Patent Publication Number: US-9412982-B2

Title: Battery pack for electric power tool

Description:
This application claims priority to Japanese patent application serial number 2013-78794, the contents of which are incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a battery pack for an electric power tool that is detachably mounted on a tool body of the electric power tool as a power source for the electric power tool. 
     2. Description of the Related Art 
     A battery pack for an electric power tool, which is detachably mounted on a tool body as a power source, has been used in electric power tools. A user can conveniently use an electric power tool when the battery pack for the electric power tool is mounted on the tool body. The battery pack for an electric power tool generally includes a case that serves as a housing as well as forming an outer package, and a battery body that is provided in the case. The case is generally formed by combining an upper and a lower case. Further, the battery body includes a plurality of battery cells that can be charged and discharged and a circuit board that controls the charging and discharging of the plurality of battery cells. The plurality of battery cells are generally held in the case by a support structure that is called a cell holder (for example, refer to Japanese Laid-Open Patent Publication No. 2008-10315). One end of an electrode member is connected to an electrode of the battery cell, which is held by the cell holder, typically using welding or the like. Current is allowed to flow via the electrode members. The opposite end of the electrode member is connected to the circuit board, which is supported by the cell holder, typically using welding or the like. 
     The above-mentioned cell holder is assembled into the case and disposed in the case. For this reason, a clearance, which is required for disposition and assembling, is needed between the inside of the case and the cell holder. 
     On the other hand, the clearance formed for this purpose generates rattling of the cell holder in the case. The rattling of the cell holder may cause the movement of the battery cells to be different from that the electrode members connected to the electrodes of the battery cells. The above movement needs to be reduced as much as possible so as not to apply a load to the above-connected areas. 
     SUMMARY OF THE INVENTION 
     Thus, there is a need in the art to provide an improved battery pack for an electric power tool that is detachably mounted on a tool body of the electric power tool serving as a power source for the electric power tool. Such a battery pack would suppress the rattling of a cell holder relative to a case in which the cell holder is assembled without increasing the number of parts while maintaining relative ease in inserting the cell holder into the case. 
     According to a first aspect of the invention, there is provided a battery pack for an electric power tool that is detachably mounted on a tool body of the electric power tool serving as a power source for the electric power tool and includes a case, battery cells, and a cell holder that is assembled into the case and covers at least a part of the battery cells. Further, a dimensional difference interposition member serves to bridge a dimensional difference between the cell holder and the inside of the case by generating a pressing force applied to the case when the cell holder is assembled. The dimensional difference interposition member is preferably molded integrally with the cell holder. 
     For this reason, the dimensional difference interposition member is formed at the cell holder by not being formed as a separate member but being molded integrally with the cell holder. Accordingly, it is possible to suppress the rattling of the cell holder relative to the case without increasing the number of parts. 
     In the related art, when a battery pack for an electric power tool is mounted on a tool body of an electric power tool, vibration of the tool during use of the electric power tool or impact applied to the electric power tool from the outside is applied to the battery pack for an electric power tool. In this cases terminals of the electric power tool and the battery pack for an electric power tool may rub against each other, and these terminals can be worn out. For this reason, the life of the terminal may be shortened. However, in an improved battery pack for an electric power tool, it is possible to suppress the rattling of the cell holder relative to the case in which the cell holder is inserted. It is possible, therefore, to improve the contact between the case and the cell holder and to suppress the movement of the cell holder relative to the tool body. Accordingly, since it is possible to suppress wear caused by the rubbing between terminals, the life of these terminals can be lengthened 
     According to a second aspect of the invention, the dimensional difference interposition member may serve to bridge the dimensional difference while the cell holder is assembled into the case. It may also serve to completely bridge the dimensional difference when the cell holder is finally inserted into the case. 
     For this reason, the dimensional difference interposition member does not serve to bridge the dimensional difference until the time immediately before the cell holder is inserted into the case. Accordingly, it is possible to sufficiently secure a clearance for assembling until the cell holder is inserted into the case. Therefore, it is possible to maintain relative ease in inserting the cell holder into the case. Further, since the dimensional difference interposition member serves to bridge the dimensional difference while the cell holder is inserted into the case, it is possible to completely bridge the clearance for assembling when the cell holder is completely assembled. Accordingly, it is possible to suppress the rattling of the cell holder relative to the case. Therefore, while maintaining relative ease in inserting the cell holder into the case, it is possible to suppress the rattling of the cell holder relative to the case. 
     According to a third aspect of the invention, the dimensional difference interposition member may be provided at a forward position or a rearward position with respect to an assembling direction of the cell holder into the case. 
     For this reason, the dimensional difference interposition member does not cause obstruction until the cell holder is completely inserted into the case. Accordingly, it is possible to sufficiently secure a clearance for assembling until the cell holder is inserted into the case. Therefore, it is possible to maintain relative ease in inserting the cell holder into the case. 
     According to another aspect of the invention, the dimensional difference interposition member is produced by cutting out the cell holder, and serves to bridge the dimensional difference by the elasticity thereof when the cell holder is completely inserted into the case. 
     For this reason, it is possible to simply provide the dimensional difference interposition member on the cell holder. Here, since the dimensional difference interposition member serves to bridge a dimensional difference by the elasticity thereof when the cell holder is completely inserted into the case, the dimensional difference interposition member can completely bridge a clearance for assembling when the cell holder is completely inserted. Accordingly, it is possible to suppress the rattling of the cell holder relative to the case. Therefore, while maintaining relative ease in inserting the cell holder into the case, it is possible to suppress the rattling of the cell holder relative to the case in which the cell holder is assembled. 
     According to another aspect of the invention, there is provided a battery pack for an electric power tool that is detachably mounted on a tool body of the electric power tool and serving as a power source of the electric power tool which includes a case, battery cells, and a cell holder that is assembled into the case and covers at least a part of the battery cells. Further, a dimensional difference interposition member serves to bridge a dimensional difference between the cell holder and the inside of the case by generating a pressing force applied to the case when the cell holder is assembled. The dimensional difference interposition member may be molded integrally with the case. 
     For this reason, the dimensional difference interposition member can bridge a dimensional difference between the cell holder and the inside of the case by generating a pressing force that is applied to the case when the cell holder is assembled. Accordingly, it is possible to suppress the rattling of the cell holder relative to the case in which the cell holder is assembled. 
     According to another aspect of the invention, the dimensional difference interposition member may be formed in at least one of the opposite ends of the cell holder or the case along a direction orthogonal to the longitudinal direction of the battery cell. For this reason, it is possible to bridge a dimensional difference between the inside of the case and the cell holder by generating a pressing force on one side and to set the other side as a reference surface to which the cell holder is fixed. Accordingly, the cell holder can be accurately positioned in the case to produce a battery pack. 
     According to another aspect of the invention, the dimensional difference interposition member is formed in at least one opposite end of the cell holder or the case along the longitudinal direction of the battery cell. For this reason, it is possible to bridge a dimensional difference between the inside of the case and the cell holder by generating a pressing force on one side and to set the opposite side as a reference surface to which the cell holder is fixed. Accordingly, the cell holder can be accurately positioned to the case to produce a battery pack. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a view showing the upper surface of a battery pack according to a first embodiment. 
         FIG. 2  is a view showing the front surface of the battery pack of  FIG. 1 . 
         FIG. 3  is an exploded perspective view of the entire battery pack of  FIG. 1 . 
         FIG. 4  is an exploded perspective view of the battery pack of  FIG. 3  that is assembled with a battery body. 
         FIG. 5  is an exploded perspective view of the battery pack of  FIG. 3  of which the battery body is housed in a lower case. 
         FIG. 6  is a top view showing the inside of the lower case from which the battery body is detached. 
         FIG. 7  is a top view showing the inside of the lower case in which the battery body is housed. 
         FIG. 8  is a cross-sectional view taken along line (VIII)-(VIII) of  FIG. 1 . 
         FIG. 9  is a cross-sectional perspective view perspectively showing the cross-section of the battery pack of  FIG. 8 , 
         FIG. 10  is a cross-sectional perspective view showing the cross-section of the battery pack of  FIG. 9  laterally extending from the center in the left and right directions. 
