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 dated February 11, 2022 and Korean Patent Application No. 10-2022-0133592 dated 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 a system for charging a battery cell, and more particularly to a method and a system for charging a plurality of battery cells connected in series. [Background technology]

[0003] The widespread use of electric vehicles and other devices due to technological developments and increasing demand for mobile devices has led to a rapid increase in demand for secondary batteries as an energy source, with the demand for lithium secondary batteries, which have high capacity and energy density, being particularly high.

[0004] In general, a secondary battery is manufactured by placing an electrode assembly consisting of a negative electrode, a positive electrode, and a separator in a cylindrical or rectangular metal can or a pouch-type case made of an aluminum laminate sheet, and injecting an electrolyte into the electrode assembly. The secondary battery manufactured in this manner must be activated by performing a predetermined charge and discharge in order to function as a battery, and this process is called a formation process or an activation process.

[0005] The charger / discharger used in the activation process charges and discharges the battery cells according to a predetermined recipe (such as electrical electrodes for charging and discharging). The activation process is very important because the quality and non-quality of each battery cell is judged and sorted according to 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 applied to a battery cell at the beginning to quickly charge the battery cell to a set voltage, and then a constant voltage is applied to the battery cell while continuously reducing the current so that the battery cell maintains the set voltage.

[0007] When each battery cell is charged individually, CC-CV charging has the advantage of being able to accurately charge the battery cells to a set voltage, resulting in high charging accuracy.

[0008] However, in the process of mass production of secondary batteries, it is preferable for a charger / discharger to simultaneously charge and discharge a 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 number 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 complete charging of multiple battery cells increases, and the voltage distribution of multiple battery cells that have completed charging becomes very large, reducing the accuracy of charging. [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] An object of the present invention is to provide a battery cell charging method and charging system capable of shortening a charging time and improving 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. 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.

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

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

[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 a step of 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 at this time, 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 between the plurality of battery cells immediately after the second constant current is cut off to the plurality of battery cells may be smaller than a voltage distribution between the plurality of battery cells immediately after the first constant current is cut off to the plurality of battery cells.

[0019] A battery cell charging system according to an embodiment of the present invention is for a plurality of battery cells connected in series. The battery cell charging system may include a charging circuit for charging the plurality of battery cells with a constant current, a voltage sensor for measuring a voltage of the battery cells, a relay switch for bypassing 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 for controlling 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 stepped manner.

[0021] The charging circuit is capable of decreasing 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 each time a constant current drop occurs in the charging circuit. Effect 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] In addition, according to a preferred embodiment of the present invention, a correlation may be high between a capacity measured by discharging a plurality of battery cells charged 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 according to 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 only to the matters depicted in these drawings. [Brief description 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; [Diagram 2] 4 is a flowchart of a battery cell charging method according to another embodiment of the present invention. [Diagram 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. [Diagram 5] 4 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]1 is a graph showing a correlation between a measured capacity and an actual capacity of a battery cell charged by a battery cell charging method according to an embodiment of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0029] In the following, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings so that those skilled in the art can 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 description or related prior art that may unnecessarily obscure the gist of the present invention will be omitted, and in 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 the claims should not be interpreted in a limited manner to their ordinary or dictionary meanings, but should be interpreted as having meanings and concepts that are consistent with the technical ideas of the present invention, in accordance with the principle that an inventor may 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, "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 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 source or may be coupled to an external power source. 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, a person 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 amount of 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 numbers of battery cells 10 and 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 that are sequentially connected in series. The plurality of on-off circuits 20 may include a first on-off circuit 20a that turns on and off a current flowing through the first battery cell 10a based on a voltage of the first battery cell 10a, a second on-off circuit 20b that turns on and off a current flowing through the second battery cell 10b based on a voltage of the second battery cell 10b, a third on-off circuit 20c that turns on and off a current flowing through the third battery cell 10c based on a voltage of the third battery cell 10c, and a fourth on-off circuit 20d that turns on and off a current flowing through the fourth battery cell 10d based on a voltage of the fourth battery cell 10d.

