Abstract:
Disclosed is a battery installation structure for an electric automobile. A battery case guide device guides the aforementioned battery case to a battery case housing section. The battery case guide device is provided with protruding parts and guide parts. The protruding parts are provided in an area near the right front corner on the front side surface of the battery case, an area near the right front corner on the right side surface thereof, an area near the left rear corner on the rear side surface thereof, and an area near the left rear corner on the left side surface thereof. The guide parts are provided to the aforementioned automobile body. The guide parts are capable of abutting the protruding parts. Of the two categories of parts mentioned above, namely, the protruding parts and the guide parts, those in one category have each a convex curved surface, and those in the other category have each an inclined surface capable of abutting the convex curved surface.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a battery mounting structure for an electric automobile. 
     BACKGROUND OF THE INVENTION 
     Patent document 1 discloses technology related to a battery mounting structure for an electric automobile that implements a battery replacement technique. Document 1 discloses a battery replacement apparatus for an electric automobile. The battery replacement apparatus attaches and detaches a battery unit to and from a holder in a vehicle body from below the electric automobile. The battery replacement apparatus includes a lift unit that is raised and lowered while carrying the battery unit, a position detection unit that detects the position of the holder, and a positioning unit that positions and aligns the battery unit carried on the lift unit with the holder. 
     Patent document 2 also discloses technology related to a battery mounting structure for an electric automobile. Document 2 discloses a battery coupling structure for a battery device. In the coupling structure, a lock is coupled to a battery holding unit of a vehicle, and a striker, which is held by the lock, is coupled to the battery device. The lock holds the striker to couple the battery device to the holder. Further, the coupling structure includes a loosening prevention unit that prevents loosening of the battery device from the holder. 
     The loosening prevention unit includes a contact member, which is elastic and arranged on the battery device, and a seat member, which is elastic and coupled to the holder. The contact member is forced against the seat member to prevent loosening of the battery device from the holder. The seat member includes a seat surface that is inclined relative to a coupling direction of the battery device. The contact member includes a contact surface that is substantially parallel to the seat surface. The inclined seat surface and contact surface facilitate the coupling of the battery device to the holder. 
     PRIOR ART DOCUMENTS 
     Patent Documents 
     
         
         Patent Document 1: Japanese Laid-Open Patent Publication No. 6-262951 
         Patent Document 2: Japanese Laid-Open Patent Publication No. 2003-118397 
       
    
     SUMMARY OF THE INVENTION 
     Problems that are to be Solved by the Invention 
     However, even when detection of position and positioning are implemented as discussed in document 1, misalignment of from several millimeters to several centimeters occurs. In the coupling structure disclosed in document 2, however, when the battery device is guided to the holder and the battery device and holder are misaligned, the state of contact between the seat surface of the seat member and the contact surface of the contact member changes. This may vary the sliding resistance produced between the seat surface and contact surface. A variation in the sliding resistance between the seat surface and contact surface when the battery device is guided to the holder may vibrate the vehicle body or become the cause of noise. 
     It is an object of the present invention to provide a battery mounting structure for an electric automobile that guides a battery case to a battery case holder while suppressing the generation of vibration even when misalignment of the battery case and battery holder occurs before the battery case is mounted to the vehicle. 
     Means for Solving the Problem 
     A battery mounting structure for an electric automobile including a battery case, a battery case holder, a battery fastening mechanism, and a battery case guiding mechanism is provided. The battery case contains a battery cell. The battery case holder is arranged in a vehicle body of the electric automobile. The battery case holder is capable of accommodating the battery case. The battery fastening mechanism fastens the battery case, which is accommodated in the battery case holder, to the vehicle body. The battery case guiding mechanism guides the battery case to the battery case holder when the battery case is raised from below the vehicle body and accommodated in the battery case holder. The battery case includes four side surfaces facing front, rear, left, and right directions. A first corner is formed by two first side surfaces, among the four side surfaces, intersecting each other at a right angle. A second corner is formed by two remaining second side surfaces, among the four side surfaces, intersecting each other at a right angle. The battery case guiding mechanism includes a case side member and a vehicle body side member. The case side member is arranged at a portion of each of the first side surfaces near the first corner and a portion of each of the second side surfaces near the second corner. The vehicle body side member is arranged on the vehicle body. The vehicle body side member can contact the case side member. One of the case side member and the vehicle side member includes a convex-curved surface, and the other one of the case side member and the vehicle side member includes an inclined surface that can contact the convex-curved surface. 
