Abstract:
An auxiliary power supply unit which is mountable to a portable electronic device having a battery, includes at least one capacitor, and an I/F connector which is connectable to an I/F connector of the portable electronic device, the I/F connector of the portable electronic device including power terminals connected to the battery. The I/F connector of the auxiliary power supply unit includes power terminals connected to the at least one capacitor, wherein the at least one capacitor is connected in parallel to the battery when the auxiliary power supply unit is mounted to the portable electronic device via the power terminals of the I/F connectors.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an auxiliary power supply unit for a portable electronic device, and also relates to a portable electronic system which is composed of a portable electronic device, such as a digital camera, and one or more associated accessories. 
     2. Description of the Related Art 
     Various types of batteries are available on the market; each type generally has a factory-recommended final voltage which guarantees the safety of the battery. If battery continues to be used after the battery voltage drops below the final voltage, the battery may produce heat, which in turn may leak liquid, or worse explode, under some conditions. If battery leaks or explodes, a device in which the battery is loaded may malfunction or may be damaged. 
     To prevent such problems from occurring, in conventional portable electronic devices which use a battery as a power supply, the battery voltage is regularly checked to ensure the safety of the battery. For example, if the battery voltage reaches the final voltage thereof, the user can be warned that the battery is nearly exhausted, and the user is either prompted to replace the battery or the operation of the portable electronic device is stopped to prohibit the user from further using the nearly-exhausted battery. 
     However, the internal resistance of the battery varies in accordance with the servicing environment of the battery and the operating condition of the portable electronic device. When the portable electronic device operates at a low temperature or when the portable electronic device is performing an operation requiring a large current sporadically, the internal resistance of the battery appears to increase, which may cause a power-supply-voltage detecting circuit provided in the device to mistakenly determine that the battery voltage has reached the final voltage. In this case, the user is erroneously informed that the battery is nearly exhausted, and therefore he or she replaces the battery with a new one. Consequently, the energy of the battery is not used up sufficiently. 
     To prevent this problem from occurring, a battery which excels in temperature characteristics can be used, a high precision power-supply-voltage detecting circuit can be provided in the portable electronic device, or the portable electronic device can be provided therein with an additional circuit which can reduce the internal resistance of the battery. However, all of these proposals cause the portable electronic device to be increased in size, thereby increasing weight and increasing the cost of production, which are undesirable. In addition, such proposals can be wasteful since, in some cases, the final voltage is hardly ever mistakenly detected under some conditions. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide an auxiliary power supply unit and a portable electronic system which can restrain power supply voltage fluctuation of a portable electronic device and which can improve the consuming efficiency of a battery loaded in the portable electronic device. 
     To achieve the object mentioned above, according to an aspect of the present invention, an auxiliary power supply unit is provided, which is mountable to a portable electronic device having a battery, including at least one capacitor, and an I/F connector which is connectable to an I/F connector of the portable electronic device, the I/F connector of the portable electronic device including power terminals connected to the battery. The I/F connector of the auxiliary power supply unit includes power terminals connected to the at least one capacitor, wherein the at least one capacitor is connected in parallel to the battery when the auxiliary power supply unit is mounted to the portable electronic device via the power terminals of the I/F connectors. 
     Preferably, the at least one capacitor is connected in parallel to the battery so as to reduce an internal resistance of the battery when the auxiliary power supply unit is mounted to the portable electronic device via the power terminals the I/F connectors. 
     Preferably, the at least one capacitor includes at least one large-capacity capacitor, wherein, in a state where the at least one capacitor is connected in parallel to the battery, the at least one capacitor discharges to supply power to the portable electronic device when the portable electronic device is in operation, while the battery supplies power to the at least one capacitor to charge the at least one capacitor. 
     The portable electronic device can be a digital camera. 
     Preferably, the auxiliary power supply unit can be attached to the portable electronic device at a position so as not to hinder operability of the portable electronic device. 
