Method and apparatus for detecting battery capacity

A method and apparatus for detecting the capacity of a battery wherein a voltage method of measuring the voltage of the battery, to calculate the capacity of the battery based on the correlation between the voltage and the capacity of the battery, is switched to a current integrating method of integrating the current magnitude of the battery with respect to time, to calculate the capacity of the secondary battery, and vice versa, with a pre-set current magnitude as a threshold value, in order to detect the capacity of the battery. By selectively using the voltage method and the current integrating method depending on the current magnitude of the battery, the capacity of the battery can be calculated with greater accuracy.

BACKGROUND OF THE INVENTION
 1. Field of the Invention
 The present invention relates to a method for detecting the capacity of a
 battery loaded on a piece of electronic equipment, such as a portable
 personal computer, for furnishing the power. The present invention also
 relates to an associated battery pack and to an associated electronic
 equipment system.
 2. Description of the Prior Art
 In order to know the capacity (residual capacity) of a battery, such as a
 lithium ion battery, it is generally practiced to estimate the residual
 capacity from a terminal voltage of the battery or to integrate the efflux
 current to estimate the residual capacity.
 As a method for detecting the capacity (residual capacity) of such battery,
 there are currently proposed a voltage method and a current integrating
 method. The voltage method measures the voltage of the battery to
 calculate the capacity of the battery based on the correlation between the
 voltage and the capacity of the battery. The current integrating method
 integrates the current of the battery with respect to time to calculate
 the capacity of the battery. In the voltage method, the capacity
 calculation accuracy is high when the current of the battery is small,
 conversely, in the current integrating method (coulomb method), the
 capacity calculation accuracy is high when the current of the battery is
 large. Since the voltage method is based on the voltage, there is no
 integrating error, but there is a direct error. Since the current
 integrating method integrates and updates the current with respect to a
 reference value, the integration error is significant, though the direct
 error is small.
 The voltage method calculates the capacity from the correlation between the
 capacity and the terminal-to-terminal voltage of the battery (cell
 voltage). Since the battery (cell) has an internal resistance and a hence
 A the terminal voltage is fluctuated depending on the flowing current,
 correction is applied based on the current multiplied by the inner battery
 voltage. If the current is enlarged, the amount of correction becomes
 larger to increase the error.
 The current integrating method integrates the current with respect to time
 to find the electrical quantity Ah. For improving the precision, current
 measurement accuracy needs to be raised. For this reason, an operational
 amplifier or an analog-to-digital (A/D) converter of extremely high
 precision is used. Nevertheless, there is produced significant error if
 charging/discharging is repeated over an extended time interval, or if the
 current is of a small magnitude, so that contrivances such as calibration
 are occasionally required such as at the time of charging the battery to
 its fill capacity. If the calibration timing is lost, significant errors
 are inevitably produced.
 SUMMARY OF THE INVENTION
 In view of the above-mentioned problems, an object of the present invention
 is to provide a battery capacity detection method, a battery pack and an
 electronic equipment system, in which the voltage method and the current
 integration method are selectively used depending on the magnitude of the
 current through the battery to raise the calculation accuracy of the
 capacity of the battery (residual capacity).
 For solving the above problem, the present invention provides a method for
 detecting the capacity of a battery in which a voltage method of measuring
 the voltage of the battery to calculate the capacity of the battery, based
 on the correlation between the voltage and the capacity of the battery, is
 switched to a current integrating method of integrating the current
 magnitude of the battery with respect to time, to such is done calculate
 the capacity of the battery, and vice versa, with a pre-set current
 magnitude as a threshold value, in order to detect the capacity of the
 battery.
 The present invention also provides a method for detecting the capacity of
 a battery in which a voltage method of measuring the voltage of the
 secondary battery, to calculate the capacity of the battery based on the
 correlation between the voltage and the capacity of the battery, is
 switched to a current integrating method of integrating the current
 magnitude of the battery with respect to time to calculate the capacity of
 the battery, and vice versa, with a pre-set magnitude of voltage drop as a
 threshold value, in order to detect the capacity of the battery.
 With the capacity detection method according to the present invention, the
 capacity of a battery can be calculated with greater accuracy by switching
 between the voltage method and the current integrating method responsive
 to a pre-set current magnitude or to a pre-set magnitude of the voltage
 drop.
