Abstract:
There is provided with a computing device including: a coupler configured to couple to a display device having a chargeable first battery and be able to be separated from the display device, wherein charging to the first battery and wired communication with the display device can be performed via the coupler at a time of being coupled to the display device; a detector configured to detect whether the coupler is coupled to the display device; a wireless communicator configured to communicate wirelessly with the display device when separated from the display device; a connector configured to be supplied with power from an external power supply; a chargeable second battery; a further detector configured to detect whether power is supplied to the connector; and a controller configured to control charging and discharging of the first and second battery based on whether the power is supplied when coupled to the display device.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application is based upon and claims the benefit of priority from the prior Japanese Patent Applications No. 2006-94418 filed on Mar. 30, 2006, the entire contents of which are incorporated herein by reference.  
       BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates to a computing device that can be separated from and coupled to a display device, a computing device system and a power control method.  
         [0004]     2. Related Art  
         [0005]     Mobile computing is spreading as computer and communication technologies represented by personal computers and portable telephones are in progress. Above all, spread of notebook computers including wired and wireless communication devices is remarkable. The notebook computer is obtained by forming a display device and a computing device as one body. However, portability and use convenience required for mobiles and the system performance are in a trade-off relation. For example, if a high-frequency clock CPU (Central Processing Unit) or GPU (Graphics Processing Unit) is used, then the power dissipation becomes high and it becomes necessary to mount a heavier large-capacity battery. On the other hand, if the light weight and thin shape are pursued, it is inevitable to lower the performance of the CPU or GPU. In a notebook computer raised in convenience by mounting an optical drive, its total weight increase, this makes the utilization form of the personal computer close to that of a desktop personal computer placed on a desk and used, and it is therefore difficult to say that the utilization form is mobile computing. On the other hand, paying attention to the display device included in the notebook computer, its advance in thin shape and light weight is remarkable.  
         [0006]     As one method for reconciling the portability and convenience in use of the thin light-weight display device and high system performance of the computing device, it is conceivable to make the display device separable from the computing device. It can be implemented by displaying screen information sent from the computing device in a wireless form, on the display device. If it is more convenient to use the display device and the computing device as one body in input work using a keyboard, then the display device should be coupled to the computing device and the computing device should send screen information to the display device in a wired form as usual.  
         [0007]     If a display device of a computing device system such as a notebook computer which might be driven by a battery is made separable, power management of the battery for the computing device and the display device becomes an important. However, the problems have not been solved.  
         [0008]     JP-A2002-304283(KOKAI), JP-A2002-312155(KOKAI), and JP-A2004-86550(KOKAI) disclose examples in which the display device is separable, and screen information is sent out in a wireless form and displayed on the display device when the display device is separated. When the display device is separated, the display device is driven by a mounted battery in many cases. The JP-A2002-304283(KOKAI), JP-A2002-312155(KOKAI), and JP-A2004-86550(KOKAI) describe only that the computing device is driven by power supply such as an AC adapter other than the battery and the computing device can run the display device and charge a battery in the display device. In JP-A2000-99204(KOKAI), JP-A2001-5564(KOKAI), and JP-A2002-215265(KOKAI) as well, an information processing device from which a display device can be separated is described, but power management of a battery is not mentioned. On the other hand, an example in which a battery is mounted on each of a display device in a notebook computer and a computing device other than the display device is described in JP-A2002-110122(KOKAI). However, the display device is not separable, and there is no description concerning power management of the two batteries.  
