HK1072469B - Battery pack and remaining battery power calculation method - Google Patents

Description

Battery pack and battery remaining capacity calculation method

CROSS-REFERENCE TO RELATED APPLICATIONS

This document is based on the Japanese priority document JP2003-384673 filed on the sun to the patent office on 11/14/2003, the entire contents of which are incorporated herein by reference to the extent allowed by law.

Technical Field

The present invention relates to a battery pack and a battery remaining capacity calculation method, and more particularly, to a battery pack and a battery remaining capacity calculation method that allow the battery remaining capacity to be calculated based on the number of charge/discharge cycles of a battery and the temperature during use.

Background

Batteries (secondary batteries) such as liquid ion batteries have a specific capacity, and their capacity has characteristics that vary according to the use temperature of the battery.

When the battery pack is used at a low temperature, the internal resistance of the battery cells thereof increases, and when the same current as that at normal temperature is applied, a large voltage drop occurs, so that the capacity of the battery pack decreases.

Fig. 11 shows discharge characteristics of the battery pack at 25 ℃, 10 ℃, and 0 ℃. The horizontal axis represents time and the vertical axis represents voltage.

As shown in fig. 11, in the case where the discharge amount was 2.0W and the terminal voltage was 3.35V, the following measurement results were obtained: for example, if the dischargeable capacity at an ambient temperature of 25 ℃ is set to 100%, the dischargeable capacity measurement result is about 80% at an ambient temperature of 10 ℃ and about 60% at 0 ℃.

On the other hand, the battery pack has a characteristic of reduced capacity even in the case of continuous use and an increased number of charge/discharge cycles. This is because the battery pack repeatedly performs charge/discharge cycles, which may cause degradation of the battery cell such that its usable capacity is reduced, which is referred to as "cycle degradation".

The usable capacity of a battery cell undergoing many charge/discharge cycles varies by two factors: cycle degradation and temperature during use.

Heretofore, as described in Japanese laid-open patent application JP-A-2000-260488, ｃA technique of detecting the temperature of ｃA battery pack by using ｃA temperature sensor and correcting the remaining capacity of the battery has been used to correct the reduction in the capacity of the battery pack used at ｃA low temperature. (paragraph numbers [0038] to [0072])

Also, a technique of counting the number of charge/discharge cycles, determining that cycle degradation is proceeding from the counted number, and estimating that the remaining battery capacity is lower than the actual battery capacity is used to correct the battery degradation caused by the number of charge/discharge cycles.

In these conventional techniques, the correction of the usable time in the current temperature environment is performed by measuring the current temperature using a thermistor, and the correction of the usable time of the battery cell after the lapse of a plurality of charge/discharge cycles is performed by counting the number of charge/discharge cycles.

Specifically, temperature data and a temperature-dependent correction value are stored, and when the temperature data shows a low temperature, the battery remaining capacity is estimated to be lower than the actual battery remaining capacity in consideration of degradation of the discharge characteristic of the battery cell. Similarly, a correction value of cycle degradation is prepared, and when the number of charge/discharge cycles increases, the battery remaining capacity is estimated to be low. In this way, two kinds of correction values, i.e., temperature and cycle degradation correction values, are prepared and stored as data in the battery pack, thereby correcting the battery remaining capacity according to the usage environment and the number of times of use.

Disclosure of Invention

However, cycle degradation and temperature during use are correlated, and as the number of charge/discharge cycles increases, there is a characteristic that the reduction in capacity of the battery cell used at low temperatures becomes greater. That is, if correction values for correcting a capacity decrease due to temperature and a capacity decrease due to the number of charge/discharge cycles are taken as independent parameters, there is a problem in that: when the number of charge/discharge cycles increases, an error may occur between the calculated remaining battery capacity and the actual remaining battery capacity.

For example, there is a problem in that if a correction value is set such that cycle degradation at normal temperature is corrected, an error is generated in calculating the remaining amount of the battery at low temperature when the number of charge/discharge cycles increases, whereas if a correction value is set such that cycle degradation at low temperature is corrected, an error is generated in calculating the remaining amount of the battery at normal temperature.

The present invention has been conceived in view of the above-mentioned problems, and preferred embodiments of the present invention provide a battery pack and a battery remaining capacity calculation method capable of reducing a calculation error of a battery remaining capacity in consideration of a capacity reduction due to cycle degradation and temperature.