 More specifically, the present invention provides a battery pack having the
 function of detecting the capacity of a battery, wherein the battery pack
 includes voltage detection means for detecting the voltage of the battery
 to calculate the capacity of the battery based on; correlation between the
 voltage and the capacity of the battery, current detection means for
 detecting the current magnitude of the battery to calculate the capacity
 of the battery by integrating the current magnitude of the battery with
 respect to time, and control means for switching between the operation of
 calculating the capacity of the battery based on the correlation between
 the voltage and the capacity of the battery responsive to a pre-set
 current magnitude and the operation of calculating the capacity of the
 battery by integrating the current magnitude of the battery with respect
 to time.
 With the pack of the battery according to the present invention, the
 control means is responsive to the pre-set current magnitude or to the
 pre-set magnitude of the voltage drop to switch between the operation of
 calculating the capacity of the battery based on the correlation between
 the voltage and the capacity of the battery and the operation of
 calculating the capacity of the battery by integrating the current
 magnitude of the secondary battery with respect to time to improve the
 accuracy in capacity calculations.
 The electronic equipment system of the present invention resides in the
 above-described battery pack loaded on a piece of electronic equipment,
 such as a personal computer, in which the calculation accuracy of the
 battery can be improved by switching between the voltage method and the
 current integrating method during the capacity calculations.
 Additional features and advantages of the present invention are described
 in, and will be apparent from, the Detailed Description of the Preferred
 Embodiments and the Description of the Drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENTS
 FIG. 1 shows a battery pack 20 (pack of a battery E) for carrying out the
 capacity detection method for a battery according to the present invention
 and a personal computer 30 as typical of a piece of electronic equipment
 on which is loaded the battery pack 20. This personal computer 30 is, for
 example, a portable personal computer, on which battery pack 20 can be
 loaded and which is actuated by the power fed from this battery pack 20.
 To a bus line BUS of a main central processing unit (CPU) of the personal
 computer 30 are connected a variety of peripheral devices 37, a memory 38,
 such as a read-only memory (ROM) or a random-access memory (RAM), and a
 communication LSI 39, etc. A power control circuit 32 is provided with a
 power source switch 33 to perform power on/off control. In addition,
 commercial ac power from a power source plug 35 is fed to the power
 control circuit 32 via an ac adapter 34 to supply the power from the
 battery pack 20 via +terminal 12 and a -terminal 13 for battery connection
 as later explained. The charging current is fed via terminals 12, 13 to
 the battery pack 20.
 Turning to FIG. 1 for explanation of the structure of the battery pack 20,
 the battery pack 20 includes a voltage detection circuit 4, a current
 detection circuit 3, and a controller 1. The controller 1 includes a
 micro-computer 11, a storage unit 5 and a read-only memory (ROM) 10. The
 micro-computer 11 includes a communication terminal 9.
 The voltage detection circuit 4, detects the voltage of the battery E in
 order to calculate the capacity of the battery E based on the correlation
 between the voltage of the battery E and the capacity. The battery E is
 made up of four battery cells 41a, 41b, 41c, 41d and, for measuring the
 voltages of these battery cells 41a, 41b, 41c, 41d, a multiplexer 42 and
 an operational amplifier 43 are provided in the voltage detection circuit
 4. By battery cell selection control signals being fed from the
 micro-computer 11 in the controller 1 to the multiplexer 42, the four
 battery cells 41a, 41b, 41c, 41d are sequentially selected by multiplexer
 42, which then sends the terminal voltages of the four batteries to an
 operational amplifier 43. The voltage detection signals from the
 operational amplifier 43 are sent to the controller 1 and
 analog-to-digital (A/D) converted in the controller 1 so that the battery
 cell terminal voltage is retrieved as digital values by the micro-computer
 11.
 The current detection circuit 3, detects the current magnitude of the
 battery E in order to integrate the current magnitude of the battery E
 with respect to time to calculate the capacity of the battery E. This
 current detection circuit 3 includes a resistor 44 for current measurement
 which is connected to, for example, the minus side of the battery E and an
 operational amplifier 45 for detecting the voltage corresponding to the
 current flowing in the resistor 44. The current detection signal from the
 operational amplifier 45 is sent to the controller 1 where it is
 analog-to-digital (A/D) converted so that the measured current value is
 retrieved as a digital value in the micro-computer 11.
 To the multiplexer 42 and the operational amplifiers 43, 45, power is fed
 from the plus side of the battery cell set via a power saving switch 46.