     Preferably, the auxiliary power supply unit can be attached to the bottom surface of the portable electronic device. 
     According to another aspect of the present invention, a portable electronic system is provided, including a portable electronic device including a battery and a first I/F connector having terminals connected to a battery, and at least one accessory which is mountable to the portable electronic device and includes a second I/F connector, the at least one accessory being mounted to the portable electronic device via the first and second I/F connectors. One of the at least one accessory includes an auxiliary power supply unit having at least one capacitor, the second I/F connector of the auxiliary power supply unit including terminals connected to the at least one capacitor, and the at least one capacitor is connected in parallel to the battery when the auxiliary power supply unit is mounted to the portable electronic device via the first I/F connector and the second I/F connector. 
     Preferably, the at least one capacitor is connected in parallel to the battery so as to reduce an internal resistance of the battery when the auxiliary power supply unit is mounted to the portable electronic device via the first and second I/F connectors. 
     With this structure, power supply voltage fluctuation of the portable electronic device is kept low by the auxiliary power supply unit. Therefore, an error in the detection of the final voltage of the battery is prevented from occurring, and the energy of the battery can be used up sufficiently. 
     Preferably, the second I/F connector of the auxiliary power supply unit and the second I/F connector of another the at least one accessory are constructed substantially the same so as to both correspond to the first I/F connector. 
     Preferably, the first I/F connector includes control terminals used for communication between the portable electronic device and the at least one accessory. 
     The portable electronic device can be a digital camera. 
     According to another aspect of the present invention, a digital camera system is provided, including a camera body including a battery and a first I/F connector having power terminals connected to the battery, and at least one accessory which is mountable to the camera body and includes a second I/F connector, the at least one accessory being mounted to the camera body via the first and second I/F connectors. One of the at least one accessory includes an auxiliary power supply unit having at least one capacitor, the second I/F connector of the auxiliary power supply unit including power terminals connected to the at least one capacitor. The at least one capacitor is connected in parallel to the battery when the auxiliary power supply unit is mounted to the digital camera via the first and second I/F connectors. 
     Preferably, one of the at least one accessory includes a printer unit. 
     With this structure, the I/F connector of the portable electronic device can serve a multi-purpose connector not only for the auxiliary power supply unit but also for any other accessories for the portable electronic system. Therefore, it is no longer necessary for the portable electronic device to be provided with an additional connector designed specifically for the auxiliary power supply unit, which reduces the cost of production. 
     The present disclosure relates to subject matter contained in Japanese Patent Application No.2000-385185 (filed on Dec. 19, 2000) which is expressly incorporated herein by reference in its entirety. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will be described below in detail with reference to the accompanying drawings, in which: 
         FIG. 1  is a perspective view of an embodiment of a digital camera system to which the present invention is applied in a state where an auxiliary power supply unit is mounted to a camera body of the digital camera system; 
         FIG. 2  is a perspective view of the embodiment of the digital camera system shown in  FIG. 1  in a state where a printer unit is mounted to the camera body; 
         FIG. 3  is a bottom plan view of the camera body of the digital camera system shown in  FIG. 1 ; 
         FIG. 4  is a perspective view of the auxiliary power supply unit shown in  FIG. 1 , showing the top face thereof; 
         FIG. 5  is a perspective view of the printer unit shown in  FIG. 2 , showing the top face thereof; 
         FIG. 6  is a block diagram of a control system of the digital camera system shown in  FIG. 1 ; 
         FIG. 7  is an explanatory view of an I/F socket of the camera body shown in  FIG. 3 ; 
         FIG. 8A  is a block diagram of fundamental elements of the printer unit shown in  FIGS. 2 and 5 ; 