         FIG. 11  is a top view of a cell holder of the first embodiment, 
         FIG. 12  is a front view of the cell holder of  FIG. 11 . 
         FIG. 13  is a rear view of the cell holder of  FIG. 11 . 
         FIG. 14  is a right side view of the cell holder of  FIG. 11 . 
         FIG. 15  is a left side view of the cell holder of  FIG. 11 . 
         FIG. 16  is a bottom view of the cell holder of  FIG. 11 . 
         FIG. 17  is a side view of the cell holder of  FIG. 11  on which a circuit board is mounted. 
         FIG. 18  is a side view showing that battery cells are inserted into the cell holder of  FIG. 17 . 
         FIG. 19  is a perspective view perspectively showing the lower side of a battery body of a second embodiment. 
         FIG. 20  is a cross-sectional view showing the cross-section of a battery pack according to the second embodiment extending from the center in the front and rear directions. 
         FIG. 21  is a right side view of a cell holder of the  FIG. 19 . 
         FIG. 22  is a left side view of the cell holder of  FIG. 19 , 
         FIG. 23  is a perspective view perspectively showing the upper side of a battery body of a third embodiment. 
         FIG. 24  is a perspective view perspectively showing the lower side of the battery body of  FIG. 23 . 
         FIG. 25  is a cross-sectional view showing the cross-section of a battery pack according to the third embodiment extending from the center in the front and rear directions. 
         FIG. 26  is a cross-sectional view showing the cross-section of the battery pack according to the third embodiment laterally extending from the center in the left and right directions. 
         FIG. 27  is a right side view of a cell holder of  FIG. 23 . 
         FIG. 28  is a left side view of the cell holder of  FIG. 23 . 
         FIG. 29  is a perspective view perspectively showing the upper side of a battery body of a fourth embodiment. 
         FIG. 30  is a perspective view perspectively showing the lower side of the battery body of  FIG. 29 . 
         FIG. 31  is a cross-sectional view showing the cross-section of a battery pack according to the fourth embodiment that extending from the center in the front and rear directions. 
         FIG. 32  is a top view of a cell holder of  FIG. 29 . 
         FIG. 33  is a right side view of the cell holder of  FIG. 29 . 
         FIG. 34  is a left side view of the cell holder of  FIG. 29 . 
         FIG. 35  is a bottom view of the cell holder of  FIG. 29 . 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Each of the additional features and teachings disclosed above and below may be utilized separately or in conjunction with other features and teachings to provide an improved battery pack for an electrical power tool. Representative examples of the present teaching, which examples utilize many of these additional features and teachings both separately and in conjunction with one another, will now be described in detail with reference to the attached drawings. This detailed description is merely intended to teach a person of skill in the art farther details for practicing preferred aspects of the present teachings and is not intended to limit the scope of the invention. Only the claims define the scope of the claimed invention. Therefore, combinations of features and steps disclosed in the following detailed description may not be necessary to practice the invention in the broadest sense, and are instead taught merely to particularly describe representative examples of the invention. Moreover, various features of the representative examples and the dependent claims may be combined in ways that are not specifically enumerated in order to provide additional useful examples of the present teachings. 
     First Embodiment 
     A battery pack for an electric power tool according to a first embodiment of the invention will be described below with reference to  FIGS. 1 to 18 . Reference numeral  10  shown in  FIG. 1  denotes a battery pack that corresponds to a battery pack for an electric power tool according to an embodiment of the invention. The battery pack  10  is a rechargeable battery that is adapted to be detachably mounted via sliding onto tool bodies of various electric power tools typified by, for example, an electric screwdriver. The battery pack  10 , which is mounted on a tool body, functions as a power source of an electric power tool. For this purpose, when the amount of the charge of the battery pack  10  is reduced, the battery pack  10  is detached from the tool body and is mounted on a dedicated charger so as to be charged. The battery pack  10 , which has been completely charged by the dedicated charger, is mounted on the tool body again as a power source for the electric power tool. Meanwhile, the front side of the battery pack  10 , which is defined in the drawings, is set in a direction based on a direction in which the battery pack  10  is slidably mounted. Further, the upper side of the battery pack  10 , which is defined in the drawings, is set in a direction based on a direction in which the battery pack  10  is mounted on a tool body (not shown). 
     As shown in  FIGS. 1 to 7 , the battery pack  10  includes a case  11  and the battery body  30  (refer to  FIG. 4 ) that is provided in the case  11 . The case  11  functions as a housing that accommodates the battery body  30  while forming the exterior of the battery pack  10 . The case  11  is formed so as to have a structure that is divided into two (upper and lower) pieces. The upper and lower cases  12  and  21  are combined with each other in a vertical direction, to form the case  11 . As shown in  FIG. 3 , the combined state of the upper and lower cases  12  and  21 , may be maintained by screw members  19 . The case  11 , which is formed by the combination of an upper and a lower case as described above, forms a box-shaped space in which the battery body  30  can be provided. 
     As shown in  FIGS. 3 and 4 , the lower case  12  is substantially formed in the shape of a box with an open upper surface. The lower case  12  is substantially formed in the shape of a box that receives a battery portion  31  of the battery body  30  to be described below. Specifically, as shown in  FIG. 5 , the lower case  12  is formed so as to have dimensions in which five battery cells  33  arranged in parallel in a front-and-rear direction can be housed in the front-and-rear direction, a left-and-right direction, and/or an up-and-down direction. That is, the lower case  12  is formed so that a circuit board  42  (controller  41 ) of the battery body  30  protrudes upward from the housing area of the lower case  12 . 
     As shown in  FIGS. 4 and 6 , screw holes  13  for fastening are formed at the side corner edges of the lower case  12 . Further, as shown in  FIGS. 4 and 6 , drain holes  14  through which water present in the lower case is drained to the outside are formed at the bottom corner edges of the lower case  12 . Furthermore, support ribs  15  and pressure ribs  16  are formed on both left and right inner surfaces of the lower case. The support ribs  15  are formed so as to protrude in a substantially rectangular shape. The support ribs  15  are set at portions that are fitted to gap portions  561  and  571  of a cell holder  50  to be described below and restrict the movement of the cell holder  50  in the lower case  12  in the front-and-rear direction. Moreover, the pressure ribs  16  are formed so as to protrude substantially in the shape of a protrusion that extends in the up-and-down direction. The pressure rib  16  comes into contact with a lead plate  37  so as to press the lead plate  37  toward the battery cell  33 . The pressure rib  16  is sot so that an area of the pressure rib  16  coming into contact with the lead plate  37  increases toward the lower side. Meanwhile, as described below, the controller  41  disposed on the upper side of the battery portion  31  is housed in the upper case  21 . 
     As shown in  FIG. 3 , the upper case  21  is disposed on the upper side of the lower case  12 . The upper case  21  is fastened by the screw members  19  so as to be integrally coupled to the lower case  12 . That is, screw bosses  22  with which the screw members  19  are inserted into the screw holes  13  of the lower case  12  are threadably engaged are formed at the upper case  21 . The upper case  21  is formed substantially in the shape of a box which can house the controller  41  of the battery body  30  to be described below. The lower surface of the upper case  21  is opened. 
     The upper case  21  forms a connection-side exterior when the battery pack  10  is slidably mounted on the device body (the tool body of the electric power tool or the dedicated charger). Slide guide portions  231  and  232 , which are provided on the left and right sides, are formed in the middle portion of the upper case  21 . The slide guide portions  231  and  232  guide the battery pack  10  so that it can be slidably mounted on a device body such as a tool body. For this purpose, the slide guide portions  231  and  232  are formed in the shape of flanges that protrude on the left and right sides so as to extend in the front-and-rear direction in which the battery pack can be slidably mounted. Recessed portions  233  are formed at each of the slide guide portions  231  and  232 . The recessed portion  233  has the function of suppressing the transmission of vibration. Such vibration is caused by the driving of the electric power tool and is transmitted to the battery cell  33  when the battery pack  10  is mounted on the electric power tool. Further, charge/discharge terminals  44  and signal terminals  46  to be described below are formed at the upper case  21  so as to be capable of being electrically connected to terminals of the device body. That is, slits  241  for the charge/discharge terminals and slits  242  for the signal terminals are formed at the upper case  21 . These slits  241  and  242  are formed so that male external terminals formed in the shape of a plate can be inserted into the slits. Furthermore, guide slits  243  and  244 , which are used to slidably mount the battery pack  10  on the device body, are formed on the upper case  21 . 