 This switch 46 is tuned on/off by the power save signals supplied from the
 micro-computer 11. A control switch 2 for controlling the
 charging/discharging on/off is connected across the +terminal 7 of the
 battery pack 20 (plus battery terminal) and the plus side of the battery
 E. This control switch 2 includes a series connection of an FET 21, as a
 charging switching element, and an FET 33, as a discharging switching
 element. A pair of diodes 33, 34 are connected in parallel with these FETs
 21, 22. The FET 21 for charging is on/off controlled by the driver 25
 driven by the control signals supplied from the micro-computer 11. The FET
 22 for discharging is on/off controlled by the driver 26, which is driven
 by control signals supplied from the micro-computer 11 of the controller
 1.
 The controller 1, having the micro-computer 11, is configured to switch
 between the operation of calculating the capacity of the battery E, based
 on the relation between the voltage and the capacity of the battery E,
 using a prescribed current magnitude or a pre-set voltage drop value as a
 threshold or switching point, and the operation of integrating the current
 magnitude of the battery E with respect to time to calculate the capacity
 of the battery E. The relation between the cell voltage detected by the
 voltage detection circuit 4 and the capacity (%) of the battery E is shown
 in FIG. 2, in which the cell voltage CE is plotted on the ordinate and the
 current capacity to full capacity ratio, expressed in % (capacity %) is
 plotted on the abscissa. The point A in FIG. 2 denotes the time point of
 start of discharge. The cell voltage CE is decreased substantially
 linearly, while the capacity % of the battery E is also decreased
 substantially linearly. There exists a voltage value CE1 at which the cell
 voltage is decreased abruptly.
 The controller 1, in particular the micro-computer 11, calculates the
 capacity % based on a voltage detection value DE of the cell voltage CE
 obtained by the voltage detection circuit 4 based in turn on the relation
 between the cell voltage CE and the capacity % of FIG. 2.
 The battery pack 20 monitors the state of the battery E, such as its
 voltage, charging/discharging current or residual capacity, to exchange
 data with a charger, not shown, or a load, such as the personal computer
 30, by way of having the communication. To this end, the battery pack 20
 has, enclosed therein, the controller 1 having a cell monitoring and
 controlling micro-computer 11. It is possible with this battery pack 20 to
 display the states of the battery E, sent thereto from the micro-computer
 11 over the communication terminal 9, on a display provided on the load or
 on the charger, to advise the user of such states.
 The battery E, enclosed in the battery pack 20, includes a set of four
 lithium-ion-based battery cells 41a, 41b, 41d and 41d, such a battery E
 could be a Nicd battery as well. The +terminal of the battery E is
 connected via the switch 2 to a +terminal 7 of the pack (package of the
 battery pack 20), while its -terminal is connected via current detection
 circuit 3 to a -terminal 8 (GND terminal). The battery pack 20 is loaded
 in a battery housing section, not shown, provided in the personal computer
 30, whereby the +terminal 7 of the pack side is electrically connected to
 the +terminal 12 of the personal computer 30 and the -terminal 8 on the
 pack side is electrically connected to the -terminal 13 of the personal
 computer 30. When the battery pack 20 is charged, the charging current
 also flows through the +terminal 7 and the -terminal 8.
 The micro-computer 11 of the controller 1 is, for example, a central
 processing unit (CPU), and is configured for periodically receiving the
 output of the current detection circuit 3 or the voltage detection circuit
 4 to recognize the current flowing through the battery E (charging current
 and discharging current) or the voltage of the battery E. The
 micro-computer 11 controls the normally-on control switch 2, based on the
 voltage or current, to turn off the control switch 2 to interrupt the
 current (charging current and discharging current) to prohibit
 overcharging, over discharging or excess current.
 The micro-computer 11 finds the current residual capacity of the battery E,
 based on the voltage of the battery E recognized as described above. The
 micro-computer 11 also finds the integrated capacity for the charging
 capacity, based on the current capacity found as described above.
 The micro-computer 11 is connected to the communication terminal 9 of the
 pack. This communication terminal 9 is electrically connected to a
 communication terminal 14 of the personal computer 30 when the battery
 pack 20 is loaded on the personal computer 30. The communication terminal
 14 of the personal computer 30 is connected to the communication LSI 39 so
 that, when the battery pack 20 is loaded on the personal computer 30,
 communication occurs between the micro-computer 11 of the battery pack 20
 and the communication LSI 39 of the personal computer 30, via
 communication terminals 9, 14, in accordance with a pre-set communication
 sequence.