       SUMMARY OF THE INVENTION  
       [0009]     According to an aspect of the present invention, there is provided with a computing device comprising:  
         [0010]     a coupler configured to couple to a display device having a chargeable first battery and be able to be separated from the display device, wherein charging to the first battery and wired communication with the display device can be performed via the coupler at a time of being coupled to the display device;  
         [0011]     a coupling detector configured to detect whether the coupler is coupled to the display device;  
         [0012]     a wireless communicator configured to communicate wirelessly with the display device when the coupler is being separated from the display device;  
         [0013]     an external power supply connector configured to be supplied with power from an external power supply;  
         [0014]     a chargeable second battery;  
         [0015]     a connection detector configured to detect whether power is supplied to the external power supply connector; and  
         [0016]     a controller configured to control charging and discharging of the first battery and the second battery on the basis of whether the power is supplied to the external power supply connector when the coupler is being coupled to the display device.  
         [0017]     According to an aspect of the present invention, there is provided with a computing device system including a computing device and a display device which can be coupled to and separated from the computing device,  
         [0018]     the display device comprising:  
         [0019]     a chargeable first battery; and  
         [0020]     a first wireless communicator configured to communicate wirelessly with the computing device when being separated from the computing device, and  
         [0021]     the computing device comprising:  
         [0022]     a display device coupler configured to couple to the display device and be able to be separated from the display device, wherein charging to the chargeable first battery and wired communication with the display device can be performed via the display device coupler at a time of being coupled to the display device;  
         [0023]     a coupling detector configured to detect whether the display device coupler is coupled to the display device;  
         [0024]     a second wireless communicator configured to communicate wirelessly with the display device when the display device coupler is being separated from the display device;  
         [0025]     an external power supply connector configured to be supplied with power from an external power supply;  
         [0026]     a chargeable second battery;  
         [0027]     a connection detector configured to detect whether power is supplied to the external power supply connector; and  
         [0028]     a controller configured to control charging and discharging of the first battery and the second battery on the basis of whether the power is supplied to the external power supply connector when the display device coupler is being coupled to the display device.  
         [0029]     According to an aspect of the present invention, there is provided with a power control method comprising:  
         [0030]     detecting whether a computing device having a chargeable first battery and a display device having a chargeable second battery is separated or coupled each other, wherein 
        the display device and the computing device communicate wirelessly each other when being separated and perform wired communicate each other when being coupled, and     charging to the first battery from the computing device and charging to the second battery from the display device can be performed when being coupled;        
 
         [0033]     detecting whether power from an external power supply is supplied to the computing device; and  
         [0034]     controlling charging and discharging of the first battery and the second battery on the basis of whether the power is supplied from the external power supply when the display device and the computing device are being coupled each other. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0035]      FIG. 1  is a block diagram showing a configuration of a computing device system according to an embodiment of the present invention;  
         [0036]      FIG. 2  is a block diagram showing another configuration of a computing device system according to an embodiment of the present invention;  
         [0037]      FIG. 3  is a diagram showing representative modes in a computing device system according to an embodiment of the present invention;  
         [0038]      FIG. 4  is a flow chart showing an example of power management in an embodiment of the present invention;  
         [0039]      FIG. 5  is a flow chart showing another example of power management in an embodiment of the present invention; and  
         [0040]      FIG. 6  is a diagram showing an example of power use history. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0041]     Hereafter, an embodiment of the present invention will be described with reference to the drawings.  
         [0042]      FIG. 1  is a block diagram showing a configuration of a computing device system according to an embodiment of the present invention. This computing device system includes a computing device  11  and a display device  31 . The computing device  11  and the display device  31  can be separated from each other and coupled to each other. When separated, the computing device  11  and the display device  31  perform wireless communication via a wireless communication IF  17  and a wireless communication IF  36 , respectively. When coupled, the computing device  11  and the display device  31  perform wired communication via a display device coupler  23  and a computing device coupler  40 , and the computing device  11  and the display device  31  can supply power to each other via the display device coupler  23  and the computing device coupler  40 . The computing device  11  includes a CPU (Central Processing Unit)  12 , a memory  13 , a GPU (Graphics Processing Unit)  14 , a storage  15 , an external input-output IF  16 , a wireless communication IF  17 , a chargeable battery  18 , an external power supply input unit (external power supply connector)  19 , a separation/coupling detector (a coupling detector)  20 , a power mode detector  21 , a power use detector/recorder  22 , and a display device coupler  23  which couples the computing device  11  to the display device  31 . The system architecture of the computing device is not especially restricted, but, for example, a system mounted on personal computers can be utilized.  