     Moreover, a hook opening  261  and an operation opening  262  are formed on the upper case  21 . The hook opening  261  is an opening through which a hook portion  72  of a male hook mechanism  70  can protrude to the outside. The hook portion  72  protruding from the hook opening  261  can engage a female portion of an external electric power tool or dedicated charger (not shown). The operation opening  262  is an opening through which an operation portion  73  of the male hook mechanism  70  can be exposed to the outside. The engagement of the hook portion  72 , protrudes to the outside from the hook opening  261 . A user can release the engagement of the hook portion  72  using the operation portion  73  which can be operated from the operation opening  262 . 
     The male hook mechanism  70  has a structure that can be detachably locked to the device body when the battery pack  10  is slidably mounted on the device body. That is, as shown in  FIG. 3  and  FIG. 4 , the male hook mechanism  70  includes a hook-shaped structure  71  that has the shape of a hook/finger-hook, and biasing springs  75  that bias the hook-shaped structure  71  in a locking direction. The hook-shaped structure  71  includes the above-mentioned hook portion  72  and the operation portion  73 . The male hook mechanism  70 , which is formed as described above, is adapted to automatically lock the hook portion  72  to the female portion of the device body by the biasing forces of the biasing springs  75 . Further, when the operation portion  73  is pulled down against the biasing forces of the biasing springs  75 , the locking of the hook portion  72  to the female portion of the device body can be released and the battery pack  10  can be slid so as to be detached from the device body. 
     The upper case  21 , which is disposed on the upper side, is fastened to the lower case  12 , which is formed as described above, by the screw members  19 , so that the case  11  is formed. A structure, which makes the combined state of the upper and lower cases more firm, is provided between the combined lower and upper cases  12  and  21 . That is, as shown in portions that are shown in extracted circles in  FIGS. 8 to 10 , a fitting recessed groove  181  is formed at an opening edge  18  of the lower case  12 . The fitting recessed groove  181  opens upward along the opening edge  18 . The fitting recessed groove  181  is formed along the opening edge  18  at the middle portion of the opening edge  18  in a thickness direction. For this reason, the fitting recessed groove  181  is formed over the entire opening edge  18  that is formed at all the front, rear, left, and right portions of the lower case  12 . The width of the fitting recessed groove  181  is set to about a third of the thickness of the opening edge  18 . Further, the depth of the fitting recessed groove  181  is set to be substantially equal to the thickness of the opening edge  18 . 
     In contrast, a fitting protrusion  281 , which is to be fitted to the fitting recessed groove  181 , is formed at an opening edge  28  of the upper case  21  which is opened downward. The fitting protrusion  281  is formed over the entire opening edge  28  that is formed at the front, rear, left, and right portions of the upper case  21 , and is formed so as to have a protruding width and a protruding length corresponding to the depth of the fitting recessed groove  181  formed in the lower case  12 . Specifically, the protruding width of the fitting protrusion  281  is set to about a third of the thickness of the opening edge  28 . Further, the protruding length of the fitting protrusion  281  is set to be substantially equal to the thickness of the opening edge  28 . 
     According to a relationship between the fitting recessed groove  181  and the fitting protrusion  281  that are fitted to each other with these setting items, the opening edges  18  and  28  are firmly joined to each other. That is, even though a load is applied to the upper case  21  relative to the lower case  12  or a load is applied to the lower ease  12  relative to the upper case  21 , the relative displacement between the upper and lower cases can be significantly suppressed. 
     Next, the battery body  30 , which is disposed in the above-mentioned case  11 , will be described. The battery body  30  serves various functions in the rechargeable battery pack  10 . As shown in  FIG. 4 , the battery body  30  includes a battery portion  31  and a controller  41 . As shown in  FIG. 3 , the battery portion  31  includes five battery cells  33  ( 331 ,  332 ,  333 ,  334 , and  335 ). The battery cell  33  typically has a columnar shape and can be charged and discharged. The battery cells  33  are sequentially a first battery cell  331 , a second battery cell  332 , a third battery cell  333 , a fourth battery cell  334 , and a fifth battery cell  335  as arranged beginning from the front side. The five battery cells  33  extend in the left-and-right direction and are transversely arranged in parallel in the front-and-rear direction. For this reason, positive and negative electrodes  34  and  35  of the five battery cells  33  are disposed on both left and right ends of the battery cells. Meanwhile, as shown in  FIG. 3 , an insulation sheet  36  is mounted on each of the positive electrodes  34 . 
     Lead plates  37  ( 371 ,  372 ,  373 ,  374 ,  375 , and  376 ), which electrically connect the electrodes  34  and  35 , are provided on the electrodes  34  and  35  that are disposed on both left and right ends of the five battery cells  33 . That is, as shown in  FIG. 3 , a first lead plate  371 , a second lead plate  372 , a third lead plate  373 , a fourth lead plate  374 , a fifth lead plate  375 , and a sixth lead plate  376  are connected to the electrodes  34  and  35 , which are disposed on both left and right ends of the battery cells  33 , through welding. These lead plates  37  ( 371 ,  372 ,  373 ,  374 ,  375 , and  376 ) correspond to electrode members of the invention. The five battery cells  33  are held by the cell holder  50  described in detail below. Further, reference numeral  38  shown in the drawings denotes cushioning mats that are laid on an inner bottom  121  forming the inner surface of the lower case  12 . The cushioning mat  38  is molded with an elastic resin so as to have the shape of a sheet. The cushioning mat  38  is formed so as to extend over a range in which the battery cells  33  are arranged side by side in the front-and-rear direction. The cushioning mats  38  are provided with an appropriate interval interposed therebetween so as to secure the positions of cell holding portions  63  of the cell, holder  50  described in detail below. 
     The controller  41  is disposed on the upper side of the battery portion  31 . The controller  41  includes the circuit board  42  that performs various kinds of control processing. The circuit board  42  is provided with a microcomputer, monitors the states of the battery cells  33  through the lead plates  37 , and performs controls relating to charge and discharge. For this purpose, the upper ends of the lead plates  37 , which are mounted on the electrodes  34  and  35  disposed on both left and right ends of the five battery cells  33 , are electrically connected to the circuit board  42 . Specifically, the upper ends of these lead plates  37  ( 371 ,  372 ,  373 ,  374 ,  375 , and  376 ) are connected to the circuit board  42  through welding. These lead plates  37  are connected to the electrodes  34  and  35  of the battery cell  33  and the circuit board  42  through welding. 
     The charge/discharge terminals  44  and the signal terminals  46  are provided on the upper surface of the circuit board  42 . The charge/discharge terminals  44  and the signal terminals  46  function as terminals electrically connected to a tool body or a dedicated charger (not shown) serving as a device body. Leaf spring-like metal terminals are disposed so as to face each other, so that the charge/discharge terminals  44  and the signal terminals  46  are formed. The charge/discharge terminals  44  function as discharge terminals when the battery pack  10  is mounted on a tool body as a power source. Further, the charge/discharge terminals  44  function as charge terminals when the battery pack  10  is mounted on a dedicated charger. For this purpose, the charge/discharge terminal  44  includes a positive connection terminal  441  that is connected to, for example, a positive terminal of the tool body and a negative connection terminal  442  that is connected to, for example, a negative terminal of the tool body. Each of the positive connection terminal  441  and the negative connection terminal  442  is provided so that two terminals are arranged in parallel in the front-and-rear direction. 