According to a preferred embodiment of the present invention, there is provided a battery pack capable of calculating a remaining capacity of a battery based on a number of charge/discharge cycles and a temperature during use, characterized by comprising: temperature measuring means for measuring a temperature of the battery cell; charge/discharge cycle counting means for counting the number of charge/discharge cycles; a correction value storage means for storing a correction value which changes every predetermined charge/discharge cycle number and calculates the remaining battery capacity according to the measured temperature; and remaining capacity calculating means for retrieving the correction value storing means on the basis of the measured temperature and the counted number of charge/discharge cycles to specify a temperature correction value, and calculating the remaining capacity of the battery based on the specified temperature correction value.

According to this construction, the remaining power calculating means specifies a correction value for calculating the remaining power of the battery based on the measured temperature from among the correction values that vary every predetermined number of charge/discharge cycles stored in the correction value storing means, on the basis of the temperature of the battery cell measured by the temperature measuring means and the number of charge/discharge cycles counted by the charge/discharge counting means, and calculates the remaining power of the battery based on the specified correction value. The temperature correction value that varies every predetermined number of charge/discharge cycles is used instead of the different temperature correction values set for all the number of charge/discharge cycles, whereby the correction of the remaining battery capacity can be performed based on the cycle degradation and the temperature with a small number of parameters.

The battery pack according to the preferred embodiment of the present invention, which is used to calculate the correction value of the remaining battery capacity according to the temperature, is specified from among the correction values that vary according to the number of predetermined charge/discharge cycles that are stored every other on the basis of the measured temperature of the battery cells and the counted number of charge/discharge cycles, and the remaining battery capacity is calculated according to the specified correction value. Therefore, even if the number of charge/discharge cycles increases and cycle degradation occurs, the actual remaining battery capacity in various temperature environments can be calculated more accurately.

In addition, since the correction value that varies every predetermined number of charge/discharge cycles is used, the correction of the remaining amount of the battery can be performed with a small number of parameters.

The present invention is applicable to a battery pack connected to, for example, a video camera, a digital still camera, or a battery charger, and the like.

Drawings

The above and other objects, features, and advantages of the present invention will become more apparent from the following description of preferred embodiments of the present invention taken in conjunction with the accompanying drawings, in which:

fig. 1 shows a functional block diagram of the principle of a battery pack according to a preferred embodiment of the present invention;

FIG. 2 shows an example of stored temperature correction values;

fig. 3 illustrates an example of a hardware structure of a battery pack according to a preferred embodiment of the present invention;

fig. 4 is a graph showing discharge characteristics of a battery cell for different numbers of charge/discharge cycles at 25 ℃ (normal temperature);

fig. 5 is a graph showing discharge characteristics of a battery cell at 0 c (low temperature) for different numbers of charge/discharge cycles;

fig. 6 shows a capacity change characteristic diagram corresponding to the temperature and the number of charge/discharge cycles of the battery cell;

fig. 7 is a graph showing the number of charge/discharge cycles versus capacity in the course of performing correction according to cycle degradation at normal temperature;

FIG. 8 is a graph showing the number of charge/discharge cycles versus capacity during which correction is performed according to cycle degradation at low temperatures;

FIG. 9 is a schematic diagram illustrating the manner in which correction is implemented for a battery pack in accordance with a preferred embodiment of the present invention;

fig. 10 illustrates an operation flowchart of a battery pack according to a preferred embodiment of the present invention; and

fig. 11 shows a discharge characteristic diagram of the battery pack at 25 deg.c, 10 deg.c, and 0 deg.c.

Detailed Description

Preferred embodiments of the present invention will be described in detail below with reference to the accompanying drawings.

Fig. 1 shows a functional block diagram of the principle of a battery pack according to a preferred embodiment of the present invention.

The battery pack 10 according to the preferred embodiment of the present invention includes: a temperature measuring device 12 for measuring the temperature of the battery cells 11; a charge/discharge cycle counting means 13 for counting the number of charge/discharge cycles; a correction value storage means 14 for storing a correction value (hereinafter referred to as "temperature correction value") that changes according to the number of charge/discharge cycles per predetermined number and that is used to calculate the remaining battery capacity according to the measured temperature; remaining capacity calculating means 15 for specifying a temperature correction value by retrieving the correction value storing means 14 on the basis of the measured temperature and the counted number of charge/discharge cycles, and calculating a remaining capacity of the battery based on the specified temperature correction value; and a communication means 16 for transmitting the calculated remaining battery capacity to a connected device, which is not shown.