     Here, the convex-curved surface refers to a convex curved surface that comes into line contact or point contact with a flat surface and includes, for example, a spherical surface, an arcuate surface, and an elliptical surface. Further, the phrase of near a corner on each side surface refers to a portion of each side surface closer to a corner than a middle part. 
     In the present invention, even if misalignment occurs between the battery case, prior to mounting, and the battery case holder, when the case side member and vehicle body side member slide relative to each other as the battery case is guided, the state of contact between the convex-curved surface and tapered surface subtly changes, and the sliding resistance between the convex-curved surface and tapered surface subtly varies as compared with the prior art. Accordingly, even when misalignment occurs between the battery case and the battery case holder of the vehicle before mounting the battery case, the generation of vibration is suppressed when the battery case is guided to the battery case holder. 
     Preferably, the convex-curved surface is formed by a spherical surface. 
     Accordingly, the contact of the convex-curved surface, which is formed by a spherical surface, and tapered surface is always point contact. Thus, even when tilting of the battery case and battery case holder causes misalignment, the sliding resistance between the convex-curved surface and tapered surface subtly varies. 
     Preferably, the convex-curved surface is formed by an arcuate surface. 
     Accordingly, the contact of the convex-curved surface, which is formed by an arcuate surface, and tapered surface is always line contact. When misalignment occurs due to tilting of the battery case and the battery case holder in a specific direction that maintains the convex-curved surface and tapered surface in line contact, the sliding resistance between the convex-curved surface and tapered surface subtly varies. 
     Preferably, the battery case is tetragonal when viewed from above, and the first corner and the second corner are located at diagonal positions. 
     Accordingly, the battery case, which is tetragonal when viewed from above, is positioned at the first corner and second corner, which are most separated. Thus, the battery case is positioned relative to the battery case holder with high accuracy. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a side view showing battery replacement in an electric automobile according to a first embodiment of the present invention. 
         FIG. 2  is a plan view of a battery case in the first embodiment of the present invention. 
         FIG. 3(   a ) is a cross-sectional view taken along line  3 - 3  in  FIG. 2 , and  FIG. 3(   b ) is a perspective view showing a front projection. 
         FIG. 4  is a plan view showing a battery case holder in the first embodiment of the present invention. 
         FIG. 5  is a cross-sectional view taken along line  5 - 5  in  FIG. 4 . 
         FIG. 6  is a cutaway partial view illustrating the guiding of the battery case when misaligned. 
         FIG. 7  is a partial plane view showing the contact of the battery case, which is tilted and thereby misaligned, and a guide. 
         FIG. 8  is a partial cutaway view showing a battery guiding mechanism in a second embodiment of the present invention. 
         FIG. 9  is a plan view showing a battery case in a third embodiment of the present invention. 
         FIG. 10(   a ) is a partial cutaway view showing a main part of a battery guiding mechanism in a fourth embodiment of the present invention, and  FIG. 10(   b ) is a perspective view showing a front projection in the fourth embodiment. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A battery mounting structure for an electric automobile according to a first embodiment of the present invention will now be described with reference to the drawings. In the present embodiment, an electric automobile  10  implements a battery replacement technique and uses battery cells C in a battery case  20 , which is attachable and detachable, as a drive source. When the battery cells C need to be charged, the battery case  20  is removed from a battery case holder  44  of the electric automobile  10  at a battery replacement station  11  and a different battery case  20 , which accommodates charged battery cells C, is mounted on the battery case holder  44 . 
       FIG. 1  shows a battery replacement station  11 , which is for replacing the battery case  20  of the electric automobile  10 . The battery replacement station  11  includes a parking pit  12 , in which the electric automobile  10  is parked, and a battery replacer  14 , which is set on a floor F in a space S under the parking pit  12 . The parking pit  12  includes an opening  13 , which is in communication with the space S. The opening  13  has a shape and size that allows for passage of the battery case  20 . The battery replacer  14  includes a platform  15 , on which the battery case  20  is placed, a conveyor  16 , which conveys the battery case  20  between the platform  15  and a location separated from the platform  15 , and a lift  17 , which raises and lowers the platform  15  in the vertical direction. 
     The battery case  20  of the present embodiment is tetragonal when viewed from above as shown in  FIG. 2  and has four side surfaces in the front, rear, left, and right directions. The battery case  20  contains a large number of battery cells C. The battery case  20  includes a connector (not shown), which outputs power from the charged battery cells C and inputs regenerative power. With regard to the four side surfaces of the battery case  20 , the side surface facing toward the front side of the electric automobile  10  is defined as a front surface  21  (first side surface), the side surface facing toward the rear side of the electric automobile  10  is defined as a rear surface  22  (second side surface), the side surface facing toward the right side of the electric automobile  10  is defined as a right surface  23  (first side surface), and the side surface facing toward the left side of the electric automobile  10  is defined as a left surface  24  (second side surface). 