         [0043]     The display device  31  includes a processor  32 , a memory  33 , a display controller  34 , an external input-output IF  35 , a wireless communication IF  36 , a chargeable battery  37 , a separation/coupling detector  38 , a power use detector/recorder  39 , a computing device coupler  40  which couples the computing device  11  to the display device  31 , a display  41 , a touch panel  42 , and a speaker &amp; microphone  43 . The touch panel and the speaker &amp; microphone are examples of external input-output devices.  
         [0044]     In the computing device  11 , an example of a single bus is shown for simplicity. However, it is also possible to provide a plurality of buses differing in demanded rate by using a chip set or the like. The display controller is incorporated in the GPU  14 . It is also conceivable that the GPU itself is incorporated in the chip set. The memory  13  can be utilized not only as a main memory but also as a video memory. As a matter of course, a video memory may be connected to the GPU  14  besides the main memory. The external input-output IF  16  is provided to connect external input-output devices such as a USB (Universal Serial Bus), a UART (Universal Asynchronous Receiver Transmitter), an I 2 S (Inter-IC Sound), an I 2 C (Inter Integrated Circuit) and RS232C. As for the storage  15 , a hard disk drive (HDD) which is a magnetic recording device can be mentioned as a representative example. However, a nonvolatile semiconductor storage such as a compact flash may also be used. In addition, a DVD, a CD, or an optical drive called multi drive may also be included in the storage  15  as external storages. It is also possible to connect the external storage to the external input-output IF  16 . As the external input-output devices, there are a key board, a touch pad, a microphone, a speaker and various sensors.  
         [0045]     Separation and coupling between the computing device  11  and the display device  31  depend on disconnection/coupling between the display device coupler  23  in the computing device  11  and the computing device coupler  40  in the display device  31 . A state in which the computing device  11  and the display device  31  are separated from each other is referred to as separation mode, whereas a state in which the computing device  11  and the display device  31  are coupled to each other is referred to as coupling mode. Separation and coupling between the computing device  11  and the display device  31  mean disconnection and connection of at least a screen information line (wired communication path) and a power supply line respectively of the computing device  11  and the display device  31 . If the display device  31  has an input part such as a touch panel, a speaker or a microphone, then an input-output line is also included in the wired communication path. Detection of separation and coupling between the computing device  11  and the display device  31  is performed by the separation/coupling detector  20  and the separation/coupling detector  38 . However, an already known method such as a method disclosed in the above JP-A2002-304283(KOKAI) can be utilized.  
         [0046]     The battery  18  and the battery  37  are mounted on the computing device  11  and the display device  31 , respectively. In the computing device  11 , the battery  18  and an external power supply connected to the external power supply input unit  19  supply operation power to the computing device  11 . The battery  18  can be charged by using power of the external power supply connected to the external power supply input unit  19  or power supplied from the battery  37  in the display device  31 . Furthermore, the computing device  11  can supply power from the external power supply connected to the external power supply input unit  19  and power of the battery  18  to the display device  31  via the display device coupler  23 . The display device  31  can operate by using the power supplied from the computing device  11 , and can charge the battery  37 . Furthermore, the display device  31  can operate by using power of the battery  37 . The battery  18  and the battery  37  are not especially restricted, as long as they can supply power for driving the computing device  11  and the display device  31  and they can be charged. As the most representative batteries, lithium ion batteries and lithium polymer batteries can be mentioned. Nickel hydrogen batteries can also be used although the energy density is low as compared with the lithium ion batteries.  
