Battery pack and charging control circuit

Abstract

This battery pack comprises a secondary battery and a charging control circuit. The charging control circuit is controllable to charge the secondary battery by constant current charging and constant voltage charging. The charging control circuit estimates deterioration of the secondary battery by using a change in voltage of the secondary battery in the constant current charging.

Description

Battery pack and charge control circuit

[0001] The present disclosure relates to a battery pack and a charge control circuit.

[0002] Patent Literature 1 discloses a secondary battery degradation state estimation device that estimates the degradation state of a secondary battery. Patent Literature 1 discloses that the degradation state estimation device acquires a first current value when a charger switches from constant-current / constant-voltage charging to constant-voltage charging and a second current value a predetermined time after the charger switches to constant-voltage charging. The degradation state estimation device also discloses that the degradation state estimation device calculates a slope of the change from the first current value to the second current value and estimates the degradation state of the secondary battery from the slope.

[0003] Patent Document 2 discloses a battery system that includes a battery that can be charged and discharged, a voltage detector that detects the battery voltage as a detected voltage value, a current detector that detects the current flowing through the battery as a detected current value, and a control device that controls the charging and discharging of the battery and estimates the deterioration of the battery over time.

[0004] JP 2018-155706 A JP 2018-185259 A

[0005] Lithium-ion batteries, which are commonly used as secondary batteries, deteriorate over time and through charge-discharge cycles. As a result of this deterioration, the charge capacity of a lithium-ion battery decreases and its internal resistance increases. For example, the State of Health (SOH) is used as an index of lithium-ion battery deterioration.

[0006] The present disclosure provides a technique for estimating the degradation state of a secondary battery.

[0007] In one aspect of the present disclosure, a battery pack is provided that includes a secondary battery and a charge control circuit, the charge control circuit being capable of controlling the secondary battery to be charged by constant current charging and constant voltage charging, and the charge control circuit estimating deterioration of the secondary battery using a change in voltage of the secondary battery during the constant current charging.

[0008] According to the present disclosure, the deterioration state of a secondary battery can be estimated.

[0009] FIG. 1 is a diagram showing an example of a schematic of a battery pack according to the present embodiment. FIG. 2 is a diagram showing an example of the relationship between the SOC value and cell voltage of a secondary battery before and after use of the secondary battery in the battery pack according to the present embodiment. FIG. 3 is a diagram showing an example of the relationship between the charging current and charging time of the secondary battery in the battery pack according to the present embodiment. FIG. 4 is a diagram showing an example of the relationship between charging time and cell voltage in the battery pack according to the present embodiment. FIG. 5 is a flow chart showing an example of processing in the battery pack according to the present embodiment. FIG. 6 is a diagram showing an example of display on a display unit in the battery pack according to the present embodiment. FIG. 7 is a flow chart showing an example of a modified example of processing in the battery pack according to the present embodiment. FIG. 8 is a diagram showing an example of charging control of the secondary battery by a charge control unit.

[0010] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The following describes the preferred embodiments of the present invention with reference to the accompanying drawings. In the drawings, the same components are designated by the same reference numerals, and redundant explanations may be omitted.

[0011] In the directions of parallel, right-angle, orthogonal, horizontal, vertical, up-down, left-right, front-back, etc., deviations are permitted to the extent that they do not impair the effects of the embodiments. The shape of the corners is not limited to right angles and may be rounded. Parallel, right-angle, orthogonal, horizontal, and vertical may include approximately parallel, approximately right-angle, approximately orthogonal, approximately horizontal, and approximately vertical, respectively.

[0012] For example, "substantially parallel" means that even if two lines or two surfaces are not completely parallel to each other, they can be treated as parallel to each other as long as it is within a range that is permissible in manufacturing. As with "substantially parallel," "substantially right angle," "substantially perpendicular," "substantially horizontal," and "substantially vertical" are also intended to fall under the respective terms as long as the relative positional relationship between two lines or two surfaces is within a range that is permissible in manufacturing.

[0013] A battery pack according to this embodiment will be described. The battery pack according to this embodiment includes a secondary battery and a charge control circuit. The charge control circuit in the battery pack according to this embodiment is capable of controlling the secondary battery to be charged by constant current charging and constant voltage charging. The charge control circuit in the battery pack according to this embodiment also estimates deterioration of the secondary battery using changes in the voltage of the secondary battery during constant current charging.

