Patent Description:
The present invention relates to a low-voltage defect inspection method significantly reducing inspection time with regard to inspecting low voltage defective batteries, and a manufacturing method of lithium secondary battery using thereto.

As the technology development and demand for mobile devices increase, the demand for secondary batteries as an energy source is rapidly increasing, and among them, lithium secondary batteries exhibiting high energy density, high operating potential, long cycle life, and low self-discharge rate have been commercialized and widely used.

The lithium secondary battery goes through an activation process after an electrode assembly is assembled together with an electrolyte inside a battery case. The activation process stabilizes the battery structure and makes it usable through processes of charging, aging, and discharging the assembled battery.

These lithium secondary batteries may have various types of defects due to various causes during the manufacturing process or during use. Particularly, some of the secondary batteries that have been manufactured show a phenomenon in which a voltage drop behavior is higher than the self-discharge rate, and this phenomenon is referred to as low voltage.

In many cases, such a low-voltage defect phenomenon of a secondary battery is typically caused by a metal foreign material located in the inside. In particular, when a metal foreign material such as iron or copper is present in a positive electrode plate of the secondary battery, the foreign material may grow into a dendrite in a negative electrode. In addition, such dendrites cause an internal short circuit of the secondary battery, which may cause malfunction or damage of the secondary battery, and in severe cases, it may cause ignition. Accordingly, the manufacturing process of the secondary battery includes a process of screening low-voltage defective batteries.

Conventionally, in order to screen low-voltage defective batteries, OCV (Open Circuit Voltage) is measured at two selected time points during an aging process, and the change value (voltage drop) of OCV is compared to a reference value to determine the secondary battery having voltage drop that exceeds the reference value as defective.

However, since the change in voltage should be monitored for several days or tens of days to screen low-voltage defective batteries by the above method, it takes a lot of time to screen low-voltage defective batteries, resulting in a decrease in productivity. Therefore, it is necessary to develop a technology capable of reducing the time required to screen low-voltage defective batteries. Document <CIT> Al discloses examples of method for diagnosing a low voltage of a secondary cell, wherein the method includes pre-aging a battery cell, charging the battery cell according to a preset charging condition, measuring a parameter for determining low voltage failure of the battery cell, comparing the measured parameter with a reference parameter, and performing formation when the battery cell is determined to be normal.

The present invention is directed to solve the above problems, and provides a method for reducing the time required for screening low-voltage defective batteries.

An inspection method for low-voltage defect of a lithium secondary battery according to the present invention includes: a microcurrent charging/discharging process consisting of N (N is an integer of <NUM> or more) charging/discharging sections, and applying a microcurrent to a battery cell in each of the charging/discharging section; measuring the change in voltage before and after each of the charging/discharging section; and a screening process of screening low-voltage defective batteries based on the change in voltage, wherein each of the charging/discharging section includes applying a current at a current rate of <NUM>. 000001C to <NUM>.

In an exemplary embodiment of the present invention, the current rate of each of the charging/discharging section may increase stepwise or may decrease stepwise as the N increases.

In an exemplary embodiment of the present invention, the microcurrent charging/discharging process may apply a constant current during the charging/discharging through the microcurrent application.

In an exemplary embodiment of the present invention, the microcurrent charging/discharging process may perform the charging/discharging for <NUM> minute to <NUM> minutes, respectively, in one charging/discharging section.

In an exemplary embodiment of the present invention, the microcurrent charging/discharging process may further include a resting period at least one or more of between the charging/discharging or between the charging/discharging sections.

In an exemplary embodiment of the present invention, the resting period may stop charging or discharging for <NUM> minute to <NUM> minutes.

In an exemplary embodiment of the present invention, in the microcurrent charging/discharging process, the N is an integer from <NUM> to <NUM>.

In an exemplary embodiment of the present invention, the microcurrent charging/discharging process may have the same charging rate and discharging rate in one charging/discharging section.

In an exemplary embodiment of the present invention, the total time required for the microcurrent charging/discharging process may be <NUM> minutes to <NUM> hours.

In an exemplary embodiment of the present invention, the screening process may include: plotting the current applied in each charging/discharging section and the corresponding change in voltage per unit time (dV/dt); predicting the voltage drop at the point where the applied current is <NUM> in the graph derived through the above plotting process.