[0040] Each of the interrupting circuits 20 can interrupt the current based on the voltage of each of the battery cells 10. Each of the interrupting circuits 20 may be connected in parallel with each of the battery cells 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 a number of 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 constant current may be cut off for the plurality of battery cells 10 at different times.

[0047] A voltage drop may occur in the battery cell 10 where the constant current is cut off. Therefore, voltage distribution may occur between the battery cells where the constant current is cut off at different timings. In order to increase the accuracy of charging, it is preferable to reduce such voltage distribution as much as possible.

[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 interrupted from the battery cells 10. The reduction in the constant current supplied from the charging circuit 100 may be performed in a stepped manner.

[0049] More specifically, when all of the multiple battery cells 10 reach a preset voltage and the constant current is cut off, the controller 200 can lower the constant current supplied by the charging circuit 100 and open all of the relay switches 22 so that the lowered 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 can close 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] The more the 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 the plurality of battery cells 10, is repeated, the smaller the voltage distribution among the plurality of battery cells 10 may become. 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 preset 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 drop the constant current at least three times until the 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 above-described charging system will be described as another embodiment of the present invention.

[0056] A battery cell charging method (hereinafter, a charging method) according to another embodiment of the present invention may 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 the battery cells 10 that have reached a previously set voltage V1 (hereinafter, a "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 the battery cells 10 that have reached the set voltage V1 again, in which the second constant current I2 is cut off (hereinafter, a "second charging step").

[0058] The charging method may further include a step of charging the battery cells 10 with a third constant current I3 lower than the second constant current I2 when the second constant current I2 is cut off, and cutting off the third constant current I3 for the battery cells 10 that have reached the set voltage V1 (hereinafter, "third charging step"). Those skilled in the art will easily 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 a first charging stage to a fifth charging stage.

[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 characteristic 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 and a voltage drop occurs. In the voltage characteristic 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, at least some of the plurality of battery cells 10 have different capacities, so that at least some of the plurality of battery cells 10 may reach the preset voltage V1 at different times.

[0064] The controller 200 can determine whether there is a battery cell 10 that 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 by 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 battery cells 10 reach the set voltage V1 at different times, so the first constant current I1 may be cut off at different times for at least some of the 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 the more voltage drop may occur.

[0066] For ease of explanation, the following description will be given by 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 become 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. As such, since there is a difference in the degree of voltage drop among the plurality of battery cells 10, a 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 a section 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 performed in a stepwise manner.

[0070] As a result, the voltage of each battery cell 10 in which a voltage drop has 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 that at least some of the plurality of battery cells 10 can reach the set voltage V1 again at different times.

[0072] The controller 200 can determine whether there is a battery cell 10 that 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 by 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 timings, and therefore the second constant current I2 may be cut off for at least some of the plurality of battery cells 10 at different timings.

[0074] More specifically, the earlier the first constant current I1 is cut off in the first charging stage, the larger the voltage drop is, so that the later the set voltage V1 is reached in the second charging stage. That is, in the first charging stage, the first constant current I1 may be cut off in 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 in 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, the time to reach the set voltage V1 may become shorter from the first battery cell 10a to the fourth battery cell 10d in the second charging stage, and the second constant current I2 may be cut off quickly. 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 battery cells 10 may be reduced 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 the battery cells 10 to reach the set voltage V1 again in the second charging stage is shorter than the time it takes the 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 now be described. The third charging stage may be the time period 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 a voltage drop has 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 can determine whether there is a battery cell 10 that 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 by 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 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 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 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 is, so that the later the set voltage V1 is reached 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, the time it takes to reach the set voltage V1 may be longer from the first battery cell 10a to the fourth battery cell 10d, 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 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 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 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 preset 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 a first charging step, a second charging step, a third charging step, and a third charging step.