     The four signal terminals  46  are provided. When the battery pack  10  is mounted on the device body, each of the three signal terminals  46  is formed as a terminal that transmits and receives information between the device body and the battery pack. That is a first signal terminal  461  functions as a signal terminal that transmits a charge completion signal to a dedicated charger. Specifically, when the circuit board  42  determines that the voltage of the battery cell  33  becomes equal to or higher than a predetermined threshold, the first signal terminal  461  transmits a charge completion signal to the dedicated charger so as to stop the charging of the dedicated charger. Further, a second signal terminal  462  functions as a signal terminal that transmits monitoring voltage information about the temperature of the battery cell  33  to the dedicated charger. Specifically, when the temperature of the battery cell  33  reaches an abnormal temperature, the circuit board determines that the temperature of the battery cell  33  reaches an abnormal temperature and the second signal terminal  462  transmits the monitoring voltage information to the dedicated charger so as to stop the charging of the dedicated charger. Furthermore, a third signal terminal  463  functions as a signal terminal that transmits an overcharge prevention signal to the dedicated charger. Specifically, when the charge control of the dedicated charger cannot be stopped by the first signal terminal  461 , the third signal terminal  463  transmits the overcharge prevention signal to the dedicated charger to be connected from the third signal terminal  463 . When this occurs, charging of the dedicated charger is stopped so that the battery pack  10  is not overcharged. Moreover, a fourth signal terminal  464  functions as a signal terminal that transmits an over discharge prevention signal to a tool body. Specifically, when the circuit board  42  judges that the voltage of the battery cell  33  becomes equal to or lower than a predetermined threshold, the fourth signal terminal  464  transmits a discharge stop signal to the tool body so as to stop the discharging of the tool body. Meanwhile, the signal terminals  46 , which are not required to be electrically connected to each other, are arranged as fax apart as possible. Further, information, which is to be transmitted and received by the above-mentioned signal terminal  46 , is not limited to the above-mentioned signals, and may be appropriate information about the battery pack  10  or the device body. Furthermore, signals, which are to be transmitted and received, typically include information based on the control processing of the circuit board  42 . 
     Next, the cell holder  50 , which holds the above-mentioned battery cells  33 , will be described with reference to  FIGS. 8 to 18 . 
     The cell holder  50  is formed to hold the five battery cells  33  together, in order to make the five battery cells  33  ( 331 ,  33 E  333 ,  334 , and  335 ) be housed and held in the lower case  12 . Meanwhile, the cell holder  50  is assembled in the lower case  12  while covering at least a part of the battery cells  33 . That is the cell holder  50  functions to partition the inside of the lower case  12  so that the battery cells  33  can be disposed in the lower case  12 . The cell holder  50  is molded from an appropriate plastic resin. The cell holder  50  includes an outer rim body  51  that forms a framework of the cell holder  50 , and cell receiving portions  60  ( 601 ,  602 ,  603 ,  604 , and  605 ) that are supported by the outer rim body  51  and receive the battery cells  33 . 
     The outer rim body  51  has a structure that is supported by the case  11  and that supports the cell receiving portions  60 . That is, the outer rim body  51  includes a board combining portion  52  and an inner surface contact portion  53 . The board combining portion  52  functions as a portion at which the cell holder  50  is locked to the circuit board  42 . Specifically, the board combining portion  52  includes two hooks, that is, a front looking hook  521  and a rear locking hook  522  that are locked to the above-mentioned circuit board  42 . These two front and rear locking hooks  521  and  522  are formed integrally with the outer rim body  51  so as to protrude upward from side surface receiving-partitioning portions  611  which extend toward the middle from the outer rim body  51 . These two front and rear locking hooks  521  and  522  are provided at positions that can be displaced forward and rearward on the center axis of the cell holder  50  in the left-and-right direction. Specifically, the front locking hook  521  is provided on the front side on the center axis of the cell holder  50  in the left-and-right direction, and the rear locking hook  522  is provided on the rear side on the center axis of the cell holder  50  in the left-and-right direction. The front locking hook  521  is engagingly locked to a front edge  421  of the circuit board  42  so as to protrude upwards as shown in  FIG. 8 . The locked front locking hook  521  pinches the circuit board  42  between itself and upper surface receiving-partitioning portions  612  of the cell receiving portions  60  described below. Further, the rear locking hook  522  is locked to a rear edge  422  of the circuit board  42  from above, and the locked rear locking hook  522  pinches the circuit board  42  between itself and the upper surface receiving-partitioning portions  612  of the cell receiving portions  60  described below. In this way, the board combining portion  52 , which includes the front and rear locking hooks  521  and  522 , combines the cell holder  50  with the circuit board  42  so that the cell holder  50  is integrally coupled to the circuit board  42 . Since the cell holder  50  and the circuit board  42  are combined in this way by the board combining portion  52  so as to be integrally coupled with each other, screw members such as bolts for retaining the cell holder  50  and the circuit board  42  do not need to be used. Accordingly, excellent productivity is obtained. Further, since the cell holder  50  and the circuit board  42  are held together by the board combining portion  52 , ease in the manufacturing process can also be improved. 
     The inner surface contact portion  53  has the same general shape as the outer edge of the cell holder  50 . This inner surface contact portion  53  comes into contact with the inner surface of the lower case  12 . That is, the inner surface contact portion  53  includes a front wall portion  54 , a rear wall portion  55 , a right wall portion  56 , and a left wall portion  57 . The front wall portion  54  has the shape of an outer surface that can come into surface contact with a front inner surface  171  of the lower case  12 . The front wall portion  54  has the shape of an outer surface that can come into surface contact with the front inner surface  171  of the lower case  12  shown in  FIG. 6 . The rear wall portion  55  has the shape of an outer surface that can come into surface contact with a rear inner surface  172  of the lower case  12  shown in  FIG. 6 . 
     Further, the right wall portion  56  has the shape of an outer surface that can come into surface contact with a right inner surface  173  of the lower case  12  shown in  FIG. 6 . The gap portions  561  are formed on the right wall portion  56 , and the support ribs  15  formed at the above-mentioned lower case  12  are fitted to the gap portions  561 . Furthermore, the left wall portion  57  has the shape of an outer surface that can come into surface contact with a left inner surface  174  of the lower case  12  shown in  FIG. 6 . The gap portions  571  are also formed on the left wall portion  57 , and the support ribs  15  formed at the above-mentioned lower case  12  are fitted to the gap portions  571 . In this way, the relative movement of the cell holder  50  in the lower case  12  in the front-and-rear direction is restricted by the contact of the front and rear wall portions  54  and  55  so that the cell holder  50  is supported by the lower case  12 . In addition, the relative movement of the cell holder  50  in the lower case  12  in the front-and-rear direction is restricted by the contact of the support ribs  15  fitted to the gap portions  561  and  571  and by the contact of the pressure ribs  16  pressing the lead plates  37  toward the battery cells  33 . In this way, the cell holder  50  is supported by the lower case  12 . 
     Further, the receiving-partitioning portions  61  of the cell receiving portions  60  described below come into contact with the circuit board  42  from below, so that the upward movement of the cell holder  50  relative to the case  11  is restricted. Furthermore, the battery cells  33  housed in the cell receiving portions  60  described below are pressed against the cushioning mats  38 , so that the downward movement of the cell holder  50  relative to the case  11  is restricted. 
     A front pressing rib  541 , which has a small width in the left-and-right direction (width direction), is formed near the middle of the front wall portion  54  of the above-mentioned cell holder  50  in the left-and-right direction (width direction). The front pressing rib  541  corresponds to a dimensional difference interposition member. That is, the front pressing rib  541  is a portion that comes into contact with the front inner surface  171 . It generates a pressing force against the front inner surface  171  of the lower case  12  when the cell holder  50  is assembled in the lower case  12 . 