A manufacturing method of a lithium secondary battery according to the present invention includes: activating by charging an assembled battery cell; and the low-voltage defect inspection process.

In an exemplary embodiment of the present invention, the low-voltage defect inspection process is performed at any one or more selected from the following time points (<NUM>) to (<NUM>).

In an exemplary embodiment of the present invention, the low-voltage defect inspection process is performed after performing a stabilization process of stabilizing the battery cell at a certain temperature after a time point selected among (<NUM>) to (<NUM>).

The low-voltage defect inspection method according to the present invention has an effect of improving productivity by significantly reducing inspection time, which previously took several days to several tens of days, to within several hours.

Hereinafter, preferred exemplary embodiments of the present invention will be described in detail with reference to the accompanying drawings. Prior to this, the terms or the words used in this specification and claims should not be construed as being limited to ordinary or dictionary meanings, and should be interpreted as meanings and concepts consistent with the technical idea of the present invention based on the principle that the inventor can appropriately define the concept of the terms in order to explain his/her invention in the best way.

Therefore, the exemplary embodiments described in this specification and the configurations shown in the drawings are only one of the most preferred exemplary embodiments of the present invention, and do not represent all the technical ideas of the present invention, so it should be understood that there may be various equivalents and variations that may substitute them at the time of this application.

<FIG> is a flowchart of a low-voltage defect battery detection method according to an exemplary embodiment of the present invention, <FIG> is a schematic diagram showing a microcurrent pattern applied in a microcurrent charging/discharging process according to an exemplary embodiment of the present invention, and <FIG> is a schematic diagram showing a microcurrent pattern applied in a microcurrent charging/discharging process according to another exemplary embodiment of the present invention.

Referring to <FIG>, the low-voltage defect inspection method according to the present invention includes: a microcurrent charging/discharging process consisting of N (N is an integer of <NUM> or more) charging/discharging sections, and applying a microcurrent to a battery cell in each charging/discharging section S10; measuring the change in voltage before and after each of the charging/discharging section S20; and a screening process of screening low-voltage defective batteries based on the change in voltage S30, wherein each of the charging/discharging section comprises applying a current at a current rate of <NUM>. 000001C to <NUM>.

The inventors of the present invention have found that in the case of a low-voltage battery, when the battery cell is charged and discharged at a rate of <NUM>. 000001C to <NUM>. 0001C, but as shown in <FIG>, when there is a pattern to the applied current rate, the change in voltage was large compared to a normal battery, thereby leading to the present invention. Since the low-voltage defect inspection method according to the present invention charges and discharges with a very small amount of current in the microcurrent charging and discharging process, there is an effect of reducing the time required for detecting a battery cell exhibiting a voltage drop behavior to several tens of minutes to several hours. This is a drastically shortened inspection time compared to the time required for a conventional inspection method, which used to take several days to tens of days.

The microcurrent charging/discharging process S10 is a process of charging and discharging by applying a microcurrent to the battery cell, consisting of N charging/discharging sections, and each charging/discharging section performs charging and discharging. That is, the microcurrent charging/discharging process of the present invention may consist of a first charging/discharging section to an Nth charging/discharging section, and each charging/discharging section includes a charging and discharging process. In this case, N may be an integer of <NUM> to <NUM>, preferably an integer of <NUM> to <NUM>, and more preferably an integer of <NUM> to <NUM>. When there are too many charging/discharging sections, the process may be complicated, which is not preferable, and when there are too few charging/discharging sections, variation in change in voltage may be large, which is not preferable.

Referring to <FIG>, the microcurrent charging/discharging process according to an exemplary embodiment of the present invention may not have a resting period between charging and discharging, and referring to <FIG>, the microcurrent charging/discharging process according to another exemplary embodiment of the present invention may further include a resting period in at least one or more among between the charging and discharging or between charging/discharging sections.

The resting period is to stop charging or discharging, and specifically, to leave the charged/discharged battery cells for a predetermined time. The resting period may be <NUM> minute to <NUM> minutes, preferably <NUM> minutes to <NUM> minutes, and more preferably <NUM> minutes to <NUM> minutes. In the present invention, when there is no resting period between charging and discharging or between discharging and charging, noise may occur in the voltage at the beginning of changing the applied current value, so it is preferable to have a rest period in at least one or more among between charging and discharging or between charging/discharging processes.