For example, the battery cells 11 may be lithium ion batteries. The temperature measuring device 12 may be a thermistor mounted on the surface of the battery cell or on a circuit board. The charge/discharge counting means 13 and the remaining power calculating means 15 may be implemented by, for example, a microcontroller. The correction value storage device 14 may be, for example, an EEPROM (electrically erasable programmable read only memory).

In the following description of the embodiments, the term "charge/discharge cycle" refers to a process of charging a battery cell once and then discharging the battery cell to a certain voltage level, which is defined as one charge/discharge cycle.

The temperature correction value used in the preferred embodiment of the present invention is changed every predetermined number of charge/discharge cycles.

Fig. 2 shows an example of a stored temperature correction value.

For example, temperature correction values for which the number of charge/discharge cycles is 50 or less are stored in addresses 00 to 06 of the correction value storage device 14, respectively. According to different use process temperatures, the addresses 00-06 respectively store different temperature correction values, for example, temperature correction values under the conditions of 0 ℃ below, 0-10 ℃, 10-20 ℃, 20-30 ℃, 30-40 ℃, 40-50 ℃ and 50 ℃ above. For example, for 0-10 ℃, the temperature correction value for reducing the capacity at a rate of 10%/50 cycles is stored, and for 25 ℃, the temperature correction value for reducing the capacity at a rate of 4%/50 cycles is stored. In order to reduce the number of parameters, the temperature correction value may be set such that the capacity is reduced at a rate of 4%/50 cycles for all temperatures for 50 cycles or less.

Further, values obtained by changing these temperature correction values that change every predetermined number of charge/discharge cycles are stored.

As shown in FIG. 2, the temperature correction values for 51-100 charge/discharge cycles are stored in addresses 10-16, the temperature correction values for 101-150 charge/discharge cycles are stored in addresses 20-26, the temperature correction values for more than 150 charge/discharge cycles are stored in addresses 30-36, and the temperature correction values for each cycle number range are corrected. For example, the temperature correction value may be modified by modifying the capacity reduction ratio to be different from the reduction ratio of the temperature correction value for 50 cycles or less, or may be modified by increasing or decreasing the capacity from each cycle number range to the next range in a stepwise manner (this example will be described later).

The above-described temperature correction value is set based on the capacity characteristics that vary with the temperature of the battery cell 11 and the number of charge/discharge cycles, and is stored in the correction value storage device 14.

The operation of the battery pack 10 will be described below.

For example, when the battery pack 10 is connected to an apparatus such as a video camera or a digital still camera and starts to be used, the temperature of the battery cell 11 is measured by the temperature measuring device 12. The charge/discharge counting means 13 sends the currently counted number of charge/discharge cycles to the remaining capacity calculating means 15. The remaining amount calculating means 15 specifies a temperature correction value from the stored correction values as shown in fig. 2 by retrieving the correction value storing means 14 on the basis of the measured temperature and the number of charge/discharge cycles, and calculates the battery remaining amount from the specified temperature correction value. The communication means 16 transmits the calculated remaining battery power so that the usable time can be displayed on a connected device such as, for example, a video camera or a digital still camera.

In this way, a temperature correction value is specified from among temperature correction values that vary every predetermined number of charge/discharge cycles on the basis of the measured temperature of the battery cell and the counted number of charge/discharge cycles, and the battery remaining capacity is calculated based on the specified temperature correction value. Therefore, even if the number of charge/discharge cycles increases and cycle degradation occurs, the actual remaining battery capacity in various temperature environments can be calculated more accurately.

In addition, used herein are temperature correction values that vary every predetermined number of charge/discharge cycles (for example, 50 and 100 cycles), rather than different temperature correction values that are set for all the number of charge/discharge cycles. Therefore, the correction of the remaining battery capacity can be achieved by a small number of parameters.

The details of the preferred embodiment of the invention will be described below.

Fig. 3 shows an example of a hardware structure of a battery pack according to a preferred embodiment of the present invention.

The battery pack 50 includes a battery cell 51, a peripheral circuit 52, a microcontroller 53, a thermistor 54, and a communication circuit 55.

For example, the battery cell 51 may be a lithium ion battery, a nickel-metal hydride battery, or a lithium polymer battery.

