JP2010019856A - Device and method for monitoring secondary battery - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To issue an alarm before load current increases more than expected and a desired discharge duration cannot be secured. <P>SOLUTION: This device for monitoring secondary battery of an electric power feed system having a secondary battery as a backup power supply includes: a load current measuring means for measuring load current; a clocking means for measuring the measurement time by a load current measuring means; an electric quantity calculating means for calculating the electric quantity per unit time based on the load current by the load current measuring means and the measurement time by the clocking means; and a discharge sustainable time calculating means for sequentially subtracting the electric quantity per predetermined unit time by the electric quantity calculating means from the battery capacity of the secondary battery and calculating the discharge sustainable time of the secondary battery. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a secondary battery monitoring device and a secondary battery monitoring method.

  Conventionally, a DC power supply system in which a storage battery is maintained by a floating charging method is known. In the DC power supply system, the load facility and the storage battery are connected in parallel to the output side of the rectifier, and the rectifier always supplies DC power to the load, and the battery is fully charged according to the state of charge of the storage battery. A necessary charging current is supplied. When a power failure or rectifier failure occurs in the DC power supply system, the storage battery is discharged at that moment, and the power supply to the load facility is not interrupted even for a moment. This is because the storage battery capacity is selected such that power can be generated by discharging from the storage battery only during the time required by the DC power supply system.

In the case of a lead storage battery, the capacity calculation of each storage battery constituting such an assembled battery is performed by a method as shown in FIG. 18 (see, for example, Non-Patent Document 1). In a method with a high safety coefficient, the battery capacity is calculated according to the time to supply the load from the storage battery with reference to the maximum current expected to be consumed by the load.
C = (1 / L) ・ K ・ I
Here, the used storage battery is a CS-type lead storage battery, and the conditions of maintenance rate L = 0.8, minimum storage battery temperature: 5 ° C., and allowable minimum voltage: 1.8 V / cell are given. Therefore, L = 0.8, I = 50, and K = 5.6. Therefore, C = (1 / L) · K · I = (1 / 0.8) × 5.6 × 50 = 350 Ah / 10HR.

  The capacity conversion time K is obtained from the retention time-capacity conversion time characteristics shown in FIG. Moreover, the capacity conversion time K is determined based on the discharge characteristics (eg, FIG. 20) of the lead storage battery.

C = KI / θ
C: Battery capacity
K: Capacity conversion time
I: Current
θ: Maintenance rate

On the other hand, the calculation method of the storage battery capacity in consideration of the load fluctuation is as shown in FIGS. The example shown in FIG. 21 is a method of calculating the storage battery capacity when the discharge current increases with time. In this case, the used storage battery is an AH alkaline storage battery, and the conditions of maintenance rate L = 0.8, minimum storage battery temperature: 5 ° C., and allowable minimum voltage: 1.06 V / cell are given.
Therefore, L = 0.8, I1 = 10, I2 = 20, and I3 = 120. Further, T1 = 60, T2 = 20, T3 = 0.167 (10 seconds), K1 = 1.40, K2 = 0.70, and K3 = 0.255. Here, C = (1 / L) · (K1 · I1 + (K2 (I2−I1) + K3 (I3−I2)) = (1 / 0.8) × (1.40 × 10 + 0.70 (20−10)) +0.255 (120-20)) = 59 Ah / 5HR Therefore, a storage battery of 59 Ah / 5HR or more, for example, an alkaline storage battery is suitable.

  FIGS. 22 to 25 are diagrams for explaining a method of calculating the storage battery capacity when the discharge current decreases with time. In this case, the used storage battery is a CS (EF) type lead storage battery, and the maintenance rate L = 0.8, the minimum storage battery temperature: 5 ° C., and the allowable minimum voltage: 1.7 V / cell are given. In this case, it is necessary to obtain the capacity for the following three types of load characteristics.

  First, in the block shown in FIG. 23, L = 0.8, I = 1500, T = 20, K = 2.00, and Ch = (1 / L) · K · I = (1 / 0.8) × 2.00 × 1500 = 3750 Ah / 10HR. Next, in the block shown in FIG. 24, L = 0.8, I1 = 1500, I2 = 200, T1 = 180, T2 = 110, K1 = 4.38, K2 = 3.80, and Cn = (1 / L ) · (K1 · I1 + K2 (I2−I1)) = (1 / 0.8) × (4.30 × 1500 + 3.80 (200-1500)) = 1888 Ah / 10HR.