         FIG. 8B  is a block diagram of fundamental elements of the auxiliary power supply unit shown in  FIGS. 1 and 4 ; 
         FIG. 9  is a schematic connection diagram of the battery provided in the camera body, a capacitor group provided in the auxiliary power supply unit, and the load on the battery and the capacitor group; 
         FIG. 10  is a graph showing a discharge characteristic when only a battery is used, and another discharge characteristic when both the battery and the auxiliary power supply unit are used; 
         FIG. 11  is a graph showing a temperature characteristic of ESR (equivalent series resistance) when only a typical battery is used, and another temperature characteristic of ESR when both a typical battery and the auxiliary power supply unit are used; and 
         FIG. 12  is a flow chart showing fundamental operations of the digital camera system shown in  FIG. 1 . 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The overall structure of a digital camera system (portable electronic system) to which the present invention is applied will be hereinafter discussed with reference to  FIGS. 1 through 4 . The digital camera system is composed of a camera body (portable electronic device/digital camera)  10  to which a photographing lens  10   a  is fixed, and camera accessories which are mounted to the camera body  10  when in use. The camera accessories include an auxiliary power supply unit  20  shown in  FIG. 4  and a printer unit  30  shown in  FIG. 5 .  FIG. 1  shows a state of the digital camera system in which the auxiliary power supply unit  20  is mounted to the bottom of the camera body  10 , while  FIG. 2  shows another state of the digital camera system in which the printer unit  30  is mounted to the bottom of the camera body  10 . 
     The camera body  10  is provided on a bottom surface  10   b  thereof with an I/F socket (I/F connector)  11  and a pair of locking/locating slots  12  (see  FIG. 3 ). The pair of locking/locating slots  12  are used for mounting the auxiliary power supply unit  20  or the printer unit  30  to the bottom of the camera body  10  while positioning the auxiliary power supply unit  20  or the printer unit  30  accurately relative to the bottom surface  10   b  of the camera body  10 . The camera body  10  is provided in each of the pair of locking/locating slots  12  with an engaging claw  12   a . Each engaging claw  12   a  is normally biased in a direction toward the inside of the corresponding slot  12  (i.e., in a locking direction) by a spring member (not shown), and can be retracted in a direction opposite to the locking direction (i.e., in an unlocking direction) by an operation of an unlock member (not shown). 
     As shown in  FIG. 4 , the auxiliary power supply unit  20  is in the shape of a flat box, and has a substantially rectangular upper surface  20   a  which corresponds to the bottom surface  10   b  of the camera body  10 . The auxiliary power supply unit  20  serves as a part of a power circuit of the camera body  10  when mounted to the camera body  10 . The auxiliary power supply unit  20  is provided on the upper surface  20   a  thereof with an I/F plug (I/F connector)  21  which is fitted into the I/F socket  11  of the camera body  10 , and a pair of locking/locating projections  22  which are respectively fitted into the pair of locking/locating slots  12  of the camera body  10 . The I/F plug  21  is provided with a control terminal array  21   a  and a power terminal array  21   b . Each of the pair of locking/locating projections  22  is provided with an engaging recess  23  in which the corresponding engaging claw  12   a  is engaged when the pair of locking/locating projections  22  are respectively fitted into the pair of locking/locating slots  12 . Fitting the pair of locking/locating projections  22  into the pair of locking/locating slots  12  causes the I/F plug  21  to be fitted into the I/F socket  11 , respectively, and at the same time causes the pair of engaging claws  12   a  to be engaged in the pair of engaging recesses  23 , respectively, to thereby lock the auxiliary power supply unit  20  to the camera body  10 . In a state where the auxiliary power supply unit  20  is locked to the camera body  10 , if it is required that the auxiliary power supply unit  20  be dismounted from the camera body  10 , the aforementioned unlock member is operated so that the pair of engaging claws  12   a  are respectively disengaged from the pair of engaging recesses  23 , and subsequently the auxiliary power supply unit  20  is removed from the camera body  10  with the pair of engaging claws  12   a  being disengaged from the pair of engaging recesses  23 . 