The positive electrode of the battery cell 51 is connected to the positive terminal 61, and the negative electrode of the battery cell 51 is connected to the negative terminal 62 through the current detection resistor Rs and the charge control switch SW1 and the discharge control switch SW2, which are made of a power MOSFET (metal oxide semiconductor field effect transistor) and a diode, respectively.

The peripheral circuit 52 includes a circuit configuration mainly composed of a voltage comparator (comparator), and has a function of detecting a charge/discharge current value flowing through the current detection resistor Rs, and a protection function of preventing overcharge, overdischarge, or overcurrent of the battery cell 51. Specifically, when the voltage of the battery cell 51 is equal to or higher than the set voltage, the peripheral circuit 52 turns off the charge control switch SW1 to stop charging, thereby preventing overcharging. On the other hand, when the voltage of the battery cell 51 is lower than the set voltage, the peripheral circuit 52 turns off the discharge control switch SW2 to stop the discharge, thereby preventing the over-discharge.

The microcontroller 53 cumulatively adds up the charge/discharge currents detected by the peripheral circuit 52, and calculates the remaining battery capacity based on the temperature during operation and the number of charge/discharge cycles. In addition, the microcontroller 53 controls the thermistor 54 to measure the temperature of the battery cell during use. The microcontroller 53 also has a built-in EEPROM as well as correction value storage means for storing the temperature correction value as shown in fig. 2. The microcontroller 53 also has a function of controlling the communication circuit 55 to transmit the calculated value of the remaining amount of the battery to the connected device.

Before describing the operation of the battery pack 50 according to the preferred embodiment of the present invention, the correlation between the temperature and the charge/discharge cycle will be described below.

Fig. 4 shows a discharge characteristic diagram of a battery cell for different numbers of charge/discharge cycles at 25 deg.c (normal temperature).

Fig. 5 shows a discharge characteristic diagram of a battery cell for different numbers of charge/discharge cycles at 0 c (low temperature).

In fig. 4 and 5, the horizontal axis represents time and the vertical axis represents voltage, and the discharge characteristics shown in the figures are obtained by repeating charge/discharge cycles under 4.2V voltage, 0.5A current, and 2.5 hours of charge condition, and under 2w constant power discharge condition.

As shown in fig. 4, for example, in the case where the battery cell 51 is discharged to 3.35V and the discharge time of the battery cell using 0 charge/discharge cycles is set to 100% during use at, for example, 25 ℃ (normal temperature), the discharge time of the battery cell using 50 charge/discharge cycles is 94%, the discharge time of the battery cell using 100 charge/discharge cycles is 90%, and the discharge time of the battery cell using 500 charge/discharge cycles is reduced to 68%. That is, it can be observed that the capacity of the battery cell is reduced by about 5% per 50 uses due to cycle degradation.

On the other hand, as shown in fig. 5, during use at 0 ℃ (low temperature), the discharge time of the battery cell 51 using even 0 charge/discharge cycles is reduced to 60% compared to the battery cell 51 using 0 charge/discharge cycles at normal temperature as shown in fig. 4. As the number of charge/discharge cycles of the battery 10 at 0 ℃ increases, the discharge time was reduced to 46% for 50 cycles, to 39% for 100 cycles, and to 10% for 500 cycles, compared to the battery cell 51 using 0 charge/discharge cycles at normal temperature. That is, it can be seen that the cycle degradation rate of the battery cell 51 used at 0 ℃ is greater than that of the battery cell used at normal temperature.

Fig. 6 shows a capacity change characteristic diagram corresponding to the temperature and the number of charge/discharge cycles of the battery cell.

The horizontal axis represents the number of charge/discharge cycles, and the vertical axis represents the capacity. The capacity of the battery cell using 0 charge/discharge cycles at 25 c (normal temperature) was 100%.

As can be seen from fig. 6, the degree of cycle degradation due to an increase in the number of charge/discharge cycles is increased at low temperatures.

If the cycle deterioration is corrected by two parameters such as the number of charge/discharge cycles and the operating temperature, it will be difficult to calculate the correct remaining battery capacity.

Fig. 7 shows a graph of the number of charge/discharge cycles at normal and low temperatures as a function of capacity, corrected for cycle degradation at normal temperature.