Finally, as shown in FIG. 25, L = 0.8, I1 = 1500, I2 = 200, I3 = 100, T1 = 190, T2 = 170, T3 = 60, K1 = 5.5, K2 = 5 0.1, K3 = 2.75, and Cc = (1 / L) · (K1 · I1 + K2 (I2−I1) + K3 (I3−I2)) = (1 / 0.8) (5.5 × 1500 + 5.1) (200-1500) +2.75 (100-200)) = 1682 Ah / 10HR.
Therefore, among Ch, Cn, and Cc, a lead storage battery (4000 Ah / 10HR) having a maximum value of 3750 Ah / 10HR or more is appropriate.

  Thus, the amount of electricity of each block is calculated according to the change in the load current, and these values are integrated. However, as shown in FIG. 22 to FIG. 25, in the case of a load pattern with a decrease in load current, the calculation is complicated, and the calculation is performed by dividing each time when the current decreases. It is selected as a capacity. When such a battery is used, the duration of discharge from the battery when a power failure occurs is also calculated.

Battery Industry Association, “Method for calculating capacity of stationary storage battery”, SBA S 0601: 2001 (2001, 11, 27)

  However, in the case of lead-acid batteries, the amount of electricity that can be extracted varies depending on the discharge current. Therefore, the conventional capacity calculation method for the storage battery described above accurately calculates the battery capacity required for the storage battery when the load current changes over time. There is a problem that it becomes difficult to do. Further, in actual operation, when the load current increases more than expected, there is a problem that a desired discharge duration cannot be ensured and the purpose as a backup power source cannot be achieved.

  The present invention has been made in consideration of such circumstances, and the purpose thereof is to calculate the dischargeable time of the secondary battery more accurately even when the situation of the load current changes, An object of the present invention is to provide a secondary battery monitoring device and a secondary battery monitoring method capable of issuing an alarm before the load current increases more than expected and the desired discharge duration cannot be ensured.

  In order to solve the above-described problems, the present invention provides a load current measuring unit that measures a load current in a power supply system including a secondary battery as a backup power source, and a timing that measures a measurement time by the load current measuring unit. And a battery capacity calculating means for calculating a battery capacity required for the secondary battery based on a load current by the load current measuring means and a measurement time by the time measuring means.

  In order to solve the above-described problem, the present invention provides a secondary battery monitoring device for a power supply system including a secondary battery as a backup power source, the load current measuring means for measuring a load current, and the load A time measuring means for measuring a measuring time by the current measuring means, an electric quantity calculating means for calculating an electric quantity per unit time based on a load current by the load current measuring means and a measuring time by the time measuring means, and the electric quantity A discharge duration calculation means for sequentially subtracting a predetermined amount of electricity per unit time by the calculation means from the battery capacity of the secondary battery and calculating a discharge sustainability time of the secondary battery. .

The present invention is based on the above-described invention, based on the storage duration for storing the discharge duration required when the secondary battery is used as a backup power source, and the discharge sustainable time by the discharge duration calculation means. And determining means for determining whether or not the battery capacity of the secondary battery can be discharged for the discharge duration stored in the storage means, and determining that the discharge is not possible for the discharge duration by the determining means. And an alarm means for issuing an alarm when the alarm is issued.
The present invention is characterized in that in the above-mentioned invention, a correction means for correcting the discharge sustainable time based on a secular change of the secondary battery is provided.
The present invention is characterized in that, in the above-mentioned invention, a correction means for correcting the discharge sustainable time based on an ambient temperature is provided.

  In order to solve the above-described problem, the present invention provides a secondary battery monitoring method for a power supply system including a secondary battery as a backup power source, the step of measuring a load current, and the measurement of the load current A step of measuring time, a step of calculating an amount of electricity per unit time based on the load current and the measurement time, and subtracting the amount of electricity per unit time from the battery capacity of the secondary battery in sequence, Calculating a sustainable duration of the secondary battery.

  According to this invention, the electric quantity calculating means calculates the electric quantity per unit time based on the load current by the load current measuring means and the measuring time by the time measuring means, and the discharge sustainable time calculating means calculates the electric quantity. The amount of electricity per unit time determined by the means is sequentially subtracted from the battery capacity of the secondary battery to calculate the discharge sustainable time of the secondary battery. Therefore, even if a situation change occurs in the load current, it is possible to accurately calculate the discharge sustainable time of the secondary battery.