         [0047]     The power mode detector  21  detects a power mode which represents whether an external power supply (for example, an AC adapter) is connected to the external power supply input unit  19 . In the present embodiment, the case where the external power supply is connected is referred to as AC mode, and the case where the external power supply is not connected is referred to as battery mode. In other words, the computing device  11  has two major power modes: the AC mode and the battery mode. A distinction between the two modes is made depending upon whether the power supply which supplies operation power to the computing device  11  is the external power supply or the battery. In the AC mode, operation power is supplied from the external power supply. In the battery mode, operation power is supplied from the battery  18 . Power mode detection in the power mode detector  21  can be performed by using a technique such as, for example, measurement of a voltage level of a signal from the external power supply or a signal from an electric or mechanical switch installed in the external power supply input unit  19 .  
         [0048]     The power use detector/recorder  22  records the power use situation in the computing device  11  successively as power use history. The power use detector/recorder  39  in the display device  31  records the power use situation in the display device  31  successively as power use history. In other words, power use in both batteries in the computing device and the display device (use history of battery  18  and use history of battery  37 ) is measured and recorded. By the way, the recording destination may be a semiconductor storage device or a magnetic recording device. In the case of a semiconductor storage device, a nonvolatile storage device is desirable. It is desirable to transmit the power use history of the battery stored in the display device  31  to the computing device  11  via the wireless communication IF  17  and the wireless communication IF  36  in the separation state or the display device coupler  23  and the computing device coupler  40  in the coupling state and store the power use history in the display device  31  on the computing device  11  side as well.  
         [0049]      FIG. 6  shows an example of data of history of power use measured and recorded, at certain time.  
         [0050]     In  FIG. 6 , the product of “device current consumption” and “device input voltage” becomes “device power consumption” at that time point. When calculating average power consumption of a subject device, a subject time period of averaging is set by referring to “time” in recording items. When considering “power mode” changeover, “elapsed time (after power mode changeover)” should be referred to. “Battery charging/discharging charge quantity (after power mode changeover)” is a value obtained by performing time integral on a value of “battery charging/discharging current” after power mode changeover. “Battery voltage” is measured to prevent overcharging and overdischarging. “Battery temperature” is measured for a safety reason that temperature abnormality should be detected and in order to take temperature correction of charging and discharging characteristics into consideration as occasion demands. “Battery capacity,” exactly speaking, battery current capacity is a nominal battery capacity. This value may be acquired together with a nominal battery voltage on the basis of information of a battery pack at the time of battery exchange. The nominal battery voltage is omitted in  FIG. 6 . A residual battery capacity (not shown) can be calculated on the basis of the battery charging and discharging charge quantity and the battery capacity.  
         [0051]     The output scheme of a video signal from the GPU  14  in the computing device  11  and the display controller  34  in the display device  31  is not especially restricted, as long as the output scheme conforms to the input scheme of the display  41  which is the output destination. As the representative scheme, the analog RGB, digital parallel, LVDS (Low Voltage Differential Signaling), TMDS (Transmission Minimized Differential Signaling), and HDMI (High Definition Multimedia Interface) can be mentioned.  