In the microcurrent charging/discharging process S10, a constant current may be applied as the current applied in one charging/discharging section. As shown in <FIG> and <FIG>, in the microcurrent charging/discharging process, as the charging/discharging section progresses, the rate of the current being applied seems to increase gradually/stepwise, but in a single charging/discharging section, charging and discharging can be done at a constant current rate. In <FIG>, charging is indicated as a current rate having a positive value and discharging is indicated as a current rate having a negative value. Positive/negative corresponds to charging and discharging, respectively, and the absolute values of the current rates may be the same, or may be different. That is, in one charging/discharging section, the absolute value of the current rate during charging and the absolute value of the current rate during discharging may be mutually the same, or the absolute value of the current rate during charging and the absolute value of the current rate during discharging may be mutually different. However, when cases in which the absolute values of the current rates during charging and discharging are different accumulate, the voltage of the battery cell may fluctuate as a whole, so it is desirable that the absolute value of each current rate of charging and discharging are the same. That is, in a single charging/discharging section, it is desirable that the charging rate and the discharging rate are the same.

When a microcurrent is applied, the present invention alternately performs charging and discharging at a rate of <NUM>. 000001C to <NUM>. 0001C, which represents the lower limit and the upper limit of the charging/discharging rate that can be applied in the entire section of the microcurrent charging/discharging process. That is, the current rate applied in the microcurrent charging/discharging section may be a value selected from the above range.

In addition, in the microcurrent charging/discharging process of the present invention, the current rate of each charging/discharging section may increase stepwise or decrease stepwise as the N increases. As shown in <FIG> and <FIG>, current rate of the charging/discharging section increasing as the N increases means that the charging/discharging rate of the second charging/discharging section is greater than the charging/discharging rate of the first charging/discharging section, and that the charging/discharging rate of the third charging/discharging section is greater than the charging/discharging rate of the second charging/discharging section, and current rate of the charging/discharging section decreasing as N increases means the other way around.

In the microcurrent charging/discharging process, it is preferable to perform the charging/discharging in one charging/discharging section for <NUM> minute to <NUM> minutes, more preferably <NUM> minutes to <NUM> minutes, and more preferably <NUM> minutes to <NUM> minutes, respectively.

In addition, the total time required for the microcurrent charging/discharging process in which the charging-resting-discharging is alternately performed may be <NUM> minutes to <NUM> hours, preferably <NUM> minutes to <NUM> hours, and more preferably <NUM> minutes to <NUM> hours.

The process of measuring the change in voltage S20 includes a process of measuring the change in voltage before and after each charging/discharging section. Unlike the conventional screening method, the method for screening low-voltage defective batteries according to the present invention screens low-voltage batteries based on voltage response characteristics after a microcurrent is applied. In the microcurrent charging/discharging process, since charging and discharging are performed by applying microcurrent for each of the N charging/discharging sections, the process of measuring the change in voltage S20 may include a process of measuring the voltage of the battery for each of the N charging/discharging sections, calculating the change in voltage before and after the charging/discharging section, and collecting data of the change in voltage.

In the low-voltage defective battery inspection method according to the present invention, since the battery cell is charged and discharged with a microcurrent in the microcurrent charging/discharging process, the voltage measured from the change in voltage may vary depending on the material of the electrode or the capacity of the battery, but it has a unit of several microvolts to hundreds of microvolts.

The screening process S30 is a process of screening low-voltage defective batteries based on the change in voltage measured in the process of measuring the change in voltage.

<FIG> is a flowchart of a screening process according to an exemplary embodiment of the present invention. Referring to <FIG>, the screening process S30 includes: measuring voltage before and after each charging/discharging section, and collecting data of a current applied in each charging/discharging section and a change in voltage (dV/dt) per unit time corresponding thereto; and calculating the voltage drop when the applied current is <NUM> by deriving a relational expression between the applied current and the change in voltage per unit time from the collected data.

<FIG> is a graph obtained by plotting a change in voltage per unit time corresponding to an applied current value in each charging/discharging section according to an exemplary embodiment of the present invention. Referring to <FIG>, the x-axis represents the applied current of each charging/discharging section, the y-axis represents the change in voltage per unit time, and the change in voltage per hour corresponding to the applied current is indicated by dots. When a plurality of dots is connected, a formula can be derived, and the voltage drop when the applied current is <NUM> can be calculated from the derived formula. Although a plurality of dots is shown as a linear equation function in <FIG>, the correlation function between the applied current and the change in voltage per unit time is not limited thereto, and it is possible to derive a formula of the change in voltage per time according to the applied current based on the plurality of dots using a known statistical method such as a linear regression, etc..