     The battery case  20  includes four corners. The four corners are a right front corner  25 , which is formed where the front surface  21  and the right surface  23  intersect each other at a right angle, a left front corner  26 , which is formed where the front surface  21  and the left surface  24  intersect each other at a right angle, a right rear corner  27 , which is formed where the rear surface  22  and the right surface  23  intersect each other at a right angle, and a left rear corner  28 , which is formed where the rear surface  22  and the left surface  24  intersect each other. In the present embodiment, the right front corner  25  corresponds to a first corner, and the left rear corner  28  corresponds to a second corner. The right front corner  25  and left rear corner  28  are located at diagonal positions. 
     A front striker  29 , which projects in an orthogonal direction (frontward) from the front surface  21 , is arranged on the front surface  21  at a portion near the right front corner  25  and a portion near the left front corner  26 . A right striker  30 , which projects in an orthogonal direction (rightward) from the right surface  23 , is arranged on the right surface  23 . A left striker  31 , which projects in an orthogonal direction (leftward) from the left surface  24 , is arranged on the left surface  24 . The strikers  29  to  31  form part of a battery fastening mechanism that fastens the battery case  20  to the electric automobile  10 . 
     A front projection  32 , which serves as a case side member of a battery case guiding mechanism, is arranged on the front surface  21  near the right front corner  25  at the left side of the front striker  29 . A right projection  33 , which serves as a case side member of the battery case guiding mechanism, is arranged on a portion of the right surface  23  near the right front corner  25 . The front projection  32  and right projection  33  are projections arranged at portions near the right front corner  25  on the side surfaces  21  and  23  that form the right front corner  25 . Further, a rear projection  34 , which serves as a case side member of the battery case guiding mechanism, is arranged on a portion of the rear surface  22  near the left rear corner  28 . A left projection  35 , which serves as a case side member of the battery case guiding mechanism, is arranged on a portion of the left surface  24  near the left rear corner  28 . The rear projection  34  and left projection  35  are projections arranged at portions near the left rear corner  28  on the side surfaces  22  and  24  that form the left rear corner  28   
     The front projection  32 , right projection  33 , rear projection  34 , and left projection  35  basically have the same structure. Here, only the front projection  32  will be described, and the right projection  33 , rear projection  34 , and left projection  35  will not be described. As shown in  FIG. 3(   a ), the front projection  32  is fixed by a bolt  37  and nut  38  to a support plate  36 , which is fixed to the front surface  21  of the battery case  20  and has an L-shaped cross-section. The front projection  32  is formed from a hard resin, for example, a nylon hard resin. The formation of the front projection  32  from a hard resin improves the sliding capability of the front projection  32  relative to a vehicle body  10 A during replacement of the battery case  20  and suppresses the generation of noise when the front projection  32  slides along the vehicle body  10 A. 
     As shown in  FIGS. 3(   a ) and  3 ( b ), the front projection  32  includes a distal surface  39 . The distal surface  39  includes a lower end surface  39 A and an upper end surface  39 B. The lower end surface  39 A is a curved vertical surface that is curved in the horizontal direction. The upper end surface  39 B is defined by a convex-curved surface. In the present embodiment, the convex-curved surface is formed by a spherical surface. Thus, when cutting the convex-curved surface in the vertical direction or horizontal direction, the cut convex-curved surface is always arcuate. Further, in the front projection  32 , an upper portion  40 , which corresponds to the upper end surface  39 B, has an upper end that projects upward from an upper surface of the battery case  20 . Since the upper end of the upper portion  40  projects upward from the upper surface of the battery case  20 , the battery case  20  is prevented from directly interfering with the vehicle body  10 A when raising and accommodating the battery case  20  in a battery case holder  44 . 