[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, and the time it takes for the voltage to reach the set voltage again from the dropped voltage becomes shorter. It should be made clear that "t1" to "t5" shown in Figures 3 and 4 do not reflect such a time difference, but are shown uniformly to facilitate 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 with the first constant current I1 may be longer than the time t2-t1 for charging with the second constant current I2.

[0092] Also, 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 with the second constant current I2 may be longer than the time t3-t2 during which the plurality of battery cells 10 are charged with 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 duration t1 of charging the battery cells 10 with the first constant current I1 may be 90% or more of the time required to fully charge the battery cells 10.

[0095] For example, when the charging method includes a first charging stage to a fifth charging stage, 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 and comparative examples of the present invention.

[0098] The above description is applicable 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. Since this CC-CV charging recipe is a conventional technique, it will be described briefly.

[0100] According to the CC-CV charging recipe, a constant current is applied to a plurality of battery cells connected in series, and when any one of the battery cells reaches a set voltage, a constant voltage can be applied to the plurality of battery cells thereafter. The constant voltage can be a voltage that causes each battery cell to maintain a set voltage, and can vary depending on which battery cell is set as a reference. That is, the charging method according to the comparative example can include a plurality of steps of applying a constant voltage set on the basis of each battery cell, and the number of the plurality of steps can correspond to the number of the plurality of battery cells. As a result, the total charging time can 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 the 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 the 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 is 4.2V, the number of battery cells is 16, and each battery cell is a cylindrical battery having a form factor of 4680. Here, the form factor means a value indicating the diameter and height of the cylindrical battery. In the 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. That is, each battery cell is a cylindrical battery with a diameter of approximately 46 mm and a height of approximately 80 mm.

[0104] "CC-CV voltage" in Fig. 5 is data showing the final voltage of a plurality of battery cells charged by the charging method according to the comparative example, and "stepped CC voltage" is data showing the final voltage of a plurality of battery cells charged by the charging method according to the experimental example of the present invention. And, "density" in Fig. 5 indicates the ratio of the number of battery cells having a certain final voltage among the plurality of battery cells. In other words, a higher density at a specific voltage may mean a larger number of battery cells having a final voltage of the specific 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 diverse manner 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 multiple battery cells charged by the charging method according to the comparative example was 4.802, while the voltage distribution (standard deviation) of multiple battery cells charged by the charging method according to the experimental example of the present invention was 0.2062, which was 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 embodiment of the present invention.

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

[0109] When battery cells are mass-produced, 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, it was confirmed that the coefficient of determination (R-squared) between the first measured capacity and the actual capacity was 43.0%.

[0111] Meanwhile, although not shown in the figure, 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, 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 improved by approximately 143 times compared to 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 improved, 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 explain, rather than to 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 according to 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 battery cells with a first constant current, and cutting off the first constant current for a battery cell that has already reached a preset voltage; and When the first constant current is cut off for the battery cells, the battery cells are charged with a second constant current lower than the first constant current, and the second constant current is cut off for the battery cells that have reached the set voltage again. A method for charging a battery cell, including:

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

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

4. 2. The method of claim 1, wherein the drop from the first constant current to the second constant current is stepwise.

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

6. In the charging with the first constant current, the first constant current is cut off for one of the battery cells earlier than the other battery cells, 2 . The method of claim 1 , wherein 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.

7. 7. 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.

8. A battery cell charging system for a plurality of battery cells connected in series, comprising: a charging circuit for charging the plurality of battery cells with a constant current; a voltage sensor for measuring a voltage of the battery cell; a relay switch for bypassing 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 for controlling the charging circuit to reduce the constant current to recharge the battery cells when the constant current is interrupted to the battery cells; a battery cell charging system.

9. 9. The battery cell charging system of claim 8, wherein said charging circuit ramps down said constant current in a stepped manner.

10. The charging circuit includes:

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

11. 9. The battery cell charging system of claim 8, wherein the constant current is interrupted for at least some of the plurality of battery cells at different times.

12. 12. The battery cell charging system of claim 8, wherein the voltage distribution among the plurality of battery cells is reduced each time a constant current drop is applied in the charging circuit.

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