The low-voltage defective battery inspection method according to the present invention has an advantage in that it can be used in any process among the manufacturing process of lithium secondary battery because it applies a microcurrent to the battery cell, thereby having a small effect on the battery.

Hereinafter, a manufacturing method of a lithium secondary battery according to the present invention will be described.

The manufacturing method of a lithium secondary battery according to an exemplary embodiment of the present invention includes: activating by charging an assembled battery cell; and the low-voltage defect inspection process.

The assembled battery cell may refer to a battery cell manufactured through a process of accommodating an electrode assembly in which a positive electrode, a separator, and a negative electrode are sequentially stacked into a battery case, injecting an electrolyte, and sealing the battery case.

After the assembly of the battery is completed as such, a pre-aging process may be performed in which the battery cell is left for a predetermined period of time in order to sufficiently impregnate the electrode assembly with the electrolyte before starting the activation process of the battery cell.

The process of activating the battery cell is a process of forming a battery into a usable state by charging an assembled battery cell and allowing an electrode and an electrolyte constituting a battery to undergo an electrochemical reaction. This activation process includes a process of charging a battery cell, and may further include a process of discharging after charging, or may further include an aging process of aging a battery cell by leaving it for a predetermined time after charging.

<FIG> is a flowchart of a manufacturing method of a lithium secondary battery according to an exemplary embodiment of the present invention. Referring to <FIG>, the manufacturing method of a lithium secondary battery according to an exemplary embodiment of the present invention includes: a primary charging process S100 of charging the battery cell until it reaches <NUM>% to <NUM>% of the battery design capacity; an aging process S200 of aging the battery cell; an additional charging/discharging process S300 of additionally charging and discharging the battery cell; and the aforementioned low-voltage defect inspection process.

However, the present invention is not limited to the above embodiment, and the battery may be shipped after passing through the low-voltage defect inspection process after the primary charging process, and after the primary charging and the aging processes, the battery screened according to the low-voltage defect inspection process may be shipped.

Also, the low-voltage defect inspection process is performed at any one or more selected from the following time points (<NUM>) to (<NUM>).

In addition, after one time point selected among the (<NUM>) to (<NUM>), a stabilization process of stabilizing the battery cell at a constant temperature may be performed, and then the low-voltage defect inspection process may be performed.

In an exemplary embodiment of the present invention, the primary charging process S100 is a process of charging an assembled secondary battery to <NUM>% to <NUM>% of the state of charge (SOC) of the secondary battery in order to form a Solid Electrolyte Interface (hereinafter referred to as "SET") film layer of the negative electrode. The upper limit charge capacity of the primary charging may be appropriately selected considering the type of the battery, the capacity of the battery, and the characteristics of the positive and negative electrodes constituting the battery, and specifically, it may be SOC <NUM>% to SOC <NUM>%, or SOC <NUM>% to SOC <NUM>%.

The primary charging process may be performed under charging conditions according to methods known in the art. Specifically, the charging method may perform charging in a constant current method until the charging end voltage is reached. Here, the charging rate (c-rate) may be <NUM>. 01C to 2C, <NUM>. 1C to <NUM>. 5C, and <NUM>. 2C to 1C, but is not necessarily limited thereto, and may be appropriately adjusted according to the type of battery, the capacity, and the material characteristics of the positive electrode and the negative electrode.

In addition, the temperature condition of the primary charging process may be carried out at <NUM> to <NUM>, specifically <NUM> to <NUM>, and more specifically <NUM> to <NUM>, or it may be alternatively carried out under high temperature condition of <NUM> to <NUM>.

In addition, the primary charging process may perform charging while pressurizing the secondary battery. Specifically, the primary charging process may be performed while being loaded on a jig formation device capable of pressurizing the secondary battery even during charging.

Here, the pressure for pressurizing the secondary battery may be <NUM>. 1kgf/cm<NUM> to 10kgf/cm<NUM>, preferably <NUM>. 3kgf/cm<NUM> to <NUM>. 5kgf/cm<NUM>, and more preferably <NUM>. 5kgf/cm<NUM> to 5kgf/cm<NUM>.

