltage V1 using a voltage sensor 21 that detects the voltage of each battery cell 10. The controller 200 can cut off the first constant current I1 for the battery cell 10 that has reached the set voltage V1 by controlling the relay switch 22 so that the first constant current I1 bypasses the battery cell 10 that has reached the set voltage V1. As a result, a voltage drop may occur in the battery cell 10 for which the first constant current I1 is cut off.

[0065] At least some of the plurality of battery cells 10 reach the set voltage V1 at different times, and therefore the first constant current I1 may be cut off at different times for at least some of the plurality of battery cells 10. The earlier the battery cells 10 reach the set voltage V1, the longer the time for which the first constant current I1 is cut off, and therefore, a larger voltage drop may occur.

[0066] For ease of explanation, the following description will be given taking as an example a case where the capacity increases from the first battery cell 10a to the fourth battery cell 10d. The time required to reach the set voltage V1 may be longer from the first battery cell 10a to the fourth battery cell 10d, and the first constant current I1 may be cut off later. Therefore, at the end of the first charging stage, a smaller voltage drop may occur from the first battery cell 10a to the fourth battery cell 10d. Because the degree of voltage drop varies among the plurality of battery cells 10, voltage distribution may occur among the plurality of battery cells 10 at the end of the first charging stage.

[0067] To reduce such voltage distribution, a second charging stage may be performed.

[0068] The second charging stage will be described below. The second charging stage may be the period between "t1" and "t2" shown in FIGS.

[0069] When the first constant current I1 is cut off for all of the battery cells 10, the charging circuit 100 may charge the battery cells 10 with a second constant current I2 (S21, n=2). The drop from the first constant current I1 to the second constant current I2 may be stepwise.

[0070] As a result, the voltage of each battery cell 10 in which the voltage drop occurred can increase again, and can reach the set voltage V1 again.

[0071] At least some of the plurality of battery cells 10 have different amounts of voltage drop that occurred during the first charging stage, so at least some of the plurality of battery cells 10 can reach the set voltage V1 again at different times.

[0072] The controller 200 determines whether any battery cell 10 has reached the set voltage V1 again, and can cut off the second constant current I2 for the battery cell 10 that has reached the set voltage V1 (S22)(S23)(n=2). More specifically, the controller 200 can determine whether each battery cell 10 has reached the set voltage V1 again using a voltage sensor 21 that detects the voltage of each battery cell 10. The controller 200 can control the relay switch 22 so that the second constant current I2 bypasses the battery cell 10 that has reached the set voltage V1, and can cut off the second constant current I2 for the battery cell 10 that has reached the set voltage V1. As a result, a voltage drop may occur in the battery cell 10 for which the second constant current I2 is cut off.

[0073] At least some of the plurality of battery cells 10 may reach the set voltage V1 again at different times, and therefore the second constant current I2 may be cut off for at least some of the plurality of battery cells 10 at different times.

[0074] More specifically, the earlier the first constant current I1 is cut off in the first charging stage, the larger the voltage drop in the battery cell 10, and therefore the later the battery cell 10 may reach the set voltage V1 in the second charging stage. That is, in the first charging stage, the first constant current I1 may be cut off for one battery cell 10 earlier than the other battery cells 10, and in the second charging stage, the second constant current I2 may be cut off for the one battery cell 10 later than the other battery cells 10.

[0075] For example, if the voltage drop is smaller from the first battery cell 10a to the fourth battery cell 10d at the end of the first charging stage, then in the second charging stage, the time it takes to reach the set voltage V1 may become shorter from the first battery cell 10a to the fourth battery cell 10d, and the second constant current I2 may be quickly cut off. Therefore, at the end of the second charging stage, the voltage drop may be larger from the first battery cell 10a to the fourth battery cell 10d, in contrast to the end of the first charging stage.

[0076] However, the voltage distribution among the plurality of battery cells 10 may decrease at the end of the second charging stage compared to the end of the first charging stage because the voltage of each battery cell 10 is higher at the start of the second charging stage than at the start of the first charging stage, and therefore the time it takes for the plurality of battery cells 10 to reach the set voltage V1 again in the second charging stage is shorter than the time it takes for the plurality of battery cells 10 to reach the set voltage V1 in the first charging stage.

[0077] To further reduce the voltage distribution, a third charging stage may be performed.

[0078] The third charging stage will be described below. The third charging stage may be the section between "t2" and "t3" shown in FIGS.

[0079] When the second constant current I2 is cut off for all of the battery cells 10, the charging circuit 100 may charge the battery cells 10 with a third constant current I3 (S21, n=3). The drop from the second constant current I2 to the third constant current I3 may be stepwise.