     Specifically, the front pressing rib  541  is formed to come into contact with the front inner surface  171  of the lower case  12  before the cell holder  50  is assembled in the lower case  12  so that a dimensional difference between the inside of the lower case  12  and the front wall portion  54  of the cell holder  50  is bridged. That is, when the cell holder  50  is assembled in the lower case  12 , a dimensional difference between the inside of the lower case  12  and the front wall portion  54  of the cell holder  50  is completely bridged. In this case, the front pressing rib  541  generates a satisfactory pressing force against the front inner surface  171  of the lower case  12  by coming into contact with the front inner surface  171  of the lower case  12 . Meanwhile, the front pressing rib  541  is molded integrally with the cell holder  50  so as to be integrated with the front wall portion  54 . Specifically, the front pressing rib  541  is formed so as to be inclined forward while protruding upward. Further, a portion at which the front pressing rib  541  is provided is situated at a middle portion of the front inner surface  171  of the above-mentioned lower case  12  in the left-and-right direction (width direction) and faces the upper end of the front inner surface  171 . For this reason, the front pressing rib  541  is provided at an edge of the base end (base end edge) of the cell holder  50  in an assembling direction when the cell holder  50  is assembled into the lower case  12 . Meanwhile, when the cell holder  50  is assembled into the lower case  12 , an upper edge of the front pressing rib  541  is set so as to be aligned with the upper edge of the front inner surface  171  of the lower case  12  facing the cell holder. The front pressing rib  541 , which is set as described above, is formed only at the front end of the cell holder that is one end in a direction orthogonal to the direction of the extension axis of the above-mentioned battery cell  33 . 
     Next, the cell receiving portions  60 , which are provided at the cell holder  50 , will be described. The cell receiving portions  60  are portions of the cell holder  50  that receive the battery cells  33 . The cell receiving portions  60  include the receiving-partitioning portions  61  that partition a hollow structure for receiving the battery cells  33  and the cell holding portions  63  that hold the housed battery cells  33 . The receiving-partitioning portions  61  are provided so as to partition the hollow structure into first to fifth cell receiving portions  601  to  605  that are arranged side by side in the front-and-rear direction. The receiving-partitioning portions  61  are formed by connecting the side surface receiving-partitioning portions  611  with the upper surface receiving-partitioning portions  612 . The side surface receiving-partitioning portions  611  and the upper surface receiving-partitioning portions  612  are formed in the shape of partition walls extending in the left-and-right direction, and partition off the first to fifth cell receiving portions  601  to  605  that are arranged side by side in the front-and-rear direction. Specifically, the side surface receiving-partitioning portions  611  are formed in the shape of partition walls that are provided so as to partition off a space in the front-and-rear direction. Further, the upper surface receiving-partitioning portions  612  are formed in the shape of partition walls that are provided so as to partition off a space in the up-and-down direction. The side surface receiving-partitioning portions  611  and the upper surface receiving-partitioning portions  612 , as described above, partition off the cell holder  50  into first to fifth cell receiving portions  601  to  605 . The side surface receiving-partitioning portions  611  and the upper surface receiving-partitioning portions  612 , as described above, correspond to guide portions of the invention. That is, the side surface receiving-partitioning portions  611  and the upper surface receiving-partitioning portion  612  are formed so as to have a substantially C shaped-curved surface which function to guide the first to fifth battery cells  331  to  335  into the first to fifth cell receiving portions  601  to  605  of the cell holder  50 . 
     Both left and right sides of the first to fifth cell receiving portions  601  to  605 , which are formed so as to be partitioned off as described above, are configured to be open. The opening shape of the first to fifth cell receiving portions  601  to  605  are configured such that the battery cell  33  can be inserted into each of these first to fifth cell receiving portions  601  to  605  through only one of the left and right sides as shown by void arrows of  FIG. 11 . That is, opening/closing portions  62 , which restrict the insertion of the battery cells  33  by closing the opened sides, are provided at the opposite sides of both left and right sides of these first to fifth cell receiving portions  601  to  605 . The opening/closing/portions  62  are configured to be a first opening/closing portion  621 , a second opening/closing portion  622 , a third opening/closing portion  623 , a fourth opening/closing portion  624 , and a fifth opening/closing portion  625 . That is, the first opening/closing portion  621  protrudes from the right wall portion  56  so as to close a part of the right opened side of the first cell receiving portion  601 . For this reason, when the battery cell  33  is to be housed in the first cell receiving portion  601 , the battery cell  33  can be inserted and housed in the first cell receiving portions  601  only from the left side. Further, the second opening/closing portion  622  protrudes from the left wall portion  57  so as to close a part of the left opened side of the second cell receiving portions  602 . For this reason, when the battery cell  33  is to be housed in the second cell receiving portion  602 , the battery cell  33  can be inserted and housed in the second cell receiving portion  602  only from the right side. 
     Furthermore, the third opening/closing portion  623  protrudes from the right wall portion  56  so as to close a part of the right opened side of the third cell receiving portion  603 . For this reason, when the battery cell  33  is to be housed in the third cell receiving portion  603 , the battery cell  33  can be inserted and housed in the third cell receiving portion  603  only from the left side. Moreover, the fourth opening/closing portion  624  protrudes from the left wall portion  57  so as to close a part of the left opened side of the fourth cell receiving portion  604 . For this reason, when the battery cell  33  is to be housed in the fourth cell receiving portion  604 , the battery cell  33  can be inserted and housed in the fourth cell receiving portion  604  only from the right side. Further, the fifth opening/closing portion  625  protrudes from the right wall portion  56  so as to close a part of the right opened side of the fifth cell, receiving portion  605 . For this reason, when the battery cell  33  is to be housed in the fifth cell receiving portions  605 , the battery cell  33  can be inserted and housed in the fifth cell receiving portions  605  only from the left side. When the battery cells  33  are to be housed in the first to fifth cell receiving portions  601  to  605  in this way, the battery cells  33  are alternately inserted into the left and right sides of the cell receiving portions. That is, the first opening/closing portions  621  to fifth opening/closing portions  625  are configured such that the battery cell  33  can be inserted into each of the first to fifth cell receiving portions  601  to  605  only through the predetermined sides. Meanwhile, the insertion direction in which the battery cells  33  are inserted into these first to fifth cell receiving portions  601  to  605  is configured such that the positive electrode of the battery cell  33  is always positioned on the tip side in the insertion direction. 
     The cell holder  50  is provided with the cell holding portions  63  that come into contact with columnar outer peripheral surfaces  330  of the battery cells  33  so as to pinch the outer peripheral surfaces  330  and hold the battery cells  33 . Two cell holding portions  63  are provided in each of the above-mentioned first to fifth cell receiving portions  601  to  605 . The cell holding portions  63  are provided at positions which are symmetrical to each other with respect to the center axis of the cell holder  50  in the left-and-right direction. That is, the cell holding portions  63  are provided at positions that are symmetrical to each other in a longitudinal direction of the battery cell  33  with respect to the center of the battery cell  33 . Specifically, right cell holding portions  631  are disposed on the right side of the center axis of the cell holder  50  in the left-and-right direction. Further, left cell holding portions  632  are disposed on the left side of the center axis of the cell holder  50  in the left-and-right direction. 
     The cell holding portions  63  hold the battery cells  33  so as to pinch the columnar outer peripheral surfaces  330  of the battery cells  33  by the above-mentioned receiving-partitioning portions  61  (the side surface receiving-partitioning portions  611  and the upper surface receiving-partitioning portions  612 ). In other words, the cell holding portions  63  come into contact with the columnar outer peripheral surfaces  330  so as to press the battery cells  33  against the receiving-partitioning portions  61  that form the substantially C shaped-curved surfaces of the above-mentioned cell receiving portions  60 . In this case, the cell holding portions  63  act so as to press the battery cells  33  against the side surface receiving-partitioning portions  611  and the upper surface receiving-partitioning portions  612 . A contact diameter, which is formed by the cell holding portion  63  and the receiving-partitioning portion  61 , is configured to be smaller than the diameter of the columnar outer peripheral surface  330  of the battery cell  33 . That is an inner diameter, which is formed by the cell holding portion  63  and the receiving-partitioning portion  61 , is configured to be smaller than the peripheral diameter of the outer peripheral surface  330  of the battery cell  33 . For this reason, the cell holding portion  63  and the receiving-partitioning portions  61  can pinch and hold the battery cell  33 . Meanwhile, an inner diameter, which is formed by the cell holding portion  63  and the receiving-partitioning portion  61  (the side surface receiving-partitioning portions  611  and the upper surface receiving-partitioning portion  612 ), corresponds to a contact diameter of the invention. 