The aging process S200 is a process of aging the battery by leaving the battery at a constant temperature. The aging process may be performed under various conditions to accelerate the stabilization of the SEI film formed through the primary charging process. Specifically, it may include a room temperature aging process of aging the secondary battery at room temperature for a predetermined period of time, and depending on the purpose, it may further include a high temperature aging process of aging the secondary battery at a high temperature, and both the room temperature aging and the high temperature aging can be performed. The high temperature aging ages the battery in a high temperature environment, and it can accelerate the stabilization of the SEI film, and the high temperature aging and the room temperature aging processes can be sequentially performed on the initially charged battery.

In one specific example, the high temperature aging may be performed at a temperature of <NUM> to <NUM>, preferably <NUM> to <NUM>. The high temperature aging may be performed for <NUM> to <NUM> hours, preferably <NUM> to <NUM> hours. The high temperature aging accelerates the stabilization of the SEI film and reduces the voltage drop due to self-discharge of the normal battery, thereby further improving the selectivity of the battery to distinguish good batteries from low-voltage defective batteries.

In one specific example, the room temperature aging may be performed at a temperature of <NUM> to <NUM>, specifically <NUM> to <NUM>, more specifically <NUM> to <NUM>, and even more specifically <NUM> to <NUM>. The room temperature aging may be performed for <NUM> to <NUM> hours, or <NUM> to <NUM> hours.

The additional charging/discharging process S300 is a process of additionally charging and discharging the secondary battery, and specifically, it may be performed by a full discharge and a full charge process of completely discharging the secondary battery to near SOC <NUM> and then charging it to <NUM>% (SOC <NUM>%) or more of the design capacity of the discharged secondary battery; or by charging the secondary battery to <NUM>% (SOC <NUM>%) or more of the design capacity, completely discharging the secondary battery to near SOC <NUM>, and then charging back the secondary battery to <NUM>% (SOC <NUM>%) or more of the design capacity of the secondary battery; or by repeating the full discharge and the full charge processes two or more times, but is not limited thereto.

Hereinafter, the present invention will be described in detail using exemplary embodiments to aid the understanding of the present invention. However, the exemplary embodiments according to the present invention may be modified in many different forms, and it should not be construed that the scope of the present invention is limited to the following exemplary embodiments. The exemplary embodiments of the present invention are provided to more completely explain the present invention to those skilled in the art.

<NUM> parts by weight of Li[Ni<NUM>Mn<NUM>Co<NUM>]O<NUM> functioning as a positive electrode active material, <NUM> parts by weight of graphite functioning as a conductor, and <NUM> parts by weight of polyvinylidene fluoride (PVdF) functioning as a binder were mixed to prepare a positive electrode mixture. A positive electrode mixture slurry was prepared by dispersing the prepared positive electrode mixture in <NUM>-methyl-<NUM>-pyrrolidone functioning as a solvent. This slurry was coated on both sides of an aluminum foil with a thickness of <NUM>µm, dried, and pressurized to prepare a positive electrode.

<NUM> parts by weight of artificial graphite and natural graphite (weight ratio: <NUM>:<NUM>) functioning as a negative electrode active material, <NUM> parts by weight of styrenebutadiene rubber (SBR) functioning as a binder, and <NUM> parts by weight of carboxymethyl cellulose (CMC) were mixed to prepare a negative electrode mixture. A negative electrode mixture slurry was prepared by dispersing this negative electrode mixture in ion-exchanged water functioning as a solvent. This slurry was coated on both sides of a copper foil with a thickness of <NUM>, dried, and pressurized to prepare a negative electrode.

A non-aqueous electrolyte was prepared by dissolving LiPF<NUM> in an organic solvent mixed with ethylene carbonate (EC), propylene carbonate (PC), and diethyl carbonate (DEC) in a composition of <NUM>:<NUM>:<NUM> (volume ratio) so that its concentration becomes <NUM>.

The positive electrode and the negative electrode prepared above were stacked with a porous polyethylene separator interposed therebetween, and were stored in a pouch, and the electrolyte was injected to complete the assembly of the lithium secondary battery.