Further, according to the present invention, based on the discharge sustainable time by the discharge sustainable time calculating means, the determination means can discharge the battery capacity of the secondary battery power for the discharge duration stored in the storage means. If it is determined by the alarm means that discharge is not possible for the discharge duration, an alarm is issued. Therefore, even if a change in the situation occurs in the load current, the load current increases more than expected, and the desired discharge duration cannot be ensured, an effect that an alarm can be issued in advance can be obtained.
According to the present invention, the discharge sustainable time is corrected by the correcting means based on the secular change of the secondary battery. Therefore, the effect that the discharge sustainable time of the secondary battery can be calculated more accurately is obtained.
According to the invention, the discharge sustainable time is corrected based on the ambient temperature by the correction means. Therefore, the effect that the discharge sustainable time of the secondary battery can be calculated more accurately is obtained.

It is a block diagram which shows the basic composition of the secondary battery holding time determination apparatus by embodiment of this invention. It is a conceptual diagram which shows the discharge characteristic of a lithium ion secondary battery. It is a conceptual diagram which shows the discharge capacity-discharge current characteristic of a lithium ion secondary battery. It is a conceptual diagram for demonstrating the calculation method of the battery capacity in a lithium ion secondary battery. It is a conceptual diagram which shows the experiment example for demonstrating the validity of the calculation method of the battery capacity used by this embodiment. It is a conceptual diagram which shows the experiment example for demonstrating the validity of the calculation method of the battery capacity used by this embodiment. It is a conceptual diagram which shows the difference of actual discharge time and estimated discharge time. It is a conceptual diagram which shows the increase in load current, and the change (holding current pattern of a day) of holding time by this. It is a conceptual diagram which shows the increase in load current, and the change (holding current pattern of a day) of holding time by this. It is a conceptual diagram which shows the increase in load current, and the change (holding current pattern of a day) of holding time by this. 4 is a flowchart for explaining the operation of the secondary battery holding time determination device according to the present embodiment. It is a conceptual diagram for obtaining a capacity | capacitance estimated value from the capacity | capacitance fall characteristic of a lithium secondary battery. It is a conceptual diagram for estimating a capacity | capacitance from an estimation formula. It is a conceptual diagram which shows the temperature dependence of the battery capacity of a lithium secondary battery. It is a block diagram which shows the state which attached the secondary battery holding | maintenance time determination apparatus 1 by this embodiment to the power supply for communication. It is a block diagram at the time of providing the secondary battery holding | maintenance time determination apparatus 1 by this embodiment in the monitoring center. It is a block diagram which shows the example which applied the secondary battery holding time determination apparatus 1 by this embodiment to the supply power supply of alternating current power. It is a conceptual diagram for demonstrating the capacity | capacitance calculation method of each storage battery which comprises an assembled battery by a prior art. It is a conceptual diagram which shows the retention time-capacity conversion time characteristic of a lead acid battery. It is a conceptual diagram which shows the discharge characteristic of a lead storage battery. It is a conceptual diagram for demonstrating the calculation method of storage battery capacity in case the discharge current increases with progress of time by a prior art. It is a conceptual diagram for demonstrating the calculation method of storage battery capacity in case the discharge current reduces with progress of time by a prior art. It is a conceptual diagram for demonstrating the calculation method of storage battery capacity in case the discharge current reduces with progress of time by a prior art. It is a conceptual diagram for demonstrating the calculation method of storage battery capacity in case the discharge current reduces with progress of time by a prior art. It is a conceptual diagram for demonstrating the calculation method of storage battery capacity in case the discharge current reduces with progress of time by a prior art.

Hereinafter, a secondary battery holding time determination device according to an embodiment of the present invention will be described with reference to the drawings.
FIG. 1 is a block diagram showing a basic configuration of a secondary battery holding time determination device according to an embodiment of the present invention. In FIG. 1, the secondary battery holding time determination device 1 includes a calculation unit 2, a current measurement unit 3, a measurement processing unit 4, a data storage unit 5, a display unit 6, an input unit 7, a current sensor unit 8, and a power supply unit 9. Consists of. The data storage unit 5 stores various information as battery data, for example, manufacturer data, manufacture year, model, battery data for each lot, capacity aging data, capacity temperature change data, and the like.