         [0052]     As for the wireless communication IF  17  and the wireless communication IF  36 , for example, the wireless LAN, Bluetooth, UWB (Ultra Wide Band), PHS and portable telephone can be mentioned. However, the wireless communication IF  17  and the wireless communication IF  36  are not especially restricted. When the computing device  11  and the display device  31  are in the separated state, data communication is performed between the wireless communication IF  17  and the wireless communication IF  36 . When the computing device  11  and the display device  31  are in the coupled state, data communication is performed between the computing device  11  and the display device  31  via the display device coupler  23  and the computing device coupler  40 . The display controller  34  receives image information from the wireless communication IF  36  or the computing device coupler  40  via the processor  32  or directly, generates a video signal on the basis of the received image information, and outputs the video signal to the display  41 . The external input-output IF  35  outputs an input signal from the speaker &amp; microphone  43  or the touch panel  42  to the wireless communication IF  36  or the computing device coupler  40  via the processor  32  or directly. A configuration including a multiplexer  72  in the display device as shown in  FIG. 2  is also possible. The multiplexer  72  in a display device  71  is supplied with a signal which represents the separation mode or the coupling mode from a separation/coupling detector  73 . If the multiplexer  72  is supplied with a signal which represents the coupling mode, the multiplexer  72  outputs a video signal supplied from the computing device coupler  40  to the display  41 , and outputs an input signal from the speaker &amp; microphone  43  or the touch panel  42  to the computing device coupler  40 . On the other hand, if the multiplexer  72  is supplied with a signal which represents the separation mode, the multiplexer  72  outputs a video signal supplied from the display controller  34  to the display  41 , and outputs an input signal from the speaker &amp; microphone  43  or the touch panel  42  to the external input-output IF  35 .  
         [0053]     The display  41  mounted on the display device  31  is not especially restricted, as long as it can display screen information supplied from the computing device  11 . As displays mounted on a large number of notebook computers at the present time, transmissive liquid crystal display devices (LCD) can be mentioned. There are various schemes for the transmissive LCD as well. However, the schemes are not restrictive. Besides the LCD, an emissive display device such as an organic EL device or an inorganic EL device may also be used. A reflective display device may also be used as a display device which implements lower power consumption. As the reflective display device, a reflective LCD may be used, or an electrophoretic display device or a reflective display device utilizing the MEMS technique can also be utilized. In addition, not only a direct-view display device described heretofore, but also a projection display device may be utilized. Depending upon the size and shape of the computing device, a wearable display device such as a head mount wearable display device can also be used.  
         [0054]     The CPU (controller)  12  included in the computing device  11  performs power management for the computing device  11  and the display device  31 . Specifically, the CPU  12  controls charging and discharging in the battery  18  in the computing device  11  and the battery  37  in the display device  31 . As for the charging and discharging control, there are three broad charging methods: charging the battery  37  in the display device  31  preferentially (preferential charging), charging the battery  18  in the computing device  11  preferentially, and charging the battery  18  in the computing device  11  and the battery  37  in the display device  31  with good balance (balanced charging), i.e., charging without giving priority to either the battery  37  or  18 . Each of the charging and discharging methods may be controlled more finely. Supposing the battery mode, in the balanced charging, charging and discharging of the batteries  37  and  18  are controlled on the basis of the power use histories of the display device  31  and the computing device  11  so as to make discharge time periods of the batteries  37  and  18  respectively in the display device  31  and the computing device  11  equal as far as possible, i.e., so as to make drive time periods of the display device  31  and the computing device  11  equal as far as possible. In other words, charging and discharging of the batteries  37  and  18  are controlled so as to cause the residual quantity of the battery  18  and the residual quantity of the battery  37  to satisfy a balance condition. Details of the preferential charging and balanced charging will be described later.  
         [0055]     Hereafter, power management in the computing device system shown in  FIG. 1  will be described in detail.  
         [0056]      FIG. 3  represents transitions among various states of the computing device system including the display device  31  and the computing device  11 .  
         [0057]     In a state  51  of the separation mode and the battery mode, the display device  31  operates using power supplied from the battery  37  and the computing device  11  operates using power supplied from the battery  18 .  
         [0058]     If an external power supply, such as an AC adapter  61 , is connected to the computing device  11  in the state  51  of the separation mode and the battery mode, then transition to a state  52  of the separation mode and the AC mode is performed.  
         [0059]     If the display device  31  is coupled to the computing device  11  in the state  51  of the separation mode and the battery mode, transition to a state  53  of the coupling mode and the battery mode is performed.  
         [0060]     If the AC adapter  61  is connected to the computing device  11  in the state  53  or the display device  31  is coupled to the computing device  11  in the state  52 , then transition to a state  54  of the coupling mode and the AC mode is performed.  