     The contact diameter of the cell holding portions  63  is configured so that they can increase when the battery cell  33  is held. Specifically, the cell holding portions  63  are formed so as to approach or be separated from the receiving-partitioning portions  61  (the side surface receiving-partitioning portions  611  and the upper surface receiving-partitioning portions  612 ). That is the cell holding portions  63  are formed so as to be elastically deformable. Accordingly, the cell holding portions  63  can be elastically deformed so as to be capable of approaching or being separated from the receiving-partitioning portions  61 . That is, the inner diameter, which corresponds to the above-mentioned contact diameter and is formed by the cell holding portion  63  and the receiving-partitioning portion  61  (the side surface receiving-partitioning portions  611  and the upper surface receiving-partitioning portion  612 ), increase or decrease in accordance with the change of the position of the cell holding portion  63  relative to the receiving-partitioning portion  61 . 
     In this way, the contact diameter (inner diameter), which is formed by the cell holding portion  63  and the receiving-partitioning portions  61 , can increase when the battery cell  33  is held. Due to their elasticity, the cell holding portions  63  come into contact with the columnar outer peripheral surface  330  of the battery cell  33 . The cell holding portions  63  pinch the columnar outer peripheral surface  330  between themselves and the receiving-partitioning portions  61 . That is, the cell holding portions  63  act so as to press the columnar outer peripheral surfaces  330  of the battery cells  33  against the receiving-partitioning portions  61  when holding the battery cells  33 . 
     The cell holding portion  63  is molded with an appropriate plastic resin so as to be elastically deformable. That is, the cell holding portion  63  includes one contact-curved portion  64  that corresponds to the columnar outer peripheral surface  330  of the battery cell  33 , and two support-curved portions  65  and  66  that are provided at both ends of the one contact-curved portion  64 . The contact-curved portion  64  is formed so as to have a gently curved shape that corresponds to the shape of the outer peripheral surface  330  of the battery cell  33 . The contact-curved portion  64  mainly corresponds to the outer peripheral surface  330  of the battery cell  33 , but is elastically deformable. That is, the contact-curved portion  64  can be bent into a linear shape from the gently curved shape. The contact-curved portion  64 , which is bent as described above, can be elastically restored so that the shape of the contact-curved portion returns to the original gently curved shape. 
     Further, the two support-curved portions  65  and  66  are provided to extend from both ends of the contact-curved portion  64 . The support-curved portions  65  and  66  are configured to have shapes that are symmetrical to each other with respect to the contact-curved portion  64 . The support-curved portions  65  and  66  have a curved shape that is curved with a sharper angle than that of the contact-curved portion  64 . Specifically, the support-curved portions  65  and  66  are configured so as to fold the end portions of the contact-curved portion  64  and form a circle. For this reason, the support-curved portions  65  and  66  are formed so as to be more elastically deformable than the contact-curved portion  64 . The support-curved portions  65  and  66 , which are formed as described above, are adapted to be capable of elastically supporting the contact-curved portion  64 . That is, when the contact-curved portion  64  is pressed so that the above-mentioned contact diameter (inner diameter) increases, the support-curved portions  65  and  66  are bent first. The support-curved portions  65  and  66 , which are bent as described above, act to be elastically restored so that the shapes of the support-curved portions return to the original curved shape. The elastic restoration actions of the support-curved portions  65  and  66  cause the contact-curved portion  64  to return to the original position of the contact-curved portion  64 . That is, the contact-curved portion  64  returns to the original position so that the above-mentioned contact diameter (inner diameter) decreases, and presses the outer peripheral surface  330  of the battery cell  33 . 
     Furthermore, the above-mentioned cell holder  50  is provided with extending leg portions  67  that extend toward the inner bottom  121  of the lower case  12 . The extending leg portions  67  correspond to extending portions of certain embodiments of the invention. The extending leg portions  67  come into contact with the inner bottom  121  of the lower case  12 . When a force is further applied to the extending leg portions  67 , the extending leg portions  67  are bent. The bent extending leg portions  67  have elasticity that causes the extending leg portions to return to their original shape. Further, the extending leg portions  67  are provided between the first to fifth cell receiving portions  601  to  605 . Furthermore, the extending leg portions  67  are provided at three portions in the left-and-right direction, i.e. at a right portion, an intermediate portion, and a left portion in the left-and-right direction. That is a right extending leg portion  671 , an intermediate extending leg portion  672 , and a left extending leg portion  673  are provided in each space between the first to fifth cell receiving portions  601  to  605 . Meanwhile, the right extending leg portions  671 , the intermediate extending leg portions  672 , and the left extending leg portions  673  are also provided on the front wall portion  54  and the rear wall portion  55 . The right extending leg portions  671 , the intermediate extending leg portions  672 , and the left extending leg portions  673  extend downward from the outer rim body  51  of the cell holder  50  so as to be divided at portions where the cell holding portions  63  are provided. Meanwhile, the right cell holding portion  631  is disposed between the right extending leg portion  671  and the intermediate extending leg portions  672 . The left cell holding portion  632  is disposed between the intermediate extending leg portions  672  and the left extending leg portions  673 . Further, the right extending leg portion  671  and the left extending leg portion  673  are provided at positions that are symmetrical to each other in a longitudinal direction of the battery cell  33 . The extending leg portions  67  are formed so as to extend in the shape of an elastically deformable plate. The extending leg portions  67  extend from the above-mentioned outer rim body  51  and also are supported by the outer rim body  51 . 
     In the battery pack  10  described above, certain effects can be obtained. The contact diameter is formed by the cell holding portion  63  and the receiving-partitioning portions  61 . It is configured to be smaller than the diameter of the columnar outer peripheral surface  330  of the battery cell  33  and therefore the cell holding portion  63  comes into contact with the columnar outer peripheral surface  330  of the battery cell  33  so as to hold the columnar outer peripheral surface  330 . In this way, the cell holding portions  63  can hold the battery cells  33  so that the battery cells are integrally coupled with the cell holder  50  at the cell receiving portions  60 . Further, the cell holder  50  is supported by the case  11 . Therefore, even when impact or vibration is applied to the battery pack  10  from the outside, the cell holder  50  supported by the case  11  and the battery cells  33  can be integrally moved. Therefore, it is possible to suppress the movement of the battery cells  33  relative to the case  11 . Further, according to the above-mentioned battery pack  10 , the contact diameter formed by the cell holding portion  63  and the receiving-partitioning portions  61  increases when the battery cell  33  is held. When this occurs, the battery cell  33  is pressed when held. Accordingly, the battery cells  33  can be elastically held by the cell holding portions  63 . Therefore, even when impact or vibration is applied to the battery pack  10  from tire outside, the cell holder  50  supported by the case  11  can elastically support the battery cells  33  and can move integrally with the battery cells  33 . In this way, the movement of the battery cells  33  relative to the case  11  can be further suppressed. Furthermore, in the above-mentioned battery pack  10 , the cell holder  50  is provided with receiving-partitioning portions  61  that guide the battery cells  33  as they are inserted and housed. The battery cells  33  can be easily inserted and housed in the cell holder  50 . Accordingly, the ease in inserting the battery cells  33  into the cell holder  50  can be improved. Further, in the above-mentioned battery pack  10 , the receiving-partitioning portion  61  is formed so as to have a substantially C shaped-curved surface corresponding to the outer peripheral shape of the battery cell  33 . This can aid in guiding the battery cell  33  inserted into the cell holder  50 . 