[0014] The battery pack according to the present embodiment will be described in detail with reference to the drawings. Fig. 1 is a diagram showing an example of a schematic configuration of a battery pack 1, which is an example of a battery pack according to the present embodiment.

[0015] The battery pack 1 stores electricity by being charged externally. The battery pack 1 also supplies the charged power to devices. Devices in which the battery pack 1 is used include, for example, information devices such as personal computers, smartphones, and tablet terminals, vehicles such as bicycles and automobiles, home appliances, and power storage devices. The battery pack 1 is used by repeatedly charging and discharging. The battery pack 1 includes a secondary battery 10, a charge control circuit 20, and a charging circuit 30.

[0016] Each component of the battery pack 1 will now be described in detail.

[0017] [Secondary Battery 10] The secondary battery 10 is a battery that can be repeatedly charged and discharged. The secondary battery 10 is, for example, a lithium ion battery.

[0018] [Charge Control Circuit 20] The charge control circuit 20 controls charging in the charge circuit 30. The charge control circuit 20 also estimates deterioration of the secondary battery 10.

[0019] The charge control circuit 20 is, for example, a computer and includes a central processing unit (CPU), a random access memory (RAM), a read-only memory (ROM), an auxiliary storage device, etc. The CPU operates based on a program stored in the ROM or the auxiliary storage device.

[0020] The charge control circuit 20 includes a charge control unit 21 , an SOC (State Of Charge) estimation unit 22 , a cell voltage / current detection unit 23 , a deterioration level calculation unit 24 , a storage unit 25 , and a display unit 26 .

[0021] (Charging Control Unit 21) The charging control unit 21 controls the charging of the secondary battery 10. The charging control unit 21 controls the charging circuit 30 to control the charging of the secondary battery 10.

[0022] FIG. 8 illustrates an example of charging control of the secondary battery 10 by the charging control unit 21. The charging control unit 21 performs preliminary charging at the start of charging and starts main charging when the battery voltage exceeds V8_1. Rapid charging when the secondary battery's charge capacity is low accelerates deterioration of the secondary battery. Therefore, the secondary battery is charged at a controlled low current as preliminary charging. The main charging begins with constant current charging (CC charging: Constant Current charging) at a constant current. Then, when the open-circuit voltage reaches a predetermined voltage value (determination voltage V8_2), the charging control unit 21 performs constant voltage charging (CV charging: Constant Voltage charging) at a constant voltage. During constant voltage charging, the charging control unit 21 stops charging when the charging current falls below a predetermined current value (I8_1). In other words, the charging control unit 21 controls constant current charging to be performed before constant voltage charging. The charge control unit 21 then controls the secondary battery 10 to perform constant current charging when the voltage of the secondary battery 10 reaches a threshold voltage during constant current charging. CC charging has a fast charging speed, but there is a risk of the secondary battery catching fire if the voltage exceeds a certain level. On the other hand, CV charging has a slow charging speed but allows for highly accurate charging control. Therefore, after rapid charging using CC charging, when the threshold voltage V8_2 is reached, the charging control unit 21 switches to CV charging to control the charging of the secondary battery with high accuracy.

[0023] (SOC Estimation Unit 22 ) The SOC estimation unit 22 calculates the SOC value (charging rate) of the secondary battery 10 using the current value and voltage value acquired from the secondary battery 10 .

[0024] (Cell Voltage and Current Detector 23) The cell voltage and current detector 23 acquires the voltage and current values ​​from the secondary battery 10 during charging.

[0025] (Deterioration degree calculation unit 24) The deterioration degree calculation unit 24 calculates the ratio (proportion) of the integrated power value and the amount of change in the SOC value to calculate the deterioration degree of the secondary battery 10. In other words, the deterioration degree calculation unit 24 estimates the deterioration of the secondary battery 10 based on the change in the voltage of the secondary battery 10 during constant current charging.

[0026] (Storage Unit 25) The storage unit 25 stores data etc. The storage unit 25 stores, for example, a variation coefficient that serves as a criterion for determination.

[0027] (Display unit 26) The display unit 26 displays the estimated result of the deterioration of the secondary battery 10 estimated by the deterioration degree calculation unit 24. Note that a display unit may be further provided outside the charge control circuit 20, and the calculation result from the charge control circuit 20 may be sent to the external display unit.