The current sensor unit 8 detects a load current. The current measuring unit 3 measures the load current detected by the current sensor unit 8 during a predetermined measurement period. At this time, the current measuring unit 3 also measures the measurement period (time). The measurement period is arbitrarily set. Thereby, for example, a load current pattern for one day or one week is created during a predetermined measurement period.
When the data is collected, the calculation unit 2 calculates the discharge sustainable time T of the storage battery assuming the start of discharge from the designated calculation start time. The display unit 6 displays various data stored in the data storage unit 5 and the calculated discharge sustainable time T. Further, when it is determined that the discharge sustainable time T is not sufficient, the display unit 6 notifies (warns) that effect. The input unit 7 instructs input of battery data, calculation start time on the load pattern, and the like. The power supply unit 9 generates a predetermined DC voltage from the commercial power supply 10 and supplies it to each unit.

Next, a battery capacity calculation method in the secondary battery holding time determination device 1 according to the present embodiment will be described. In the case of a lithium ion secondary battery, the calculation of the battery capacity is simpler than that of a lead storage battery. FIG. 2 is a conceptual diagram showing discharge characteristics of the lithium ion secondary battery. As shown in the figure, there is almost no change in the amount of electricity that can be extracted from a small current range (0.1 CA) to a large range (3 CA). FIG. 3 is a conceptual diagram showing the discharge capacity-discharge current characteristics of the lithium ion secondary battery. As shown in this figure, the discharge end voltage is not limited by the discharge current in any of these currents as in the lead storage battery. That is, it can be seen that there is no difference in the amount of electricity that can be taken out by discharging at any current.
Therefore, in the present embodiment, even if an arbitrary load current pattern in which the current changes with time is obtained from the above relationship, the electric current obtained from the simple product of the current and the time as shown in Expression (1). The battery capacity required for the secondary battery is obtained by sequentially adding the amounts.

In addition, C is the time divided according to storage battery capacity (or dischargeable capacity), Ii: load current, and Ti: Ii.
Therefore, unlike a lead storage battery, a lithium ion secondary battery can integrate the amount of electricity divided by time even for a load whose current changes over time (see, for example, FIG. 4). In the present embodiment, the battery capacity is calculated according to this principle. In addition, when calculating the discharge sustainable time from the load current, the reverse calculation is performed.

Next, FIG. 5 and FIG. 6 are examples of experiments performed to confirm the validity of the above principle. In the experiment, since the battery capacity to be used is determined, the discharge duration is obtained in advance and compared with the discharge duration when the actual battery is discharged along the load pattern. The actual discharge time and the estimated discharge time agreed with an error of about 1%. A similar experiment was performed for a plurality of patterns to obtain an error, which was about 1%. An example of this case is shown in FIG.
Therefore, it can be seen that the battery capacity calculation method and the discharge sustainable time calculation method proposed in this embodiment are appropriate. If safety is expected, the design is performed with the maximum current consumed by the load, but the battery capacity is likely to be excessive. Therefore, it is desirable to calculate the capacity in consideration of the load current change, and it can be easily understood that the battery capacity calculation method according to the present embodiment is easy and has a very high effect because of its high accuracy. In the present embodiment, the calculation method based on the product of the current and the elapsed time described above is a first feature.

Next, the discharge sustainable time after battery installation will be described. The capacity of the battery to be installed is selected by the process described above. However, the capacity selection is based solely on the design current value before battery installation, and the actual current consumed by the load can be sufficiently different from the design value.
If the load current is lower than the design value, the time that can be backed up by discharging from the battery will be extended, and the problem will be on the safety side, except that an excessively large battery is installed. No. The problem is when the actual load current becomes larger than the design value. In the case of communication, etc., if the demand for communication increases or the number of communication facilities connected to the power supply facilities increases. , Etc. In such a case, the actual current consumption of the load has increased from the initial value of the battery installation, and when backup is performed by discharging from the storage battery, it is difficult to secure the initial backup time, resulting in service failure. There is a fear.