         [0061]     Thus, the computing device system including a separable display device has two modes (the separation mode and the coupling mode) concerning the coupling relation between the computing device and the display device and two modes (the AC mode and the battery mode) concerning the power supply form. As a result, the computing device system can assume the four states  51  to  54  obtained by combining the modes.  
         [0062]     The batteries  18  and  37  are mounted on the computing device  11  and the display device  31 , respectively. Power management concerning the battery charging and discharging becomes extremely important in ensuring the continuous operation time of the computing device system. For example, if the computing device system continues to be utilized in the state  51  of the separation mode and the battery mode, then it is usually desirable that the computing device  11  supplied with power from the battery  18  becomes nearly in operation time to the display device  31  supplied with power from the battery  37 . Therefore, it becomes necessary to control the charging and discharging of the batteries  18  and  37  in the coupling mode (the states  53  and  54 ) by taking, for example, the residual battery capacities of the batteries  18  and  37  and power consumption of the computing device  11  and the display device  31  into consideration. In particular, since the power consumption quantity of the computing device  31  varies largely according to its utilization method, it is desirable to reflect the power use history over a certain definite time period into the power management.  
         [0063]     In general, it is indispensable in obtaining high convenience of use of the system that the user can utilize the computing device system while separating and coupling the display device and performing transition between the battery mode and the AC mode without being conscious of the power management. On the other hand, there are various demands for the power management according to different user&#39;s utilization methods, and it also becomes important that the user can perform power management setting manually as occasion demands.  
         [0064]      FIGS. 4 and 5  are flow charts showing examples of a power management processing according to an embodiment of the present invention.  FIG. 4  shows the case where the charging and discharging control is exercised automatically.  FIG. 5  shows the case where the charging and discharging control is exercised manually.  
         [0065]     In  FIG. 4 , the separation/coupling mode which represents whether the display device  31  and the computing device  11  are in the separated state or in the coupled state (S 11 ).  
         [0066]     If the separation mode is detected (separation mode at S 11 ), charging and discharging of the batteries  37  and  18  respectively mounted on the display device  31  and the computing device  11  should be controlled independently (S 12 ). By the way, power is measured at all times, and its result is recorded. In the case of the separation mode, the battery  37  and the battery  18  are electrically independent. Even if the separation mode and the AC mode are combined, therefore, charging and discharging control should be performed on respective batteries as usual. For example, in the charging and discharging control of the computing device in the state of the separation mode and the AC mode, the charging current should be controlled while detecting the battery voltage, the charging and discharging charge quantity and the battery temperature considered in charging of the ordinary secondary battery. Charging control differs depending upon the kind of the battery. Typically, in the case of the lithium ion battery usually utilized in PCs (Personal Computers), quick charging using constant current control is performed over a range of approximately 80 to 85% of the nominal battery capacity, and then full charging using constant voltage control with the full charging voltage of the battery is performed. The quick charging is performed typically with a current quantity of 0.5 C to 1 C. A current quantity at which the discharging finishes after one hour is referred to as 1 C. For example, if the nominal battery capacity is 1,100 mAh as shown in  FIG. 6 , 1,100 mA becomes 1 C.  
         [0067]     On the other hand, if the coupling mode is detected (coupling mode at S 11 ), the power mode is detected as the next step (S 13 ). If the AC mode is detected (AC mode at S 13 ), the battery  37  in the display device  31  is charged preferentially (S 14 ). The reason is that the computing device  11  is operated in the AC mode in some cases whereas the display device  31  is operated basically in the battery mode from the viewpoint of improvement of convenience in user&#39;s use.  
         [0068]     If the battery mode is detected (battery mode at S 13 ), then it is typically desirable that the display device  31  supplied with power from the battery  37  becomes nearly equal in operation time to the computing device  11  supplied with power from the battery  18  as described above. Therefore, balanced charging is performed on the basis of the power use histories of the display device  31  and the computing device  11  (S 15 ).  