[0028] [Charging Circuit 30] The charging circuit 30 supplies power to the secondary battery 10 in order to charge the secondary battery 10. The charging circuit 30 charges the secondary battery 10 with a current or voltage based on the control of the charging control unit 21 in the charging control circuit 20.

[0029] <Processing in charge control circuit 20> First, a description will be given of the relationship between the cell voltage and the SOC value of the secondary battery 10. Fig. 2 is a diagram showing an example of the relationship between the SOC value and the cell voltage of the secondary battery 10 before and after use of the secondary battery 10 in the battery pack 1, which is an example of a battery pack according to this embodiment.

[0030] 2, the horizontal axis represents the SOC value (unit: percent), and the vertical axis represents the cell voltage (unit: volts). Line L1 shows the relationship between the SOC value and the cell voltage when an unused secondary battery 10 is initially charged. That is, line L1 shows the relationship between the SOC value and the cell voltage when the secondary battery 10 is charged for the first time. Line L2 shows the relationship between the SOC value and the cell voltage of the secondary battery 10 when it is charged for the 100th time from an unused state.

[0031] 2, the relationship between the cell voltage and the SOC value does not change even when the battery is repeatedly charged and discharged. Therefore, the SOC value can be estimated by measuring the cell voltage.

[0032] For example, by using the results shown in FIG. 2, when the cell voltage is voltage Va (e.g., 3.6 volts), the SOC value can be estimated to be 20% regardless of the number of charges. Furthermore, when the cell voltage is voltage Vb (e.g., 4.0 volts), the SOC value can be estimated to be 60%. In other words, when the cell voltage changes from voltage Va to voltage Vb, the SOC value can be estimated to change by ΔSOC. Furthermore, based on the results shown in FIG. 2, for example, when the SOC value is in the range of 10% to 90%, the cell voltage and the SOC value can be considered to be proportional to each other.

[0033] For example, if the SOC value when the cell voltage is voltage V is SOC(V), the voltage V and SOC(V) satisfy the relationship of Equation 1, where α and β are constants.

[0034] SOC (V) = α V + β ... (Formula 1)

[0035] From equation 1, the SOC value when the cell voltage is voltage Va, that is, SOC(Va), is given by equation 2. Also, from equation 1, the SOC value when the cell voltage is voltage Vb, that is, SOC(Vb), is given by equation 3.

[0036] SOC (Va) = α Va + β ... (Formula 2) SOC (Vb) = α Vb + β ... (Formula 3)

[0037] Therefore, the amount of change ΔSOC in the SOC value when the cell voltage changes from voltage Va to voltage Vb is given by Equation 4.

[0038] ΔSOC = SOC (Vb) - SOC (Va) = α (Vb - Va) ... (Formula 4)

[0039] Next, the relationship between charging time and charging current in constant current charging will be described. Fig. 3 is a diagram showing an example of the relationship between charging current and charging time of the secondary battery 10 in the battery pack 1, which is an example of the battery pack according to this embodiment.

[0040] 3, the horizontal axis represents the time (unit: seconds) elapsed since the start of constant current charging, and the vertical axis represents the charging current (unit: amperes). Line L11 shows the relationship between time and charging current when charging secondary battery 10.

[0041] 3, the battery pack 1 is charged with a constant current from 0 seconds to t0 seconds, and from t0 seconds onwards, the battery pack 1 is charged with a constant voltage.

[0042] The integrated amount of power (amount of charged charge) ΔQ when charging for a time Δt (=tb−ta) from time ta to time tb is given by Equation 5. The charging current is assumed to be current I0.

[0043] ΔQ = I0 × Δt = I0 × (tb-ta) ... (Formula 5)

[0044] From equations 4 and 5, equation 6 holds true.

[0045] ΔQ / ΔSOC = I0(tb-ta) / α(Vb-Va)...(Formula 6)

[0046] The relationship between cell voltage and charging time will now be described. Fig. 4 is a diagram showing an example of the relationship between time and cell voltage during charging in battery pack 1, which is an example of a battery pack according to this embodiment.