     Similar to the auxiliary power supply unit  20 , the printer unit  30  is in the shape of a flat box, and has a substantially rectangular upper surface  30   a  which corresponds to the bottom surface  10   b  of the camera body  10 . The printer unit  30  serves as a printer for producing prints of digital images when mounted to the camera body  10 . The printer unit  30  is provided on one end face thereof with an ejection slot  30   b  (see  FIGS. 1 and 5 ) from which prints are ejected. As shown in  FIG. 5 , the printer unit  30  is provided on the upper surface  30   a  thereof with an I/F plug (I/F connector)  31  which is fitted into the I/F socket  11  of the camera body  10 , and a pair of locking/locating projections  30   c  which are respectively fitted into the pair of locking/locating slots  12  of the camera body  10 . Each of the pair of locking/locating projections  30   c  is provided with an engaging recess  30   d  in which the corresponding engaging claw  12   a  is engaged when the pair of locking/locating projections  30   c  are respectively fitted into the pair of locking/locating slots  12 . The I/F plug  31  is provided with a control terminal array  31   a  and a power terminal array  31   b.    
     In the present embodiment, the I/F plug  21  of the auxiliary power supply unit  20  and the I/F plug  31  of the printer unit  30  are made under the same standard. In other words, the I/F socket  11  of the camera body  10  is made to serve as a multi-purpose socket  11  for various accessories for the digital camera system. Furthermore, ease of operability of the camera is not sacrificed (i.e., not hindered) when the auxiliary power supply unit  20  or the printer unit  30  is attached to the bottom surface  10   b  of the camera body  10 . 
     The control system of the present embodiment of the digital camera system will be hereinafter discussed in detail with reference to  FIGS. 6 through 8 .  FIG. 6  is a block diagram of a control system of the present embodiment of the digital camera system. The digital camera system is provided with an MPU  100  serving as a controller which comprehensively controls the overall operation of the digital camera system. The MPU  100  is provided therein with a ROM (not shown) in which various control programs are stored, and a RAM (not shown) in which various data are temporarily stored. The digital camera system is further provided with a power circuit  102 , setting switches  103 , a DSP (digital signal processor)  104 , a bus line  105 , an image pick-up device (e.g., a CCD image sensor)  106 , a first A/D converter  107 , an information indicator  108 , an image memory  109 , a potential divider  110 , and a second A/D converter  111 . 
     A constant voltage is supplied to the MPU  100  from the battery  101  via the power circuit  102 . The MPU  100  operates with the power supplied via the power circuit  102 , and performs various processes corresponding to a state of setting switches  103 . Each of the setting switches  103  is operated upon operation thereof by the user. The MPU  100  monitors the battery voltage via the potential divider  110  and the second A/D converter  111  to warn the user that the battery  101  is nearly exhausted before the battery voltage reaches a predetermined final voltage of the battery  101 . 
     The first A/D converter  107 , the information indicator  108  and the image memory  109  are connected to the DSP  104 . The first A/D converter  107  converts analog image signals output from the image pick-up device  106  into a digital image signal. The information indicator  108  visually indicates digital images taken by the image pick-up device  106 , and various photographic information. The information indicator  108  is, e.g., a color LCD panel provided at the back of the camera body  10 . The image memory  109  stores digital image signals output from the first A/D converter  107 . The image memory  109  is, i.e., a cache memory incorporated in the camera body  10  or a removable nonvolatile memory card such as CompactFlash or SmartMedia. The MPU  100  controls the indicating process performed via the DSP  104 , and the MPU  100  also controls the image pick-up process, which is performed by the image pick-up device  106 , via the DSP  104  and the first A/D converter  107 . 