Here, FIGS. 8 to 10 are conceptual diagrams showing an increase in load current and a change in holding time (a load current pattern of one day) due to this. As shown in FIG. 8, for example, in the power failure from 19:30, as shown in FIG. 9, the discharge duration at the time of design is estimated to be 14 hours. On the other hand, when the load current increases during actual use, when a power failure occurs from the same time, the discharge duration is shortened to 11 hours as shown in FIG. This is a time reduction of 21%, and if applied to battery replacement due to a decrease in battery capacity, the battery needs to be replaced. In such a case, since the initial discharge duration cannot be secured, there may be a problem accompanying maintenance.
Therefore, it is necessary to periodically measure the load current and check whether the capacity of the installed storage battery is suitable for the required backup time. In the present embodiment, such a battery capacity is confirmed, and when time is insufficient, for example, a notification (alarm) is sent to the monitoring device.

  Next, the operation of the secondary battery holding time determination device according to the present embodiment will be described. Here, FIG. 11 is a flowchart for explaining the operation of the secondary battery holding time determination device according to the present embodiment. First, storage battery data is input from the input unit 7 (S1). The data includes manufacturer, year of manufacture, production lot, and number of years installed. Next, the calculation start time of the discharge sustainable time is set from the input unit 7 (S2), and a warning level (for example, 70% or 80%) is input (S3). Next, a desired discharge duration (required discharge time) is input from the input unit 7 (S4).

  When the input of various data is completed, the load current is detected by the current sensor unit 8, and the load current is measured by the current measurement unit 3 during a predetermined measurement period (S5). Next, a pattern of the measured load current during the measurement period is created (S6), and the discharge sustainable time is calculated (S7). Next, the calculated value of the discharge sustainable time is displayed on the display unit 6 (S8), and an alarm is issued when the discharge sustainable time is insufficient with respect to the discharge duration (required discharge time). (S9).

  Details will be described below. The calculation method of the discharge sustainable time is from the capacity of the installed secondary battery until the time when the remaining amount becomes zero by sequentially subtracting the product of the load current and the elapsed time. That is, assuming that the discharge was performed along the measured load current, as shown in Equation (2), from the capacity of the installed storage battery, the amount of electricity (the product of current and time. In some cases, this is the time until it becomes zero after subtracting the sum of the electric quantities obtained for each current step.

  Here, T: discharge duration time, Td: divided unit time, Ii: load current at each Td, C: storage battery capacity (or dischargeable capacity).

When the load pattern is considered by dividing the time in a fixed time unit, the load pattern is up to “n” time units that satisfy the formula (2). The load current pattern may be obtained as a pattern for each day of the week as an average of several consecutive days. The above-described calculation method of subtracting the amount of electricity based on the product of current and time from the capacity of the storage battery is a second feature.
In the initial calculation of the battery capacity, it is required to secure a predetermined duration at the assumed load current. However, when the load current increases, it is difficult to secure this time.
Therefore, if such a state is detected by the above calculation, an alarm is issued.

  As a criterion for issuing an alarm, 70% or 80% of the initially set time is considered appropriate. This is because the renewal standard when the battery is deteriorated and the capacity is reduced is set to these values. That is, when the battery capacity is reduced to 70% or 80%, even if the load does not change from the initial value, the holding time is reduced to 70% or 80% of the original design value. However, any value can be set.

As shown in FIG. 12, the battery capacity decreases with time after installation. Therefore, in the present embodiment, the remaining battery capacity is obtained in consideration of the secular change at the time of calculation. The capacity reduction may be calculated from the estimation formula as shown in FIG. 13 from the relationship proportional to the square root of the service life, or can be obtained by extrapolation of FIG. Furthermore, since the battery capacity is affected by temperature, the temperature of the capacity obtained by the above method is corrected. For this, the temperature dependence relationship shown in FIG. 14 is used. Then, based on the capacity thus obtained, the discharge sustainable time by the load current pattern is obtained.
When the secondary battery holding time determination device 1 (or its function) according to the present embodiment is provided in the monitoring center, it is possible to set an interval between current measurement and dischargeable time calculation in advance. is there.