[0080] As a result, the voltage of each battery cell 10 in which the voltage drop occurred can increase again, and can reach the set voltage V1 again.

[0081] At least some of the plurality of battery cells 10 have different amounts of voltage drop that occurred during the second charging stage, so that at least some of the plurality of battery cells 10 can reach the set voltage V1 again at different times.

[0082] The controller 200 determines whether any battery cell 10 has reached the set voltage V1 again, and can cut off the third constant current I3 for the battery cell 10 that has reached the set voltage V1 (S22)(S23)(n=3). More specifically, the controller 200 can determine whether each battery cell 10 has reached the set voltage V1 again using a voltage sensor 21 that detects the voltage of each battery cell 10. The controller 200 controls the relay switch 22 so that the third constant current I3 bypasses the battery cell 10 that has reached the set voltage V1, and can cut off the third constant current I3 for the battery cell 10 that has reached the set voltage V1. As a result, a voltage drop may occur in the battery cell 10 for which the third constant current I3 is cut off.

[0083] At least some of the plurality of battery cells 10 may reach the set voltage V1 again at different times, and therefore the third constant current I3 may be cut off for at least some of the plurality of battery cells 10 at different times.

[0084] More specifically, the earlier the second constant current I2 is cut off in the second charging stage, the larger the voltage drop in the battery cell 10, and therefore the later the battery cell 10 may reach the set voltage V1 in the third charging stage. That is, in the second charging stage, the second constant current I2 may be cut off in one battery cell 10 earlier than the other battery cells 10, and in the third charging stage, the third constant current I3 may be cut off in the one battery cell 10 later than the other battery cells 10.

[0085] For example, if a larger voltage drop occurs from the first battery cell 10a to the fourth battery cell 10d at the end of the second charging stage, in the third charging stage, it may take longer for the first battery cell 10a to the fourth battery cell 10d to reach the set voltage V1, and the third constant current I3 may be cut off later. Therefore, at the end of the third charging stage, in contrast to the end of the second charging stage, a smaller voltage drop may occur from the first battery cell 10a to the fourth battery cell 10d.

[0086] However, the voltage distribution among the plurality of battery cells 10 may be further reduced at the end of the third charging stage compared to the end of the second charging stage because the voltage of each battery cell 10 is higher at the start of the third charging stage than at the start of the second charging stage, and therefore the time it takes for the plurality of battery cells 10 to re-arrive at the set voltage V1 in the third charging stage is shorter than the time it takes for the plurality of battery cells 10 to re-arrive at the set voltage V1 in the second charging stage.

[0087] The charging method may repeat such charging steps a predetermined number of times (n) to complete charging of the plurality of battery cells 10. As an example, if the charging steps are repeated three times, the charging method may include first to third charging steps.

[0088] As another example, if the charging step is repeated five times, the charging method may include charging steps 1 to 5. Those skilled in the art will be able to easily understand the fourth and fifth charging steps from the above description of charging steps 1 to 3.

[0089] By repeating such charging steps, the voltage distribution among the battery cells 10 can be reduced, and the accuracy of charging can be improved.

[0090] On the other hand, the duration of each charging stage may become shorter as the charging stages are repeated. This is because the amount of voltage drop in each battery cell 10 decreases as the charging stages are repeated, shortening the time it takes for the voltage to reach the set voltage again from the dropped voltage. It should be noted that "t1" to "t5" shown in Figures 3 and 4 do not reflect such time differences and are shown uniformly for ease of understanding.

[0091] The duration t1 of the first charging stage may be longer than the duration t2-t1 of the second charging stage. More specifically, the time t1 for charging the plurality of battery cells 10 at the first constant current I1 may be longer than the time t2-t1 for charging at the second constant current I2.

[0092] Furthermore, the duration t2-t1 of the second charging stage may be longer than the duration t3-t2 of the third charging stage. More specifically, the time t2-t1 during which the plurality of battery cells 10 are charged at the second constant current I2 may be longer than the time t3-t2 during which the plurality of battery cells 10 are charged at the third constant current I3.

[0093] Meanwhile, the first charging stage may be a stage for quickly charging the plurality of battery cells 10, and the stages after the first charging stage (second charging stage, third charging stage, etc.) may be stages for reducing the voltage distribution among the plurality of battery cells 10.

[0094] The duration t1 of the first charging stage may be 90% or more of the time required to fully charge the battery cells 10. More specifically, the time t1 for charging the battery cells 10 at the first constant current I1 may be 90% or more of the time required to fully charge the battery cells 10.