In fig. 6 showing the relationship between the number of charge/discharge cycles and the capacity at normal and low temperatures, fig. 7 shows the corrected parameters set to linearly decrease the capacity by 4% every 50 cycles according to the cycle degradation at 25 ℃ (normal temperature). In this case, if the capacity of the battery cell using 0 charge/discharge cycles at 0 ℃ (low temperature) is set to 65%, the error of the theoretical value increases as the number of charge/discharge cycles increases, as shown in fig. 7. That is, there may be a problem that although the calculated remaining battery capacity is still a usable value, the actual remaining battery capacity is lower than the calculated remaining battery capacity and cannot be used.

Fig. 8 shows a graph of the relationship between the number of charge/discharge cycles and the capacity at normal and low temperatures in the case of correction according to cycle degradation in the case of low temperature.

In fig. 6 showing the relationship between the number of charge/discharge cycles and the capacity at normal and low temperatures, according to cycle degradation at 0 ℃ (low temperature) (for example, 0 to 100 charge/discharge cycles), the capacity using 0 charge/discharge cycles was set to 60% in fig. 8, and correction was performed using a parameter set to linearly decrease the capacity by 10% every 50 cycles. In this case, as the number of charge/discharge cycles increases at 25 ℃ (normal temperature), the error of theoretical value increases. Therefore, there may occur a problem that the calculated remaining battery capacity is lower than the actual remaining battery capacity, although the battery pack having the increased number of charge/discharge cycles can be actually used at normal temperature.

If a correction value for correcting the cycle deterioration and a correction value for correcting the capacity decrease caused by the temperature are made in combination, it is possible to calculate the correct battery remaining capacity in both the normal temperature and the low temperature, but there is a problem in that the number of parameters increases and it is difficult to perform the setting.

On the other hand, the battery pack 50 according to the preferred embodiment of the present invention may solve this problem by using the temperature correction value that is changed every predetermined number of charge/discharge cycles (e.g., 50 and 100 times) as shown in fig. 2.

The operation of the battery pack 50 according to the preferred embodiment of the present invention will be described below.

When the positive terminal 61 and the negative terminal 62 of the battery pack 50 are connected to an apparatus such as a video camera or a digital still camera, and the battery pack 50 starts to be used, the microcontroller 53 cumulatively sums up the charge/discharge current values detected by the peripheral circuit 52, and calculates the remaining battery capacity. In addition, the microcontroller 53 counts the number of charge/discharge cycles and stores the number of charge/discharge cycles in, for example, an internal EEPROM of the microcontroller 53.

In the battery remaining capacity calculation process, the microcontroller 53 specifies a temperature correction value by retrieving, for example, an EEPROM storing the temperature correction value as shown in fig. 2, on the basis of the number of charge/discharge cycles and the temperature measured by the thermistor 54, and calculates the battery remaining capacity based on the specified temperature correction value.

Fig. 9 shows a schematic diagram of the manner in which correction is achieved for a battery pack according to a preferred embodiment of the invention.

In fig. 9, on the basis of fig. 6 showing the relationship between the number of charge/discharge cycles and the capacity at normal and low temperatures, cycle correction for linearly decreasing the capacity by 4% every 50 cycles was performed in accordance with cycle deterioration at 25 ℃ (normal temperature). In this case, the cycle correction is performed to reduce the capacity of the battery pack used at low temperature every 50 cycles (in the example of fig. 9, 150 cycles or less) in a stepwise manner. That is, the temperature correction values for reducing the capacity in a stepwise manner every time the number of charge/discharge cycles reaches 50, 100, 150 times are set. In the other stages, the temperature correction value is set to linearly decrease the capacity by 4%, as in the case of the ordinary temperature. Since the temperature correction value used is changed every predetermined number of charge/discharge cycles, the correction of the remaining battery capacity can be achieved by a small number of parameters.

In the above description, the case of the cyclic correction in which the capacity linearly decreases by 4% every 50 cycles according to the cyclic degradation at normal temperature has been referred to. However, during normal temperature use, cyclic correction to increase capacity in a stepwise manner every 50 cycles may be performed, so that cyclic correction to decrease capacity by 10% every 50 cycles may be performed according to cyclic degradation at low temperatures.

The communication circuit 55 transmits the battery remaining capacity calculated in this manner to the connected devices under the control of the microcontroller 53, so that, for example, a video camera and a digital still camera can inform the user of an approximate value of the actual discharge time of the battery pack 50 at the current temperature.