     The MPU  100  controls operations of an accessory (e.g., the auxiliary power supply unit  20  or the printer unit  30 ) connected to the I/F socket  11  via the bus line  105 . As shown in  FIG. 7 , the I/F socket  11  is provided with a control terminal array  11   a  and a power terminal array  11   b , which are brought into contact with the control terminal array  21   a  or  31   a  and the power terminal array  21   b  or  31   b  when the I/F plug  21  or  31  of the mounted accessory (e.g., auxiliary power supply unit  20  or printer unit  30 ) is fitted into the I/F socket  11 , respectively. The control terminal array  11   a  is connected to the bus line  105  so that control signals are transmitted between the camera body  10  and the mounted accessory (e.g., auxiliary power supply unit  20  or printer unit  30 ) via the control terminal array  11   a  and the corresponding control terminal array  21   a  or  31   a . The power terminal array  11   b  is connected between the terminals of the power circuit  102  so that the camera body  10  gives power to (or gives and receives power to and from) the mounted accessory (e.g., auxiliary power supply unit  20  or printer unit  30 ) via the power terminal array  11   b  and the corresponding power terminal array  21   b  or  31   b.    
     The MPU  100  identifies the type of accessory mounted immediately after an accessory is mounted to the camera body  10  via the I/F socket  11 . Namely, immediately after an accessory is mounted to the camera body  10  via the I/F socket  11 , the MPU  100  outputs an accessory-type identifying signal to the mounted accessory and receives an ID signal from the mounted accessory. The ID signal is predetermined for each associated accessory, so that the MPU  100  identifies the type of the accessory mounted to the camera body  10  in accordance with the received ID signal. 
     As shown in  FIG. 8A , the printer unit  30  is provided with a power circuit  30   c , a bus line  33 , a CPU  34 , a FIFO (first-in first-out) circuit  35 , a printer  36 , a sensor  37 , a motor controller  38  and a head controller  39 . If the printer unit  30  is mounted to the camera body  10  via the I/F socket  11  and the I/F plug  31 , the bus line  105  of the camera body  10  is connected to the bus line  33  of the printer unit  30  via the control terminal arrays  11   a  and  31   a , and the battery  101  of the camera body  10  is connected to the power circuit  32  of the printer unit  30  via the power terminal arrays  11   b  and  31   b  (see  FIG. 8A ). The power circuit  32  regulates the D.C. voltage output from the battery  101  to supply the battery voltage to each circuit of the printer unit  30  as a constant voltage. The CPU  34  and the FIFO circuit  35  are connected to the bus line  33 . 
     The CPU  34  is provided therein with a ROM (not shown) in which various control programs are stored, and a RAM (not shown) in which various data are temporarily stored. The aforementioned ID signal, which is unique to the printer unit  30 , is stored in the ROM. The CPU  34  communicates with the MPU  100  of the camera body  10  via the bus line  33 . Upon inputting a printing request from the MPU  100  of the camera body  10 , the CPU  34  inputs image data for printing via the bus line  33  to store the input image data in the FIFO circuit  35 . The FIFO circuit  35  serves as a buffer memory. Immediately after a predetermined volume of image data is stored in the FIFO circuit  35 , the CPU  34  reads out image data from the FIFO circuit  35  in the same order that the FIFO circuit  35  was input, to print images on paper (not shown) through the printer  36 . This printing process of the printer  36  is controlled by the CPU  34 . The CPU  34  operates the motor controller  38  and the head controller  39  while detecting the position of a printing head (not shown) to perform the printing operation. The basic structure of the printer unit  30  is that of a typical printer unit known in the art, and therefore a further description about operations of the printer unit  30  are omitted. 
     As shown in  FIG. 8B , the auxiliary power supply unit  20  is provided with a capacitor group  24  and a logic circuit  25  which are connected to the I/F plug  21 . If the auxiliary power supply unit  20  is connected to the camera body  10  via the I/F socket  11  and the I/F plug  21 , the logic circuit  25  is connected to the MPU  100  via the bus line  105  of the camera body  10 , and the control terminal arrays  11   a  and  21   a , while the capacitor group  24  is connected in parallel to the battery  101  of the camera body  10  via the power terminal arrays  11   b  and  21   b  (see  FIG. 8B ). The capacitor group  24  consists of a plurality of capacitors, preferably large-capacity electric double layer capacitors. The logic circuit  25  outputs the aforementioned ID signal, which is unique to the auxiliary power supply unit  20 , to the MPU  100  of the camera body  10 . 