     Furthermore, according to the above-mentioned battery pack  10 , the cell holding portion  63  can come into contact with the battery cell  33  and support the battery cell  33  by the contact-curved portion  64  which is elastically supported by two support-curved portions  65  and  66  has a curved shape. Molding is used to create elastic cell holding portions  63  which can hold the battery cells  33 . In this way, manufacturing costs can be reduced in creating elastic cell holding portions  63 . Moreover, in the above-mentioned battery pack  10 , the extending leg portions  67  are adapted to be capable of coming into elastic contact with the inner bottom  121  of the lower case  12 . For this reason, when the cell holder  50  is supported from the inner bottom  121  of the lower case  12  with the extending leg portions  67  interposed therebetween, the cell holder  50  can be supported using cushioning. Even if impact or vibration applied from the outside is received, the cell holder  50  can be sufficiently supported by the case  11  and unification of the cell holder  50  and the case  11  can be maintained. Meanwhile, the above-mentioned extending leg portions  67  may be adapted to support the cell holder  50  with cushioning when receiving impact or vibration. For this purpose, the extending leg portions  67  may be adapted to be capable of coming into elastic contact with the inner surface of the case  11 . The extending leg portions  67 , however, do not necessarily need to constantly come into contact with the inner surface of the case  11 . Further, since the extending leg portion  67  is provided as a part of the cell holding portion  63 , it is possible to reduce space that is required for the extending leg portions  67 . In this way, it is possible to efficiently provide the extending leg portions  67  without increasing the size of the battery pack  10 . Furthermore, since the cell holding portions  63  and the extending leg portions  67  are provided at positions that are symmetrical to each other in the longitudinal direction of the battery cell  33 , the battery cell  33  can be held in good balance. 
     As the front pressing rib  541  is formed integrally with the cell holder  50 , the front pressing rib  541  can bridge a dimensional difference between the inside of the case  11  to be assembled and the cell holder by generating a pressing force on the front side. In other words, the cell holder  50  is pressed rearward in the case  11  by a pressing force of the front pressing rib  541  that is applied to the inside of the case  11 . In this way, it is possible to bridge a dimensional difference between the case and the cell holder  50  in the case  11 . The front pressing rib  541  is formed in the cell holder  50  by being molded integrally with the cell holder  50 . It is preferably not formed as a separate member. In this way, it is possible to suppress the rattling of the cell holder  50  within the case  11 , without increasing the number of parts. The front pressing rib  541  acts so as to bridge the dimensional difference from a time immediately before the cell holder  50  is completely assembled into the case  11 . Contrastingly, the front pressing rib  541  does not act so as to bridge a dimensional difference until the time immediately before the cell holder  50  is assembled into the case  11 . Therefore, it is possible to secure a time period for inserting the cell holder  50  is assembled into the case  11  before complete assembly is accomplished. Therefore, it is possible to easily put the cell holder  50  into the case  11 . Moreover, since the front pressing rib  541  acts so as to bridge the dimensional difference when the cell holder  50  inserted into the case  11 , the front pressing rib can bridge the clearance for complete assembly once when the cell holder  50  is inserted. Accordingly, it is possible to suppress the rattling of the cell holder  50  relative to the case  11 . Therefore, while maintaining ease in insertion of the cell holder  50  into the case  11 , it is possible to suppress the rattling of the cell holder  50 . Further, since the front pressing rib  541  is provided at the base end of the cell holder in the assembling direction, the front pressing rib  541  does not cause interference until the cell holder  50  is completely assembled into the case  11 . Accordingly, it is possible to sufficiently secure a clearance for assembly until after the cell holder  50  is inserted into the case  11 . Therefore, it is possible to easily insert the cell holder  50  into the case  11 . Furthermore, since the front pressing rib  541  is formed at the cell holder  50  by being molded integrally with the cell holder  50 , it is possible to form the front pressing rib  541  in a simple manner. The front pressing rib  541  is formed at the front end of the cell holder in a direction orthogonal to the longitudinal direction of the battery cell  33 . In this way, it is possible to bridge a dimensional difference between the inside of the case  11  and the cell holder by generating a pressing force on the front side and to set the rear side as a reference surface to which the cell holder  50  is fixed. Accordingly, the cell holder  50  can be accurately positioned to the case  11  to produce a battery pack. 
     Second Embodiment 
     Next, a battery pack  10 A according to a second embodiment of the invention will be described below with reference to  FIGS. 19 to 22 . As described below in the second and fourth embodiments, battery packs  10 A to  10 C are created by modifying a part of the structure of the cell holder  50  of the battery pack  10  of the first embodiment. Differences between portions of cell holders  50 A to  50 C of the battery packs  10 A to  10 C in the second to fourth embodiments and the above-mentioned cell holder  50  will be described. Portions of the battery packs  10 A to  10 C according to the second to fourth embodiments, which are the same as portions of the battery pack  10  according to the above-mentioned first embodiment, are denoted in the drawings using the same reference numerals, and the description thereof will be omitted. 
     As shown in  FIGS. 19 to 22 , the cell holder  50 A of the second embodiment is different from the cell holder  50  of the first embodiment in terms of the curved shapes of support-curved portions  65 A and  66 A. That is the curved shapes of the support-curved portions  65 A and  66 A of the second embodiment are formed so as to further protrude downward. Specifically, the support-curved portions  65 A and  66 A are formed so as to draw arcs more deeply when being turned over from the end portions of the contact-curved portion  64  at an angle of about 180°. Further, the support-curved portions  65 A and  66 A draw arcs to the lower side of the inner bottom  121  forming the inner surface of the lower case  12 . The support-curved portions  65 A and  66 A, which are formed as described above, exhibit the same function as the extending leg portions  67  of the first embodiment. 
     Furthermore, the support-curved portions  65 A and  66 A draw arcs to the deeply lower side until being seated on the inner bottom.  121  of the lower case  12 . For this reason, the bending amount of the support-curved portions  65 A and  66 A, when being bent, are configured to be larger than those of the support-curved portions  65  and  66  of the first embodiment when being bent. That is, the support-curved portions  65 A and  66 A, which are seated on the inner bottom  121  of the lower case  12 , are configured to support the lower case  12 , so that the cell holder  50  itself is elastically supported. For this reason, the support-curved portions  65 A and  66 A, which form a part of the cell holding portion  63 , correspond to extending portions in certain embodiments, and the elastic restoration of the support-curved portions  65 A and  66 A is caused by the repulsion of the support-curved portions  65 A and  66 A. That is, even when a force is applied to the cell holder  50  and the support-curved portions  65 A and  66 A are bent, the cell holder  50  returns to the original state by elastic restoring forces of the support-curved portions  65 A and  66 A. That is, the cell holder  50  is elastically supported from the inner bottom  121  of the lower case  12  by the elastic restoration of the support-curved portions  65 A and  66 A. 
     In the battery pack  10 A, since the support-curved portions  65 A and  66 A of the cell holding portion  63  are provided as the extending portions of the invention, other molded portions do not need to be added when the extending portions are provided. Accordingly, it is possible to simplify the molding of the cell holder  50 . Further, since the elastic restoration of the support-curved portions  65 A and  66 A is caused by repulsion, it is possible to obtain elasticity through the molded shapes of the support-curved portions  65 A and  66 A. In this way, it is possible to provide the support-curved portions  65 A and  66 A, which function as the extending portions of the invention, in the cell holder  50  in a simple manner. Furthermore, the support-curved portions  65 A and  66 A can elastically support the contact-curved portion  64 , the cell holder  50 A, and the support-curved portions  65  and  66  of the above-mentioned first embodiment. 
     Third Embodiment 
     Next, a battery pack  10 B for an electric power tool according to a third embodiment of the invention will be described below with reference to  FIGS. 23 to 28 . 
     As shown in  FIGS. 23 to 28 , the cell holder  503  of the third embodiment is different from the cell holder  50  of the first embodiment in terms of the arrangement structure of the dimensional difference interposition member of the invention. That is, in the first embodiment, only the front pressing rib  541  is provided as the dimensional difference interposition member at the front side of the cell holder  50 . In contrast, dimensional difference interposition members of the third embodiment are formed at portions corresponding to four directions. They are formed on the front side, the rear side, the left side, and the right side of the cell holder  50 . That is, the same front pressing rib  541  of the above-mentioned first embodiment is formed at the front wall portion  54  of the cell holder  50 B of the third embodiment. Further, similar to the front pressing rib  541 , a rear pressing rib  551  is also formed on the rear wall portion  55 . Furthermore, similar to the front pressing rib  541 , a right pressing rib  563  is also formed at the right wall portion  56  and a left pressing rib  573  is also formed at the left wall portion  57 . The rear pressing rib  551 , the right pressing rib  563 , and the left pressing rib  573 , which are formed as described above, have substantially the same function as that of the above-mentioned front pressing rib  541 . That is, the front pressing rib  541 , the rear pressing rib  551 , the right pressing rib  563 , and the left pressing rib  573  can bridge a dimensional difference between the cell holder SOB and the inside of the case  11 . 