 Specifically, the controller 1, in particular the micro-computer 11,
 performs preset processing responsive to the data (commands etc) sent
 thereto from the communication terminal 14 of the personal computer 30, or
 transmits the cell voltage, charging/discharging current, residual
 capacity of the battery E or the integrated capacity, to the communication
 terminal 14 of the personal computer 30 via communication terminal 9. The
 battery E is for example, a lithium ion cell. The voltage of the battery
 E, termed an open voltage or a cell voltage, is related with the residual
 capacity in a manner as shown in FIG. 2 such that, if the cell voltage is
 found, the residual capacity, expressed in terms of % relative to the full
 capacity, can be found. Thus, the micro-computer 11 finds the residual
 capacity of the battery B, such as the lithium-ion-cell, based on the cell
 voltage, as described above.
 The control switch 2 shown in FIG. 1 operates under control of the
 micro-computer 11 to turn the charging/discharging current on or off. The
 current detection circuit 3 detects the current flowing therein, that is
 the discharging current of the battery E, as well as the charging current
 to the battery E, to send the detected results to the micro-computer 11.
 The storage unit 5 includes a register for storage of the integrated
 capacity. A display unit 6 is for example, a liquid crystal display which
 displays the information such as the integrated capacity under control by
 the micro-computer 11.
 In the ROM (read-only memory) 11 are stored programs or data necessary for
 the operation of the micro-computer 11. That is, the micro-computer 11
 refers to the data stored in the ROM 10 as the occasion may demand to
 execute the programs stored in the ROM 10 to perform a variety of
 operations.
 When the parallel flat plate 20 is loaded in normal fashion on the personal
 computer 30, the +terminal 7, -terminal 8 and the communication terminal 9
 are electrically connected to the terminals 12 to 14 of the personal
 computer 30, respectively. The personal computer 30 operates with the
 battery pack 20 as the picture signals, with the discharge current of the
 battery E flowing through the +terminals 7, 12, personal computer 30 and
 through the -terminals 13, 8.
 In the battery pack 20, the current detection circuit 3 or the voltage
 detection circuit 4 detects the current (charging or discharging current)
 flowing in the battery E, or the cell voltage, respectively. These current
 and voltage values are periodically received in the micro-computer 11. The
 micro-computer 11 verifies, on the basis of these current or voltage
 values, whether or not the battery E is in the overcharged or over
 discharged state or in the overcurrent state. If the battery E is in the
 overcharged or over discharged state, the control switch 2 is turned off
 to break the current (charging or discharging current).
 The micro-computer 11 calculates the current capacity (residual capacity of
 the battery E), based on the cell voltage of the battery E, and further
 calculates the integrated capacity, as reference is had to a register unit
 5, based on the calculated current capacity. The micro-computer 11
 transmits the integrated capacity, calculated as described above, the
 current value supplied from the current detection circuit 3, and the
 voltage value supplied from the voltage detection circuit 4, via the
 communication terminal 9, responsive to the request from the personal
 computer 30. The micro-computer 11 also sends the calculated integrated
 capacity to the display unit 6 for display thereon.
 A preferred typical example of the current integrating method is explained
 with reference to FIGS. 3 and 4. In the current integrating method, in
 which the flowing current is integrated to find Ah (ampere/hour), the
 current magnitude needs to be measured accurately. For example, if the
 maximum current for measurement is 10 A and the minimum measurable current
 is 1 mA, the magnitude which represents 10 A, referred to 1 mA as the
 minimum unit, is 10000, such that, for representing 10000 decimal, 14 bits
 (2.sup.14 =16384) are required. Therefore, for representing data of
 measured values in the central processing unit (CPU) of the micro-computer
 11 of FIG. 1, 14 bits are theoretically required. That is, a 14-bit
 analog-to-digital (A/D) converter is required.
 If, in integrating the 14-bit data, the integration spacing is decreased
 excessively, the memory capacity required for integration is increased.
 Thus, the minimum integration resolution is set to find a practical
 integration spacing.
 If, for example, 1 mA is set as the resolution (minimum integration
 resolution), since the maximum current is 10 A, the minimum integration
 spacing is
EQU 1[mAh]/10[A]=3600[mAsec]/10000[mA]=0.36[sec]
 or 0.36 sec.
 If the capacity of the battery is 4800 mAh, since 13 bits binary (2.sup.13
 =8192) are required for representing 4800 decimal, the number of bits
 required for representing the integrated data value is 14 bits+13 bits=27
 bits, which may be said to be a practical level of the required memory
 capacity.