       FIG. 9  is a schematic connection diagram of the battery  101 , the capacitor group  24 , and a load on the battery  101  and the capacitor group  24 . In the illustrated embodiment shown in  FIG. 9 , the capacitor group  24  consists of two pairs of electric double layer capacitors. Two capacitors of each pair are connected in series, while the two pairs are connected in parallel. 
     In a state where the capacitor group  24  is connected in parallel to the battery  101 , the battery  101  supplies power to charge the capacitor group  24 . At the same time the capacitor group  24  discharges to supply power to the camera body  10  when the camera body  10  is in operation. The discharge of the capacitor group  24  keeps voltage fluctuation of the battery  101  at the lowest possible level when large current is consumed. Consequently, the camera body  10  operates with stability, while the final voltage of the battery  101  is detected accurately. This makes it possible to use up the energy of the battery  101  sufficiently. In the case where a battery and a capacitor are used as a power supply, it can be understood from the graph shown in  FIG. 10  that the lifetime of the battery can be assertively increased in comparison with the case where only the battery is used as a power supply. 
       FIG. 11  shows an ESR temperature characteristic (equivalent series resistance) when only a typical battery is used, and another ESR temperature characteristic when both the typical battery and the auxiliary power supply unit are used. If the temperature drops below zero centigrade, ESR increases rapidly, which makes it difficult to derive power from battery. Therefore, if only the battery  101  is used as a power supply, the battery  101  cannot supply a sufficient electric power to the camera body  10  at a low temperature. Namely, the camera body  10  cannot operate properly with only the battery  101  at a low temperature. 
     However, if a combination of the battery  101  and the capacitor group  24  which are connected in parallel is used as a power supply as shown in  FIG. 9 , the ESR of the power supply (i.e., a combination of the battery  101  and the capacitor group  24 ) can appear to be reduced since the ESR of the capacitor group  24  (“ESRc” shown in  FIG. 9 ) is much smaller than the ESR of the battery  101  (“ESRb” shown in  FIG. 9 ). Therefore, if the battery  101  and the capacitor group  24  which are connected in parallel are used as a power supply as shown in  FIG. 9 , the ESR of the battery  101  can be prevented from increasing, which makes it possible to supply large current to the camera body  10  at a low temperature. 
     Operations of the present embodiment of the digital camera system will be hereinafter discussed in detail with reference to the flow chart shown in  FIG. 12 . 
     Upon the power of the camera body  12  being turned ON (YES at step S 1 ), the MPU  100  starts operating, so that a start-up operation of the camera body  10  is performed (step S 2 ). During this start-up operation, the MPU  100  performs a communication operation via the control terminal array  11   a  to determine if an accessory is mounted to the camera body  10  via the I/F socket  11  (step S 3 ). If it is determined that no accessory is mounted to the camera body  10  (NO at step S 4 ), control returns to step S 1 . On the other hand, if it is determined that an accessory is mounted to the camera body  10  (YES at step S 4 ), the MPU  100  outputs the aforementioned accessory-type identifying signal to the mounted accessory, e.g., the auxiliary power supply unit  20  or the printer unit  30  (step S 5 ). Upon receipt of the accessory-type identifying signal, the mounted accessory outputs the ID signal thereof to the MPU  100  (step S 6 ). Subsequently, the MPU  100  identifies the type of the mounted accessory in accordance with the received ID signal (step S 7 ). 
     If the printer unit  30  is mounted to the camera body  30  (if YES at step S 8 ), communications are carried out between the MPU  100  of the camera body  10  and the CPU  34  of the printer unit  30  (step S 9 ). Subsequently, upon inputting a printing request from the MPU  100  of the camera body  10  (YES at step S 10 ), the CPU  34  performs an arithmetic process (step S 11 ), and the printer  36  operates to perform a printing operation thereof (step S 12 ). Subsequently, upon completion of the printing operation (YES at step S 13 ), control returns to step S 1  to again perform the above described operations at and after step S 1 . 