A lithium secondary battery was prepared as in the above Preparation Example, but copper particles having a diameter of <NUM>µm were introduced to complete the assembly of the defective battery so that the micro short-circuit resistance during the process of assembling the positive electrode, the negative electrode, and the separator, was <NUM>Ω.

Ten assembled lithium secondary batteries of Preparation Example <NUM> were prepared and were pre-aged by aging for <NUM> hours at room temperature of <NUM>. The pre-aged secondary batteries were loaded to a formation jig, and a primary charging process was performed by charging the secondary battery until it reached SOC <NUM>% while pressurizing it at a temperature of <NUM>. The battery that has gone through the primary charging was aged for <NUM> hours at a high temperature of <NUM>, and then was aged for <NUM> hours at room temperature of <NUM>.

Thereafter, a microcurrent charging/discharging process consisting of three charging/discharging sections was performed, but a resting period was included between charging and discharging (first charge - resting period - first discharge - resting period - second charge - resting period - second discharge- resting period - third charge - resting period - third discharge). Here, the microcurrent charging/discharging process was performed by setting the first charge and the first discharge to be charged and discharged for <NUM> minutes at a rate of <NUM>. 000001C, respectively, the second charge and the second discharge to be charged and discharged for <NUM> minutes at a rate of <NUM>. 000002C, respectively, the third charge and the third discharge to be charged and discharged for <NUM> minutes at a rate of <NUM>. 000004C, respectively, and the resting periods to be <NUM> minutes, respectively.

In addition, the voltage of the battery was measured before and after each charging/discharging section, and a graph was obtained by plotting the change in voltage per hour corresponding to the applied current in each charging/discharging section. In the graph, the voltage drop when the applied current is <NUM> was calculated (predicting process of voltage drop), and the average value and the deviation of the voltage drop for ten batteries were shown in Table <NUM>.

Ten assembled lithium secondary batteries of Preparation Example <NUM> were prepared, and in the same manner as in Example <NUM>, pre-aging, primary charging, aging process, microcurrent charging/discharging process, measuring change in voltage, and predicting process of voltage drop were performed, and the average value and the deviation of voltage drop were shown in Table <NUM>.

Thereafter, a microcurrent charging/discharging process consisting of three charging/discharging sections was performed, but a resting period was included between charging and discharging (first charge - resting period - first discharge - resting period - second charge - resting period - second discharge- resting period - third charge - resting period - third discharge). Here, the microcurrent charging/discharging process was performed by setting the first charge and the first discharge to be charged and discharged for <NUM> minutes at a rate of <NUM>. 000001C, respectively, the second charge and the second discharge to be charged and discharged for <NUM> minutes at a rate of <NUM>. 000003C, respectively, the third charge and the third discharge to be charged and discharged for <NUM> minutes at a rate of <NUM>. 000006C, respectively, and the resting periods to be <NUM> minutes, respectively.

Then, in the same manner as in Example <NUM>, the voltage drop when the applied current is <NUM> was calculated (predicting process of voltage drop), and the average value and the deviation of the voltage drop for ten batteries were shown in Table <NUM>.

Ten assembled lithium secondary batteries of Preparation Example <NUM> were prepared, and in the same manner as in Example <NUM>, aging process, microcurrent charging/discharging process, measuring change in voltage, and predicting process of voltage drop were performed, and the average value and the deviation of voltage drops were shown in Table <NUM>.

An open circuit voltage of the battery cell was measured immediately after the aging (OCV1), and an open circuit voltage of the battery was measured when <NUM> hours have elapsed (OCV2) to calculate the change in voltage per unit time (dV/dmin) and the average value and the deviation were shown in Table <NUM>.

Ten assembled lithium secondary batteries of Preparation Example <NUM> were prepared, and in the same manner as in Example <NUM>, pre-aging, primary charging, high temperature aging, room temperature aging, and calculating change in voltage per unit time were performed, and the average value and the deviation were shown in Table <NUM>.

Claim 1:
An inspection method for low-voltage defect of a lithium secondary battery, comprising:
a microcurrent charging/discharging process consisting of N, N is an integer of <NUM> or more, charging/discharging sections, and applying a microcurrent to a battery cell in each of the charging/discharging section;
measuring the change in voltage before and after each of the charging/discharging section; and
a screening process of screening low-voltage defective batteries based on the change in voltage, wherein
each of the charging/discharging section comprises applying a current at a current rate of <NUM>.000001C to <NUM>.0001C.