FIG. 15 is a block diagram illustrating a state in which the secondary battery holding time determination device 1 according to the present embodiment is attached to a communication power source. In this case, in the circuit of the commercial power source 30, the rectifier 31, and the load device 32, the discharge current to the load device 32 is measured by the secondary battery holding time determination device 1 according to this embodiment, and the discharge of the backup battery 20 can be continued. Perform time calculations.
FIG. 16 is a block diagram when the secondary battery holding time determination device 1 according to the present embodiment is provided in the monitoring center. In this case, load current data from the storage battery facilities 40 to 43 of each communication building is arbitrarily collected by the monitoring center 45, and the secondary battery holding time determination device 1 according to the present embodiment collectively calculates the discharge sustainable time. .

  FIG. 17 is a block diagram illustrating an example in which the secondary battery holding time determination device 1 according to the present embodiment is applied to an AC power supply. In this case, the secondary battery according to the present embodiment is converted into a direct current from the commercial power source 30 by the AC / DC conversion unit 51, converted to an alternating current again by the DC / AC conversion unit 52, and supplied to the load device 32. The holding time determination device 1 measures a DC discharge current to the load device 32 and calculates the discharge sustainable time of the backup battery 20. At this time, if necessary, the calculation is performed by replacing the alternating current with the direct current in consideration of the efficiency of the DC / AC converter 52.

  According to the above-described embodiment, even after the installation of the battery, even when the change in the load current occurs and the initial holding time cannot be secured, there may be a problem in maintenance. By measuring the load current periodically and checking whether the required backup time is secured, the discharge sustainable time (battery capacity) can be calculated, and the discharge sustainable time Since the alarm is issued when the shortage occurs, the relationship between the battery and the load can always be properly managed, and a highly reliable power supply system can be constructed.

  In the above-described embodiment, the series of processes performed by the arithmetic unit 2, the measurement processing unit 4, and the like may be realized by a program stored in a computer-readable recording medium. In this case, the above processing is performed by the computer reading and executing the program. That is, the calculation unit 2, the measurement processing unit 4, and the like may be realized by a central processing unit such as a CPU reading the program into a main storage device such as a ROM or a RAM and executing the calculation process. Alternatively, the computer program may be distributed to the computer via a communication line, and the computer that has received the distribution may execute the program.

1 Secondary battery retention time determination device (secondary battery capacity calculation device, secondary battery monitoring device)
2 Calculation unit (battery capacity calculation means, electric quantity calculation means, discharge sustainable time calculation means, determination means, alarm means, correction means)
3 Current measurement unit (load current measurement means, time measurement means)
4 Measurement processing section 5 Data storage section (storage means)
6 Display (alarm means)
7 Input section 8 Current sensor section (load current measuring means)
DESCRIPTION OF SYMBOLS 9 Power supply part 10 Commercial power supply 30 Commercial power supply 31 Rectifier 32 Load apparatus 40-43 Storage battery equipment 45 Monitoring center 51 AC / DC conversion part 52 DC / AC conversion part

Claims (5)

A secondary battery monitoring device of a power supply system equipped with a secondary battery as a backup power source,
Load current measuring means for measuring the load current;
A time measuring means for measuring a measurement time by the load current measuring means;
An electric quantity calculating means for calculating an electric quantity per unit time based on a load current by the load current measuring means and a measurement time by the time measuring means;
A discharge sustainable time calculating means for sequentially subtracting a predetermined amount of electricity per unit time by the charge calculating means from the battery capacity of the secondary battery and calculating a discharge sustainable time of the secondary battery; A secondary battery monitoring device. Storage means for storing a discharge duration required when the secondary battery is used as a backup power source;
Determining means for determining whether or not the battery capacity of the secondary battery power can be discharged only for the discharge duration stored in the storage means, based on the discharge sustainable time by the discharge duration calculating means; ,
The secondary battery monitoring device according to claim 1, further comprising: an alarm unit that issues an alarm when it is determined by the determination unit that discharge is not possible for the discharge duration.   The secondary battery monitoring apparatus according to claim 2, further comprising a correction unit that corrects the discharge sustainable time based on a secular change of the secondary battery.   The secondary battery monitoring apparatus according to claim 2, further comprising a correcting unit that corrects the discharge sustainable time based on an ambient temperature. A secondary battery monitoring method for a power supply system provided with a secondary battery as a backup power source,
Measuring the load current; and
Measuring the load current measurement time;
Calculating an amount of electricity per unit time based on the load current and the measurement time;
Subtracting the amount of electricity per unit time from the battery capacity of the secondary battery in order, and calculating the discharge sustainable time of the secondary battery.

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