     In the present embodiment of the digital camera system, if the auxiliary power supply unit  20  is mounted to the camera body  10 , voltage fluctuation of the battery  101  is kept low since the capacitor group  24  is connected in parallel to the battery  101  of the camera body  10 . Consequently, the camera body  10  operates with stability, while an error in the detection of the final voltage of the battery  101  is prevented from occurring. If there is little possibility of an error in the detection of the final voltage of the battery  101  occurring, the energy of the battery  101  can be used up sufficiently. This improves the consumption efficiency of the battery  101 , and also prevents the internal resistance of the battery  101  from increasing, to thereby make it possible to supply large current to the camera body  10  at a low temperature. 
     In the present embodiment of the digital camera system, since an accessory which can be freely mounted to the camera body  10  is provided with a function to reduce the internal resistance of the battery  101  provided in the camera body  10 , the precision in detection of the final voltage of the battery  101  can be improved when needed by simply mounting the auxiliary power supply unit  20  to the camera body  10  without increasing size, weight and cost of production of the camera body  10 . 
     In the present embodiment of the digital camera system, although the battery  101  supplies power to charge the capacitor group  24  when the capacitor group  24  is connected in parallel to the battery  101 , it is possible that the battery  101  be made so as to be electrically connected in parallel to the capacitor group  24  only when the power of the camera body  10  is ON. The number of capacitors which constitute the capacitor group  24  is preferably changed as appropriate in accordance with the type of portable electronic device to which the auxiliary power supply unit  20  is to be mounted and/or the type of battery to be used by the portable electronic device. 
     In the present embodiment of the digital camera system, since the I/F plug  21  of the auxiliary power supply unit  20  is made under the same standard as that of the I/F plug (e.g., the I/F plug  31 ) of a conventional accessory (e.g., the printer unit  30 ), it is not necessary for the camera body  10  to be provided with an additional connector (e.g., a socket or a plug) designed specifically for the auxiliary power supply unit  20 , which reduces the cost of production. Even when an accessory such as the printer unit  30  is not used, the accessory can serve as a protection cover for covering the I/F socket  11  if mounted to the camera body  10 . The auxiliary power supply unit  20  also serves as a protection cover for covering the I/F socket  11 . 
     In the present embodiment of the digital camera system, since each of the auxiliary power supply unit  20  and the printer unit  30  is shaped so as to fit the shape of the camera body  10 , the printer unit  30  mounted to the camera body  10  does not feel unusual to the user nor hinders the operability of the digital camera system. 
     In the present embodiment of the digital camera system, although the auxiliary power supply unit  20  and the printer unit  30  are provided as accessories for the camera body  10 , any other device such as a GPS unit can be provided as an accessory for the camera body  10 . In addition, the printer unit  30  or the above-mentioned GPS unit, etc., can be provided therein with one or more large-capacity electric double layer capacitors so that a function similar to that of the auxiliary power supply unit  20  can be provided to the printer unit or the GPS unit. 
     In the above descriptions, although a digital camera system has been discussed as a portable electronic system to which the present invention is applied, the present invention can be applied to any other portable electronic system. It is preferable that the prevent invention be applied to portable electronic systems in which load changes drastically (i.e., a heavy current variation occurs between standby and operating conditions), to make the most of the effectiveness of the prevent invention. 
     As can be understood from the above description, an auxiliary power supply unit and a portable electronic system which can restrain power supply voltage fluctuation of a portable electronic device and which can improve the consuming efficiency of the battery can be achieved. 
     Obvious changes may be made in the specific embodiment of the present invention described herein, such modifications being within the spirit and scope of the invention claimed. It is indicated that all matter contained herein is illustrative and does not limit the scope of the present invention.