[0095] For example, if the charging method includes first to fifth charging stages, the duration t1 of the first charging stage may be 90% or more of the sum of the duration t1 of the first charging stage, the duration t2-t1 of the second charging stage, the duration t3-t2 of the third charging stage, the duration t4-t3 of the fourth charging stage, and the duration t5-t4 of the fifth charging stage.

[0096] Since the first charging stage, in which the plurality of battery cells 10 are charged with the largest constant current, i.e., the first constant current I1, accounts for the majority of the entire charging process, the plurality of battery cells 10 can be charged quickly, and the time required to complete charging can be reduced.

[0097] FIG. 5 is a graph showing voltage distributions of a plurality of battery cells charged by the battery cell charging methods according to the experimental examples and comparative examples of the present invention.

[0098] The above description applies to the charging method according to the experimental example of the present invention.

[0099] The charging method according to the comparative example may refer to a constant current-constant voltage (CC-CV) charging recipe. This CC-CV charging recipe is a conventional technique and will be briefly described below.

[0100] According to a CC-CV charging recipe, a constant current is applied to a plurality of battery cells connected in series. When one of the battery cells reaches a set voltage, a constant voltage is subsequently applied to the plurality of battery cells. The constant voltage may be a voltage that maintains each battery cell at a set voltage, and may vary depending on which battery cell is used as a reference. That is, a charging method according to a comparative example may include multiple steps of applying a constant voltage set based on each battery cell, and the number of steps may correspond to the number of battery cells. As a result, the total charging time may be longer as the number of battery cells connected in series increases.

[0101] The current flowing through a battery cell to which a constant voltage is applied may decrease continuously.

[0102] A battery cell that has become a constant voltage setting reference in each stage may have current cut off in the subsequent stage. Therefore, as charging continues, the number of battery cells to which current is applied decreases. However, a voltage drop occurs in the battery cell from which current is cut off while other battery cells are being charged. As a result, at the end of charging the battery cells, the voltage distribution among the battery cells may become very large.

[0103] In the experimental examples and comparative examples of the present invention, the set voltage was 4.2 V, the number of battery cells was 16, and each battery cell was a cylindrical battery with a form factor of 4680. Here, the form factor refers to values ​​indicating the diameter and height of the cylindrical battery. In the numerical value indicating the form factor, the first two digits indicate the diameter of the battery, and the next digit indicates the height of the battery. In other words, each battery cell was a cylindrical battery with a diameter of approximately 46 mm and a height of approximately 80 mm.

[0104] The "CC-CV voltage" in Figure 5 represents data on the final voltages of a plurality of battery cells charged by the charging method according to the comparative example, and the "stepped CC voltage" represents data on the final voltages of a plurality of battery cells charged by the charging method according to the experimental example of the present invention. The "density" in Figure 5 indicates the ratio of the number of battery cells having a particular final voltage among the plurality of battery cells. In other words, a higher density at a particular voltage may indicate a larger number of battery cells having a final voltage of the particular voltage.

[0105] Referring to FIG. 5, it can be seen that the final voltages of the battery cells charged by the charging method according to the comparative example are distributed in a wide range and have a large voltage distribution, while the final voltages of the battery cells charged by the charging method according to the experimental example of the present invention are concentrated in a narrow range (around approximately 4.196 V) and have a small voltage distribution.

[0106] Quantitatively, the voltage distribution (standard deviation) of the battery cells charged by the charging method according to the comparative example was 4.802, while the voltage distribution (standard deviation) of the battery cells charged by the charging method according to the experimental example of the present invention was 0.2062, which was confirmed to be an improvement of approximately 23 times.

[0107] FIG. 6 is a graph showing the correlation between the measured capacity and the actual capacity of a battery cell charged by a battery cell charging method according to an experimental example of the present invention.

[0108] 6 may refer to a capacity measured by individually charging each battery cell at a constant current and a constant voltage and then discharging the battery cell (hereinafter referred to as "actual capacity"). And, the "stepped CC capacity" may refer to a capacity measured by charging a plurality of series-connected battery cells using a charging method according to an example of the present invention and then discharging the battery cell (hereinafter referred to as "first measured capacity").

[0109] When mass-producing battery cells, the higher the correlation between the measured capacity and the actual capacity, the more reliable the determination and sorting of the quality of the battery cells can be.

[0110] 6, it can be seen that the first measured capacity has a positive correlation with the actual capacity. Quantitatively, the coefficient of determination (R-squared) between the first measured capacity and the actual capacity was found to be 43.0%.

[0111] Meanwhile, although not shown, it was confirmed that the coefficient of determination between the capacity measured by charging and then discharging multiple battery cells connected in series using a charging method according to a comparative example (hereinafter referred to as the "second measured capacity") and the actual capacity was 0.3%.