     The pressing ribs  541 ,  551 ,  563 , and  573 , which are formed as described above, may be produced by cutting out a part of the respective wall portions  54 ,  55 ,  56 , and  57  of the cell holder  50 B. In this case, it is possible to simply form the pressing ribs  541 ,  551 ,  563 , and  573  on the cell holder  50 . Further, the pressing ribs may be adapted to act so as to bridge a dimensional difference by the elasticity thereof when the cell holder  50  and the pressing ribs  541 ,  551 ,  563 , and  573  are completely assembled into the case  11 . Accordingly, it is possible to suppress the rattling of the cell holder  50 B relative to the case  11 . Therefore, it is possible to suppress the rattling of the cell holder  50 B relative to the case  11  in which the cell holder  50 B has been assembled. 
     Fourth Embodiment 
     Next, a battery pack  10 C for an electric power tool according to a fourth embodiment of the invention will be described below with reference to  FIGS. 29 to 35 . 
     As shown in  FIGS. 29 to 35 , the cell holder  50 C of the fourth embodiment is different from the cell holder  50  of the first embodiment in that the half of each of the extending leg portions  67  of the first embodiment is substituted with a flexible leg portion  68 . As shown in  FIG. 35 , a flexible leg portion  68  may serve as a substitute for the following: each of a portion (denoted by reference numeral  680  in  FIG. 35 ) of the right extending leg portion  671  that is connected to the front wall portion  54 , a portion (denoted by reference numeral  681  in  FIG. 35 ) of the right extending leg portion  671  that is provided between the second and third cell receiving portions  602  and  603 , a portion (denoted by reference numeral  682  in  FIG. 35 ) of the right extending leg portion  671  that is provided between the third and fourth cell receiving portions  603  and  604 , and a portion (denoted by reference numeral  683  in  FIG. 35 ) of the right extending leg portion  671  that is connected to the rear wall portion. Further, the flexible leg portion  68  may serve as a substitute for the following: each of a portion (denoted by reference numeral  684  in  FIG. 35 ) of the intermediate extending leg portion  672  that is provided between the first and second cell receiving portions  601  and  602  and a portion (denoted by reference numeral  685  in  FIG. 35 ) of the intermediate extending leg pardon  672  that is provided between the fourth and fifth cell receiving portions  604  and  605 . Furthermore, the flexible leg portion  68  may serve as a substitute for the following: each of a portion (denoted by reference numeral  686  in  FIG. 35 ) of the left extending leg portions  673  that is connected to the front wall portion  54 , a portion (denoted by reference numeral  687  in  FIG. 35 ) of the left extending leg portions  673  that is provided between the second and third cell receiving portions  602  and  603 , a portion (denoted by reference numeral  688  in  FIG. 35 ) of the left extending leg portions  673  that is provided between the third and fourth cell receiving portions  603  and  604 , and a portion (denoted by reference numeral  689  in  FIG. 35 ) of the left extending leg portions  673  that is connected to the rear wall portion  55 . 
     Portions where the extending leg portion  67  are not substituted with the flexible leg portion  68  are to be the same as the extending leg portions  67  of the first embodiment. For this reason, the flexible leg portions  68  substituted for the extending leg portions  67  are provided so as to protrude downward from the cell holder  50 . As shown in  FIG. 35 , the flexible leg portions  68 , which are disposed as described above, are alternately disposed on the left and right sides not so as to be disposed adjacent to each other in the left-and-right direction. That is, the flexible leg portions  68  are configured to be arranged in a so-called zigzag pattern. 
     The flexible leg portion  68  is configured to extend below the extending leg portion  67 . Further, the flexible leg portion  68  is formed so as to be tapered toward the lower end. At the lower ends of the flexible leg portions  68 , bendable portions  691  and  692 , which have a so-called bending inclination, are provided. The bendable portions  691  and  692  are formed so that the bending directions of the lower ends are guided when the lower ends of the flexible leg portions  68  are bent. Specifically, the bendable portions  691  and  692  are formed to be gently bent forward or rearward. The bendable portions  691  and  692  include two kinds of easily bendable portions, that is, forward bendable portions  691  and rearward bendable portions  692 . 
     Since the forward bendable portions  691  are formed so that the bending direction of the lower end of the flexible leg portion  68  is directed to the front side, the bending direction of the lower end is guided so as to be directed to the front side. In contrast, the rearward bendable portions  692  are formed so that the bending direction of the lower end of the flexible leg portion  68  is directed to the rear side, and the bending direction of the lower end is guided so as to be directed to the rear side. Further, the bendable portions  691  and  692  are configured so that the flexible leg portions  68  arranged in parallel in the front-and-rear direction are bent so as to face each other. Specifically, reference numeral  680 , reference numeral  682 , reference numeral  684 , reference numeral  686 , and reference numeral  688  of the flexible leg portion  68  are configured to be the rear easily bendable portions  692 . Further, reference numeral  681 , reference numeral  683 , reference numeral  685 , reference numeral  687 , and reference numeral  689  of the flexible leg portion  68  are configured to be the forward bendable portions  691 . 
     The forward bendable portions  691  and the rearward bendable portions  692  are provided at the flexible leg portions  68 , which are formed as described above, so as to face each other. Accordingly, even when the cell holder  50  is moved by a reaction against the bending of the flexible leg portions  68 , the forces of the flexible leg portions can be cancelled. That is, the flexible leg portions  68  can sufficiently provide a cushioning function with regard to the movement of the cell holder  50 . Meanwhile, when the flexible leg portions  68  are configured as described above, the extending leg portions  67 , which are not substituted with the flexible leg portions  68 , function to prevent the battery cells  33  from reaching the bottom. That is, when the cell holder  50  is moved up and down by the flexible leg portions  68 , the lower ends of the extending leg portions  67  reach the inner bottom  121  of the lower case  12  at the position of the lower end of the cell holder  50  before the battery cells  33  reach the inner bottom  121  of the lower case  12 . Accordingly, the battery cells  33  are prevented from reaching the bottom. 
     The battery pack for an electric power tool according to the invention is not limited to the above-mentioned embodiments, and appropriate portions thereof may be modified as follows: that is, the number of the battery cells is five in the above-mentioned embodiment, but the number of the battery cells of the invention is not limited thereto and may be appropriately determined. Further, the number of the lead plates to be disposed may be determined according to the number of the battery cells. 
     Furthermore, in the above-mentioned embodiments, the front pressing rib  541  is provided at the edge of the base end (base end edge) of the cell holder  50  in an assembling direction when the cell holder  50  is assembled into the lower case  12 . However, the dimensional difference interposition member of the invention may be provided at another portion of the cell holder  50  in the assembling direction. Further, as long as the cell holder  50  can be inserted with relative ease into the case  11  while a clearance for assembling can be sufficiently secured when the cell holder  50  is assembled into the ease  11 , the dimensional difference interposition member may be appropriately provided at any position. Furthermore, the dimensional difference interposition member of the invention is not limited to an example where the dimensional difference interposition member is provided at the cell holder  50  as in the above-mentioned embodiments, and may be provided in the case  11 . Even in this case, the dimensional difference interposition member may be provided at any position within the case  11 . 
     Moreover, in the above-mentioned embodiments, the charge/discharge terminal  44  serves as a discharge terminal when the battery pack  10  is mounted on a tool body as a power source, and a charge terminal when the battery pack  10  is mounted on a dedicated charger. That is, the charge/discharge terminal  44  is used to charge and to discharge the battery pack  10 . However, the charge/discharge terminal is not limited to such a common terminal, and a discharge terminal and a charge terminal may be separately prepared.