The operation of the battery pack according to the preferred embodiment of the present invention will be summarized below in the form of a flowchart.

Fig. 10 illustrates a flowchart of the operation of the battery pack according to the preferred embodiment of the present invention.

Step S1: temperature measurement

The microcontroller 53 controls the thermistor 54 to measure the temperature of the battery cell 51. The subsequent steps S2 to S8 are a process of specifying a temperature correction value in accordance with the measured temperature and the number of charge/discharge cycles. In the description thereof, reference is made to the case where 50 charge/discharge cycles, 50 to 100 charge/discharge cycles, 101 to 150 charge/discharge cycles, and 151 or more charge/discharge cycles are stored in the EEPROM built in the microcontroller 53 for not more than 50 charge/discharge cycles.

The microcontroller 53 determines whether the number of charge/discharge cycles is greater than 50 (step S2), and if the number of charge/discharge cycles is not greater than 50, the microcontroller 53 retrieves the EEPROM and sets a temperature correction value for not greater than 50 charge/discharge cycles according to the temperature measured in step S1 in step S3. If the number of charge/discharge cycles is greater than 50, the microcontroller 53 determines whether the number of charge/discharge cycles is greater than 100 in step S4. In step S4, if the number of charge/discharge cycles is not more than 100, then in step S5, the microcontroller 53 sets a temperature correction value for 51 to 100 charge/discharge cycles based on the temperature measured in step S1. If the number of charge/discharge cycles is greater than 100, the microcontroller 53 determines whether the number of charge/discharge cycles is greater than 150 in step S6. In step S6, if the number of charge/discharge cycles is not more than 150, then in step S7, the microcontroller 53 sets a temperature correction value for 101 to 150 charge/discharge cycles in accordance with the temperature measured in step S1. If the number of charge/discharge cycles is greater than 150, the microcontroller 53 sets a temperature correction value for more than 150 charge/discharge cycles in accordance with the temperature measured in step S1 in step S8.

After the temperature correction value is set in step S3, S5, S7, or S8, the microcontroller 53 calculates the battery remaining amount based on the set temperature correction value in step S9. The microcontroller 53 controls the communication circuit 55 to transmit the calculated value of the remaining amount of the battery to a connected device, which is not shown.

The above-mentioned process can be implemented by software provided in the microcontroller 53, and thus the cost can be considered not to be significantly increased compared to the existing products.

The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention, and various modifications and changes may be made by those skilled in the art. Any modification, equivalent replacement, or improvement made within the spirit and principle of the present invention should be included in the scope of the claims of the present invention.

Claims (4)

1. A battery pack (10) comprising a battery cell (11) and means for calculating a remaining battery charge from a number of charge/discharge cycles and a temperature during operation, the remaining charge calculating means comprising:

a temperature measuring device (12) for measuring the temperature of the battery cell (11);

a charge/discharge cycle counting means (13) for counting the number of charge/discharge cycles;

a correction value storage means (14) for storing a correction value for calculating a remaining amount of battery power corresponding to the temperature corresponding to each of a plurality of charge/discharge cycle ranges, each range including a predetermined number of charge/discharge cycles; and

remaining capacity calculating means (15) for specifying a temperature correction value by searching the correction value storing means on the basis of the measured temperature and the counted number of charge/discharge cycles, and calculating the remaining capacity of the battery based on the specified temperature correction value.

2. The battery pack (10) according to claim 1, wherein the correction value includes a value that varies with the number of charge/discharge cycles according to a capacity variation characteristic that occurs in response to the temperature of the battery cell and the number of charge/discharge cycles.

3. The battery pack (10) according to claim 1, further comprising communication means (16) for transmitting the calculated remaining battery power to a connected device.

4. A remaining battery capacity calculation method for calculating a remaining battery capacity based on the number of charge/discharge cycles and a temperature during operation, comprising the steps of:

measuring the temperature of the battery cells;

specifying a temperature correction value for calculating a remaining amount of battery power corresponding to the measured temperature, wherein the correction value is obtained from a plurality of stored values, each corresponding to a temperature and a charge/discharge cycle range, each range including a predetermined number of charge/discharge cycles, the obtained correction value being selected on the basis of the measured temperature and a current number of charge/discharge cycles; and

the remaining battery capacity corresponding to the correction value is calculated.