         [0069]     In  FIG. 5  which shows the case where the charging and discharging control is exercised manually, if the separation mode is detected (separation mode at S 21 ) as a result of the detection of the separation/coupling mode (S 21 ), then charging and discharging of the batteries  37  and  18  respectively mounted on the display device  31  and the computing device  11  are controlled independently in the same way as automatic control (S 22 ).  
         [0070]     If the coupling mode is detected (coupling mode at S 21 ), the user specifies whether to perform preferential charging (S 23 ). If specification data input from the user indicates that preferential charging should not be performed (no at S 23 ), then balanced charging is performed (S 24 ).  
         [0071]     On the other hand, if specification data indicates that preferential charging should be performed (yes at S 23 ), the user inputs specification data which specifies a subject of the preferential charging (S 25 ).  
         [0072]     If the computing device  11  is specified (computing device at S 25 ), the battery  18  in the computing device  11  is charged preferentially (S 26 ). If the display device  31  is specified (display device at S 25 ), the battery  37  in the display device  31  is charged preferentially (S 27 ). At this time, specified one may be charged with preference of 100% (charged preferentially until the battery capacity reaches a prescribed value), or the degree of preference may be changed manually by the user.  
         [0073]     Hereafter, a processing flow in the case where the charging and discharging control shown in  FIG. 4  is exercised automatically will further be described with reference to a concrete example.  
         [0074]     It is now supposed that a TFT color LCD with 12.1 inch XGA, an electromagnetic touch panel, a wireless LAN circuit conforming to IEEE 802.11g, a video processing circuit, an LCD control circuit, a power supply circuit, and a 10-Wh lithium ion battery are mounted on the display device.  
         [0075]     It is now supposed that a central processing unit (CPU) having an operation frequency of 1.5 GHz, a 512-MB main memory, a graphics processing unit (GPU) having an operation frequency of 400 MHz, an 80-GB hard disk drive (HDD), a DVD multi-drive, a keyboard, a touch pad, a speaker, a wireless LAN circuit conforming to IEEE 802.11g, and a 40-Wh lithium ion battery are mounted on the computing device.  
         [0076]     It is first supposed that the display device is separated from the computing device and the separation mode is detected (separation mode at S 11 ). The separation/coupling mode detection is performed by judging a state of an electric switch attached to a connector which couples the display device to the computing device. If the display device is separated from the computing device, it becomes possible for the user to hold only the display device and use it. Screen data is sent from the computing device to the display device via the wireless LAN. Control signals from the electromagnetic touch panel and data signals are also transmitted and received between the display device and the computing device via the wireless LAN. The display device is supplied with power from the lithium ion battery included therein and the computing device is supplied with power from the lithium ion battery included therein.  
         [0077]     It is then supposed that the display device is coupled to the computing device and the coupling mode is detected (coupling mode at S 11 ). In the coupling mode, it becomes possible to utilize the computing device and the display device as an ordinary all-in-one notebook computer. Screen data is sent from the computing device to the display device in a wire form. Control signals from the electromagnetic touch panel and data signals are also transmitted and received between the display device and the computing device in a wire form. Furthermore, a power line of the display device is connected to a power line of the computing device.  
         [0078]     If the coupling mode is detected, then it is detected whether an AC adapter is connected, i.e., the power mode is detected by judging the state of the electric switch included in an AC adapter connector (the external power supply input unit) in the computing device (S 13 ).  
         [0079]     If the AC mode is detected as the power mode (AC mode at S 13 ), then power is supplied from the AC adapter to bring the display device and the computing device into operation and the battery mounted on the display device is charged preferentially (S 14 ). In other words, the battery in the computing device is not charged, but the battery in the display device is subjected to quick charging. After charging corresponding to 80% or more of the nominal battery capacity has been completed, the battery in the computing device is subject to quick charging.  