 As another example of converting the analog current magnitude for measuring
 the integrated current value in the current integrating method, there is
 such a method which uses an analog integrator 100 in combination, as shown
 in an illustrative circuit in FIG. 3. This circuit, gives an example of an
 integrator for carrying out the current integrating method and the concept
 of integration. It is noted that the circuit of FIG. 3 stands for one of
 the charging direction or the discharging direction. Two of the circuits
 shown in FIG. 3 are required for doing both charging and discharging.
 The analog integrator 100 has an input terminal 101 and a reset switch 102.
 An output of the analog integrator 100 is connected to a voltage
 comparator 103. An output pulse of the voltage comparator 103 to be
 integrated is 1 mAh and is used for actuating a reset switch 102.
 In the present example of the current integrating method, an output pulse
 104 shown in FIG. 3 is outputted each time the analog integrator 100
 overflows that is, each time the output reaches the level L and output
 pulses 104 are integrated, as shown in FIG. 4. That is, the time when the
 analog integrator 100 overflows, or the time when the output of the analog
 integrator 100 reaches the level L, is equivalent to the time point 1 mAh
 is measured. Thus, the current magnitude in mAh can be obtained by
 integrating the pulses from the voltage comparator.
 Since the integration is a continuous operation, it is unnecessary to take
 the resolution into account. However, it is necessary to take account of
 the offset or drift proper to the analog integrator 100 which tends to
 deteriorate calculation accuracy.
 Meanwhile, the aforementioned accuracy of the order of 14 bits is realized
 in certain ones of the currently marketed A/D converters. The
 aforementioned use of the 14-bit A/D converter is for theoretical
 considerations only and the precision realized in practical application is
 not so high as is contemplated in the present example. If current
 measurement is achieved to this order of accuracy, the current of the
 battery below 1 mA cannot be accommodated. There are occasions wherein no
 battery is used or the current cannot be measured based on the
 self-discharging of the battery. This point is taken into consideration in
 the present embodiment by using the voltage method in the small current
 range in place of the current integrating method so that the
 aforementioned high accuracy is not required. For example. A 10-bit A/D
 converter suffices for practical application.
 Referring to FIGS. 5 and 6, an instance of switching between the voltage
 method and the current integrating method based on a pre-set switching
 point is explained. In the present embodiment, the current integrating
 method and the voltage method are used for the larger and smaller
 magnitudes of the battery E for highly accurate calculation of the
 capacity of the battery E, respectively. This enables the calculation
 accuracy of the battery E to be improved even if the precision of the
 device used in the voltage detection circuit 4 for the voltage method or
 in the current detection circuit 3 for the current integrating method is
 not that high.
 First, the system of setting a predetermined current value as a threshold
 value as the switching point (see FIG. 5) is explained. In this case, the
 threshold value as the switching point is set by the current value itself.
 Since the current value (threshold value) is fixed, the current
 measurement accuracy (error) of the current integrating method can be
 calculated easily. For example, the internal resistance of the battery E
 is increased with lowering in temperature such that, at -10.degree. C.,
 the internal resistance is occasionally several times that at ambient
 temperature. If, for example, the internal resistance is increased by a
 factor of four, is 250 m.OMEGA. at ambient temperature and 400 mA is set
 as the current magnitude corresponding to the switching point, the voltage
 drop of the battery E, which is 0.1V at ambient temperature, is as low as
 0.4V at lower temperatures. If the accuracy for the voltage drop of 0.1V
 is good and that for the voltage drop of 0.4V is stringent, the current
 magnitude corresponding to the switching point needs to be set to a lower
 current magnitude, such as 100 mA. This renders the current measurement
 accuracy more stringent.
 In the present method, if the current magnitude of the battery E exceeds a
 predetermined current magnitude, as set as the switching point (threshold
 value), the capacity integration of the capacity of the battery E is
 switched to the current integrating method. Conversely, if the current
 magnitude of the battery E is lower than the predetermined current
 magnitude (threshold value) as the switching point, the capacity
 integration of the capacity of the battery E is switched to the voltage
 method.
 Next, the case of setting a predetermined voltage drop value of the battery
 E as the threshold value as this switching point (see FIG. 6) is
 explained. If the predetermined voltage drop value of the battery E is set
 as the threshold value as this switching point, the precision condition
 for the voltage method is easy to set, because the magnitude of the
 voltage drop (threshold value) is constant. If, for example, the magnitude
 of the voltage drop as the switching point is set to 0.1V, the current is
 400 mA and 100 mA at ambient temperature and at lower temperature,
 respectively.