[0047] 4, the horizontal axis represents the time (unit: seconds) elapsed since the start of constant current charging, for example, and the vertical axis represents the cell voltage (unit: volts). Line L21 shows the relationship between time and cell voltage when the secondary battery 10 is charged for the first time. Line L22 shows the relationship between time and cell voltage when the secondary battery 10 is charged for the tenth time.

[0048] As shown in Figure 4, the charging time and cell voltage become proportional after a certain time has passed. Furthermore, as is clear from a comparison of lines L21 and L22, the slope becomes steeper as the number of charging times increases, i.e., as the secondary battery 10 deteriorates. In other words, as the number of charging times increases, i.e., as the secondary battery 10 deteriorates, the coefficient of change (rate of change) becomes higher.

[0049] From the above results, during constant current charging, the voltage V(t) at time t is expressed by the following equation 7: where r and n are constants.

[0050] V(t) = r×t + n... (Formula 7)

[0051] Here, Equation 8 is established from Equation 6 and Equation 7. Note that r may be referred to as a change coefficient or a change rate. Δ is the ratio between the integrated amount of power and the amount of change in the SOC value.

[0052] Δ = ΔQ / ΔSOC = I0(tb-ta) / (α×r) (Formula 8)

[0053] That is, ΔQ / ΔSOC is uniquely determined.

[0054] For example, if the reference ratio Δ is Δ0, the change coefficient at that time is change coefficient r0, the ratio Δ when making the judgment is Δj, and the change coefficient at that time is change coefficient rj, then the deterioration degree γ (unit: percent) is expressed by Equation 9.

[0055] γ = Δj / Δ0 = {I0(tb-ta) / (α×rj)} / {I0(tb-ta) / (α×r0)} = r0 / rj... (Formula 9)

[0056] Next, a description will be given of specific processing in the charge control circuit 20. Fig. 5 is a flow chart showing an example of processing in the battery pack 1, which is an example of a battery pack according to this embodiment.

[0057] The variation coefficient rj is an example of a first variation coefficient, and the variation coefficient r0 is an example of a second variation coefficient.

[0058] (Step S10) The charge control circuit 20 starts constant current charging. Note that when constant current charging starts, the voltage of the secondary battery 10 is at least less than a first voltage value V1. The first voltage value V1 is, for example, the voltage value when the SOC value is 20%. Note that when the voltage is less than the first voltage value V1, the preliminary charging shown in FIG. 8 is performed, and therefore the relationship between cell voltage [V] and SOC [%] is not linear as shown by lines L1 and L2 in FIG. 2 . On the other hand, when the voltage is equal to or greater than the first voltage value V1, constant current charging is performed, and therefore the voltage of the secondary battery 10 increases in proportion to the time of constant voltage charging.

[0059] (Step S20) The charge control circuit 20 acquires the voltage of the secondary battery 10. Then, the charge control circuit 20 determines whether the voltage of the secondary battery 10 is equal to or greater than the first voltage threshold V1a. If the voltage of the secondary battery 10 is equal to or greater than the first voltage threshold V1a (YES in step S20), the charge control circuit 20 proceeds to step S30. If the voltage of the secondary battery 10 is less than the first voltage threshold V1a (NO in step S20), the charge control circuit 20 returns to step S20 and repeats the process.

[0060] (Step S30) Next, the charge control circuit 20 records the time T1 at which it is determined that the voltage of the secondary battery 10 is equal to or greater than the first voltage threshold V1a. Then, the charge control circuit 20 measures the voltage of the secondary battery. The charge control circuit 20 records the result of measuring the voltage of the secondary battery as a first voltage value V1. Note that the above process is performed as soon as possible after the process of step S20. Then, the charge control circuit 20 calculates a first SOC value SOC1, which is the SOC value at time T1, from the first voltage value V1. The charge control circuit 20 calculates the SOC value based on Equation 1.

[0061] (Step S40) Next, the charge control circuit 20 acquires the voltage of the secondary battery 10. The charge control circuit 20 then determines whether the voltage of the secondary battery 10 is equal to or greater than a second voltage threshold V2a. The second voltage threshold V2a is higher than the first voltage threshold V1a. If the voltage of the secondary battery 10 is equal to or greater than the second voltage threshold V2a (YES in step S40), the charge control circuit 20 proceeds to step S50. If the voltage of the secondary battery 10 is less than the second voltage threshold V2a (NO in step S40), the charge control circuit 20 returns to step S40 and repeats the process. Here, the first voltage threshold V1a and the second voltage threshold V2a are preferably set to cell voltages corresponding to an SOC value in the range of 10% to 80%. This is because CC charging is performed in this range, and the relationship between the SOC value and the cell voltage is approximately linear.