         [0080]     If the battery mode is detected as the power mode (battery mode at S 13 ), balanced charging and discharging are performed on the basis of power use histories of the display device and the computing device recorded at the time of the separation mode, so as to make the operation time of the display device nearly equal to that of the computing device in the separation mode at the next time (S 15 ). For example, if the battery in the display device needs to be charged, the battery in the display device is charged from the battery in the computing device and the battery in the computing device supplies power to both the computing device and the display device for their operation. On the contrary, if the battery in the computing device needs to be charged, the battery in the computing device is charged from the battery in the display device and the battery in the display device supplies power to both the computing device and the display device for their operation.  
         [0081]     Hereafter, details of the balanced charging and the preferential charging will be described.  
         [0082]     First, a method of balanced charging will now be described with respect to the lithium ion battery ordinarily utilized in personal computers. The balanced charging means charging and discharging the batteries in the display device and the computing device so as to satisfy the balance condition and a content of the balance condition is for example stored in the storage  15 . Hereafter, a detailed example of a balanced charging method will be described.  
         [0083]     An average power consumption of each of the display device and the computing device is calculated on the basis of the power use history obtained by recording measurement results of power use as shown in  FIG. 6 . A sum of them is an average power consumption of the whole system. A subject time period of the averaging may be set manually by the user or may be preset automatically equal to, for example, 30 minutes. On the other hand, the residual battery capacities of the batteries respectively in the display device and the computing device are found from  FIG. 6 . The product of the residual battery capacity and the nominal voltage becomes a residual power capacity. The sum of the residual power capacities of the display device and the computing device becomes a residual power capacity of the whole system at the time when the display device is coupled to the computing device.  
         [0084]     With respect to the battery in the display device, the target of balanced charging in the battery mode becomes a power capacity found as “residual power capacity of the whole system”×(average power consumption in display device÷average power consumption in whole system). A value obtained by dividing the target power capacity by the nominal voltage becomes a target residual battery capacity. With respect to the battery in the computing device as well, a target residual battery capacity is calculated in the same way. Charging or discharging is performed on the basis of comparison with the residual battery capacity at that time point. As for charging, quick charging using constant current control or full charging using constant voltage control is performed according to the above-described criterion. If the display device is coupled to the computing device and the system is in operation, then a battery having a residual battery at that time point greater than the target residual battery capacity supplies power utilized in the whole system, i.e., in both the display device and the computing device and charges the other battery. After the target residual battery capacity is reached, each device is supplied with power from the battery mounted on the device.  
         [0085]     The target of balanced charging in the AC mode is that the ratio between the residual power capacities of the batteries in the display device and the computing device becomes the ratio between average power consumptions of the devices. As a matter of course, if one of the batteries is fully charged, then the other is charged to be fully charged. As long as an external power supply capable of supplying the sum total of the power consumption of the whole system and power capable of performing quick charging on both batteries is connected, however, both batteries in the display device and the computing device may be subject to quick charging.  
         [0086]     In the embodiment of the present invention, preferential charging is charging one of the batteries in the display device and the computing device. One of the batteries is subject to the above-described quick charging, and the other is not charged. In the case of the lithium ion batteries in the AC mode, the battery subjected to preferential charging is shifted to the above-described constant voltage control and thereafter the other battery is subject to quick charging. As a matter of course, both batteries in the display device and the computing device may be subject to quick charging, as long as an external power supply capable of supplying the sum total of the power consumption of the whole system and power capable of performing quick charging on both batteries is connected. In the battery mode, the battery that is not the subject of the preferential charging supplies the power consumption of the whole system and charges the battery that becomes the subject of the preferential charging.  
         [0087]     According to the embodiment of the present invention, it becomes possible to effectively perform power management on batteries respectively mounted on the display device and the computing device which can be separated from and coupled to each other, as heretofore described.