[0112] That is, it was confirmed that the first measured capacity was 43%, which was an improvement of approximately 143 times over the second measured capacity of 0.3%.

[0113] In conclusion, according to the present invention, the correlation between the capacity measured by discharging a plurality of charged battery cells and the actual capacity measured by individually charging and discharging each battery cell can be increased, thereby improving the reliability of determining and sorting the quality of each battery cell.

[0114] The above description is merely an illustrative example of the technical concept of the present invention, and various modifications and variations may be made by a person having ordinary skill in the art to which the present invention pertains without departing from the essential characteristics of the present invention.

[0115] Therefore, the embodiments disclosed in the present invention are intended to illustrate, rather than limit, the technical idea of ​​the present invention, and the scope of the technical idea of ​​the present invention is not limited by such embodiments.

[0116] The scope of protection of the present invention should be interpreted by the following claims, and all technical ideas within the scope equivalent thereto should be interpreted as being included in the scope of the present invention. [Explanation of symbols]

[0117] 10: Battery cell 20: Intermittent Circuit 21: Voltage sensor 22: Relay switch 100: Charging circuit 200: Controller

Claims

1. A battery cell charging method for a plurality of battery cells connected in series, comprising: charging the plurality of battery cells with a first constant current, and cutting off the first constant current for battery cells that have reached a preset voltage; and When the first constant current is cut off for the plurality of battery cells, the plurality of battery cells are charged with a second constant current lower than the first constant current, and at this time, the second constant current is cut off for the battery cells that have reached the set voltage again. Including, In the charging with the first constant current, the first constant current is cut off for one battery cell among the plurality of battery cells earlier than the other battery cells; In the charging with the second constant current, the second constant current is cut off for the one battery cell later than the other battery cells; The plurality of battery cells are started to be charged with the second constant current when a voltage drop amount of one battery cell is larger than a voltage drop amount of the other battery cells. Battery cell charging method.

2. 2. The method of claim 1, wherein the time for charging the plurality of battery cells with the first constant current is longer than the time for charging the plurality of battery cells with the second constant current.

3. 2. The battery cell charging method of claim 1, wherein the time required to charge the plurality of battery cells with the first constant current is 90% or more of the time required to fully charge the plurality of battery cells.

4. 2. The method of claim 1, wherein the drop from the first constant current to the second constant current is in a stepped manner.

5. 2. The battery cell charging method of claim 1, further comprising: charging the plurality of battery cells with a third constant current lower than the second constant current when the second constant current is cut off for the plurality of battery cells; and cutting off the third constant current for the battery cells that have reached the set voltage.

6. 6. The battery cell charging method of claim 1, wherein a voltage distribution among the plurality of battery cells immediately after the second constant current is cut off to the plurality of battery cells is smaller than a voltage distribution among the plurality of battery cells immediately after the first constant current is cut off to the plurality of battery cells.

7. A battery cell charging system for a plurality of battery cells connected in series, a charging circuit for charging the plurality of battery cells with a constant current; a voltage sensor for measuring the voltage of the battery cell; a relay switch that bypasses the constant current to cut off the constant current to the battery cell when the voltage of the battery cell measured via the voltage sensor reaches a preset voltage; and a controller that controls the charging circuit to reduce the constant current to recharge the battery cells when the constant current is interrupted; Including, the controller controls the charging circuit to charge the plurality of battery cells with a first constant current; the relay switch cuts off the first constant current for a battery cell that has reached the set voltage when the plurality of battery cells are being charged with the first constant current; the controller controls the charging circuit to charge the battery cells with a second constant current lower than the first constant current when the first constant current is cut off; the relay switch cuts off the second constant current for a battery cell that has reached the set voltage when the battery cells are being charged with the second constant current; the relay switch cuts off the first constant current to one of the battery cells earlier than the other battery cells when the battery cells are being charged with the first constant current; the relay switch cuts off the second constant current to the one battery cell later than the other battery cells when the plurality of battery cells are being charged with the second constant current; The controller controls the charging circuit to start charging the battery cells with the second constant current when a voltage drop in one battery cell is greater than a voltage drop in the other battery cells. Battery cell charging system.

8. 8. The battery cell charging system of claim 7, wherein said charging circuit reduces said constant current in a stepped manner.

9. The charging circuit 8. The battery cell charging system of claim 7, wherein the constant current is decreased at least three times until the plurality of battery cells are fully charged.

10. The battery cell charging system according to claim 7 , wherein the constant current is interrupted at different times for at least some of the plurality of battery cells.

11. 11. The battery cell charging system of claim 7, wherein the voltage distribution among the plurality of battery cells decreases with each constant current drop in the charging circuit.

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