[0062] (Step S50) Next, the charge control circuit 20 records the time T2 at which it is determined that the voltage of the secondary battery 10 is equal to or greater than the second voltage threshold V2a. Then, the charge control circuit 20 measures the voltage of the secondary battery. The charge control circuit 20 records the result of measuring the voltage of the secondary battery as a second voltage value V2. Note that the above process is performed as soon as possible after the process of step S40. Then, the charge control circuit 20 calculates a second SOC value SOC2, which is the SOC value at time T2, from the second voltage value V2. The charge control circuit 20 calculates the SOC value based on Equation 1.

[0063] (Step S60) Next, the charge control circuit 20 calculates the integrated amount of power ΔQ from time T1 to time T2. The charge control circuit 20 calculates the integrated amount of power ΔQ based on Equation 5.

[0064] (Step S70) Next, the charge control circuit 20 calculates the ratio Δ between the integrated amount of power ΔQ and the amount of change ΔSOC in the SOC value. The charge control circuit 20 calculates the ratio Δ based on Equation 8.

[0065] (Step S80) Next, the charge control circuit 20 obtains the deterioration degree γ using the reference ratio Δ0 stored in the storage unit 25. The deterioration degree is calculated based on Equation 9.

[0066] (Step S90) Next, based on the calculated deterioration level γ, the charge control circuit 20 displays the result on the display unit 26. Fig. 6 is a diagram showing an example of a display on the display unit 26 of the battery pack 1, which is an example of the battery pack according to this embodiment.

[0067] As shown in FIG. 6 , for example, when the degradation level is between 1 and 0.9, the charge control circuit 20 controls the LEDs to light up five times. When the degradation level is between 0.9 and 0.8, the charge control circuit 20 controls the LEDs to light up four times. When the degradation level is between 0.8 and 0.6, the charge control circuit 20 controls the LEDs to light up three times. When the degradation level is between 0.6 and 0.4, the charge control circuit 20 controls the LEDs to light up two times. When the degradation level is 0.4 or less, the charge control circuit 20 controls the LEDs to blink five times.

[0068] In this way, since the deterioration of the secondary battery can be estimated during CC charging, it can be estimated without waiting for full charge. As a result, when estimating the deterioration of the secondary battery, there is no need to fully charge it, and power consumption can be reduced. Furthermore, the charge control circuit 20 estimates the deterioration state using CC charging, which is performed before CV charging. Therefore, deterioration can be determined at an early stage of charging. As a result, it is possible to control CV charging by referring to the degree of deterioration. For example, if the degree of deterioration is high, the charging time can be shortened by increasing the voltage value of CV charging.

[0069] Note that time T1 is an example of a first time, time T2 is an example of a second time, the first SOC value SOC1 is an example of a first charging rate, and the second SOC value SOC2 is an example of a second charging rate.

[0070] Next, a description will be given of a modified example of the processing in the charge control circuit 20. Fig. 7 is a flow chart showing an example of a modified example of the processing in the battery pack 1, which is an example of the battery pack according to this embodiment.

[0071] In a modified example of the processing in the charge control circuit 20, step S120 is provided instead of step S20, and step S140, step S141, and step S142 are provided instead of step S40 in the processing shown in Fig. 5. For processing common to the processing shown in Fig. 5, the above description should be referred to and a description thereof will be omitted here.

[0072] (Step S120) After processing step S10, the charge control circuit 20 acquires the voltage of the secondary battery 10. Then, the charge control circuit 20 records the acquired voltage value as a first voltage value V1. The charge control circuit 20 also records the time when the voltage is acquired as time T1. Note that the charge control circuit 20 may also process step S120 after a predetermined time has elapsed after processing step S10.

[0073] (Step S140) After processing step S120, the charge control circuit 20 determines whether a predetermined time has elapsed. The predetermined time is, for example, the time required for the SOC value to change by 60 percent. If the predetermined time has elapsed (YES in step S140), the charge control circuit 20 proceeds to step S141. If the predetermined time has not elapsed (NO in step S140), the charge control circuit 20 returns to step S140 and repeats the process.