 If the magnitude of the voltage drop of the battery E is larger than the
 threshold value as the switching point (with the predetermined magnitude
 of the voltage drop being, for example, 0.1V), more current is flowing, so
 that the current integrating method is used for detecting the capacity of
 the battery E. If the magnitude of the voltage drop of the battery E is
 smaller than the predetermined magnitude of the voltage drop of the
 switching point (threshold value), the voltage method is used as the
 method for detecting the capacity of the battery E.
 The above-mentioned current magnitude or the magnitude of the voltage drop
 as the switching point is given merely as illustration. It is, however,
 possible to switch between the current integrating method and the voltage
 method only gradually or stepwise instead of completely switching between
 the current integrating method and the voltage method. It suffices then to
 provide a width to each of the preset current magnitude or magnitude of
 the voltage drop for switching and to switch gradually between cell
 capacity detection by the current integrating method for the larger
 current magnitude of the battery and that by the voltage method for the
 smaller current magnitude. Alternatively, plural threshold values are
 provided as the preset current magnitude or the magnitude of the voltage
 drop to switch between cell voltage detection by the voltage method and
 that by the current integrating method stepwise responsive to plural
 threshold values.
 If, due to abrupt changes in the current magnitude or switching at a sole
 threshold value, switching between the current integrating method and the
 voltage method has occurred at one time, it is desirable to make
 processing for gradually changing the integrated capacity value. If the
 accuracy error is zero in the voltage method or in the current integrating
 method, this processing is not required. Since there exists a slight
 difference between the two systems, this processing is to be performed.
 In the current integrating method, the cell capacity is calculated based on
 the integrated magnitude of the current that has flowed (in Ah or
 coulombs). In the voltage method, the cell capacity is determined by a
 pre-formulated table showing the cell voltage potted against the capacity
 (%) (see FIG. 2). In the current integrating method, the current magnitude
 obtained is subtracted from the full charged capacity of the battery E to
 find the residual capacity, conversely, in the voltage method, the current
 magnitude obtained is multiplied with the full charged capacity of the
 battery E to find the residual capacity. Thus, calculation errors,
 conversion errors or table errors represent influencing factors.
 Meanwhile, integration in the current integrating method is by integration
 with the capacity value obtained with the voltage method as the base
 point.
 That is, when switching from the voltage method to the current integrating
 method, the ultimate cell capacity is found by integrating the current
 magnitude calculated by the current integrating method to the current cell
 capacity corresponding to the current cell capacity (cell capacity at the
 switching point), as found by the voltage method, as the base point.
 Conversely, in switching from the current integrating method to the
 voltage method, there are occasions when an error is produced between the
 current cell capacity (at the switching point) as found by current
 integration and the cell capacity directly after switching as found on the
 basis of the correlation between the voltage and the cell capacity. It is
 therefore desirable to take the error between the cell capacities into
 account to switch gradually from the cell capacity value as detected by
 the current integrating method directly before switching to the cell
 capacity value as detected by the voltage method directly after the
 switching.
 It is also possible to estimate the full charging capacity or deterioration
 from the coulomb values between pre-set voltages or to construct the table
 of the voltage method from the highly accurate information of the current
 magnitude obtained by the current integrating method in case the current
 magnitude is constant and large such as during charging. The voltage
 method exploits the voltage-current characteristics of the lithium ion
 secondary battery to estimate the capacity % from the voltage of the cell
 terminals, as described above. If the magnitude of the voltage drop is
 significant, such as when the current is large, the voltage method is low
 in capacity measurement accuracy of the battery E because of overlap of
 various errors. Conversely, with the current integrating method, the
 integration error is large if the current is small, whereas, if the
 voltage fluctuations are large, the capacity accuracy is low.
 With the cell capacity detection method according to the present invention,
 it is possible to obviate the deficiencies of the voltage method and the
 current integrating method reciprocally to detect the capacity of the
 battery E highly accurately by switching between these two methods at a
 preset switching point. The switching point may be the current magnitude
 or the magnitude of the voltage drop, as described above.
 The present invention also may be applied to the cell pack (battery pack)
 to which the above-described cell capacity detection method is applied,
 and to an electronic equipment system which includes the cell pack and the
 electronic equipment for loading the cell pack.
 With the above-described battery capacity detection method, battery pack
 and the electronic equipment system, of the present invention, the
 calculation accuracy of the capacity of the battery (residual capacity)
 can be increased by selectively using the voltage method and current
 integrating method depending on the current magnitude in the battery.
 Although the present invention has been described with reference to
 specific embodiments, those of skill in the art will recognize that
 changes may be made thereto without departing from the spirit and scope of
 the invention as set forth in the hereafter appended claims.