[0074] (Step S141) Next, the charge control circuit 20 determines whether constant current charging is in progress. After a predetermined time has elapsed, constant current charging may have ended and constant voltage charging may have begun. Therefore, the charge control circuit 20 determines whether constant current charging is in progress. If constant current charging is in progress (YES in step S141), the charge control circuit 20 proceeds to step S142. If constant current charging is not in progress, i.e., if constant voltage charging is in progress (NO in step S141), the charge control circuit 20 proceeds to step S90. Note that if constant current charging is not in progress (NO in step S141), the charge control circuit 20 displays in step S90 that degradation determination could not be made. The charge control circuit 20 may also display the result of the previous determination.

[0075] (Step S142) The charge control circuit 20 acquires the voltage of the secondary battery 10. Then, the charge control circuit 20 records the acquired voltage value as a second voltage value V2. The charge control circuit 20 also records the time when the voltage is acquired as time T2.

[0076] According to the battery pack of this embodiment, the deterioration state of the secondary battery can be estimated.

[0077] The battery pack of the present disclosure has been described above based on an embodiment, but the battery pack of the present disclosure is not limited to the above embodiment and various modifications are possible within the scope of the claims.

[0078] This application claims priority from basic patent application No. 2024-042049, filed with the Japan Patent Office on March 18, 2024, the entire contents of which are incorporated herein by reference.

[0079] REFERENCE SIGNS LIST 1 Battery pack, 10 Secondary battery, 20 Charging control circuit, 21 Charging control unit, 22 SOC estimation unit, 23 Cell voltage / current detection unit, 24 Deterioration degree calculation unit, 25 Storage unit, 26 Display unit, 30 Charging circuit

Claims

1. A battery pack comprising: a secondary battery; and a charge control circuit, wherein the charge control circuit is capable of controlling the secondary battery to be charged by constant current charging and constant voltage charging, and the charge control circuit estimates deterioration of the secondary battery using a change in voltage of the secondary battery during the constant current charging.

2. The battery pack according to claim 1, wherein the charging control circuit detects the time when the voltage of the secondary battery reaches a first voltage as the first time, and detects the time when the voltage of the secondary battery reaches a second voltage as the second time.

3. The battery pack according to claim 2, wherein the charge control circuit estimates a first charging rate from the first voltage, estimates a second charging rate from the second voltage, calculates a charging charge from the first time period, the second time period, and a current value during constant current charging, and calculates a first change coefficient from the first charging rate, the second charging rate, and the charging charge.

4. The battery pack according to claim 3, wherein the charging control circuit further comprises a memory unit, the memory unit stores a second variation coefficient of a reference secondary battery, and the deterioration of the secondary battery is estimated based on the ratio between the first variation coefficient and the second variation coefficient.

5. The battery pack according to claim 4, further comprising a display unit, wherein the charge control circuit sends the ratio to the display unit.

6. The battery pack according to any one of claims 1 to 5, wherein the charge control circuit performs the constant current charging before the constant voltage charging, and performs the constant voltage charging when the voltage of the secondary battery reaches a determination voltage during the constant current charging.

7. A charge control circuit that controls charging of a secondary battery, capable of controlling the secondary battery to be charged by constant current charging and constant voltage charging, and that estimates deterioration of the secondary battery using changes in voltage of the secondary battery during the constant current charging.

8. The charge control circuit according to claim 7, wherein the time when the voltage of the secondary battery reaches a first voltage is detected as the first time, and the time when the voltage of the secondary battery reaches a second voltage is detected as the second time.

9. The charge control circuit according to claim 8, wherein a first charging rate is estimated from the first voltage, a second charging rate is estimated from the second voltage, a charged charge amount is calculated from the first time period, the second time period and the current value during constant current charging, and a first change coefficient is calculated from the first charging rate, the second charging rate and the charged charge amount.

10. A charging control circuit as described in claim 9, further comprising a memory unit that stores a second change coefficient of a reference secondary battery, and determines deterioration of the secondary battery based on the ratio between the first change coefficient and the second change coefficient.

11. A charge control circuit according to any one of claims 7 to 10, wherein the constant current charging is performed before the constant voltage charging, and the constant voltage charging is performed when the voltage of the secondary battery reaches a determination voltage during the constant current charging.

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