     The battery case holder  44  of the electric automobile  10  will now be described. As shown in  FIG. 1 , the battery case holder  44 , which includes a void that accommodates the battery case  20 , is formed in a lower portion of the vehicle body  10 A of the electric automobile  10 . As shown in  FIG. 4 , the battery case holder  44  is defined by a tetragonal frame  45  arranged in the vehicle body  10 A. The tetragonal frame  45  includes a front frame segment  46 , a rear frame segment  47 , a right frame segment  48 , and a left frame segment  49 . A front latch mechanism  51  is arranged near each of the two ends of the front frame segment  46 . The front latch mechanism  51  includes a latch  51 A, which is engaged with the front striker  29  of the battery case  20 , and a drive unit  51 B, which locks (fixes) the latch  51 A in engagement with the front striker  29  or forcibly disengages the latch  51 A from the front striker  29 . The latch  51 A is arranged on an inner surface  46 A of the front frame segment  46  that faces the battery case holder  44 . The drive unit  51 B is arranged on an outer surface  46 B of the front frame segment  46 . 
     A right latch mechanism  52  is arranged on the right frame segment  48 . The right latch mechanism  52  includes a latch  52 A, which is engaged with the right striker  30  of the battery case  20 , and a drive unit  52 B, which locks (fixes) the latch  52 A in engagement with the right striker  30  or forcibly disengages the latch  52 A from the right striker  30 . The latch  52 A is arranged on an inner surface  48 A of the right frame segment  48  that faces the battery case holder  44 . The drive unit  52 B is arranged on an outer surface  48 B of the right frame segment  48 . A left latch mechanism  53  is arranged on the left frame segment  49 . The left latch mechanism  53  includes a latch  53 A, which is engaged with the left striker  31  of the battery case  20 , and a drive unit  53 B, which locks (fixes) the latch  53 A in engagement with the left striker  31  or forcibly disengages the latch  53 A from the left striker  31 . The latch  53 A is arranged on an inner surface  49 A of the left frame segment  498  that faces the battery case holder  44 . The drive unit  53 B is arranged on an outer surface  49 B of the left frame segment  49 . The latch mechanisms  51  to  53  form the battery fastening mechanism together with the strikers  29  to  31 . 
     A front guide  55 , which serves as a vehicle body side member of the battery case guiding mechanism, is arranged on the inner surface  46 A of the front frame segment  46 . A right guide  56 , which serves as a vehicle body side member of the battery case guiding mechanism, is arranged on the inner surface  48 A of the right frame segment  48 . The front guide  55  and the right guide  56  are members that can respectively contact the front projection  32  and right projection  33 , which are arranged at portions near the right front corner  25  on the side surfaces  21  and  23  forming the right front corner  25  of the battery case  20 . Further, a rear guide  57 , which serves as a vehicle body side member of the battery case guiding mechanism, is arranged on an inner surface  47 A of the rear frame segment  47 . A left guide  58 , which serves as a vehicle body side member of the battery case guiding mechanism, is arranged on an inner surface  49 A of the left frame segment  49 . The rear guide  57  and the left guide  58  are members that can respectively contact the rear projection  34  and left projection  35 , which are arranged at portions near the left rear corner  28  on the side surfaces  22  and  24  forming the left rear corner  28  of the battery case  20 . 
     The front guide  55 , right guide  56 , rear guide  57 , and left guide  58  of the present embodiment basically have the same structure. Here, only the front guide  55  will be described, and the right guide  56 , rear guide  57 , and left guide  58  will not be described. As shown in  FIG. 5 , the front guide  55  is fixed by a bolt  60  to a support plate  59 , which is fixed to the inner surface  46 A and outer surface  46 B of the front frame segment  46  and has a wedge-shaped cross-section. The front guide  55  is formed from a hard resin. This improves the sliding capability of the front projection  32  during replacement of the battery case  20  and suppresses the generation of noise when sliding the front projection  32 . 
     The front guide  55  includes an inner surface  61 , which faces the battery case holder  44 . The inner surface  61  includes an upper inner surface  61 A, and a lower inner surface  61 B. The upper inner surface  61 A is a vertical surface, and the lower inner surface  61 B is an inclined surface inclined outward from the front frame segment  46 . The vertical surface  61 A extends further downward than a bottom surface  46 C of the front frame segment  46 . The inclined surface  61 B, which extends continuously from the vertical surface  61 A, is inclined so as to be located further outward from the front frame body at downward positions. The front guide  55  has a lower end located between the inner surface  46 A and the outer surface  46 B (more specifically, near the outer surface  46 B) below the bottom surface  46 C of the front frame segment  46 . The inclined surface  61 B and vertical surface  61 A of the front guide  55  contact the front projection  32  during replacement of the battery case  20 . 
     In this manner, the battery mounting structure of the present embodiment includes the battery case  20 , the battery case holder  44 , the battery fastening mechanism (strikers  29  to  31  and latch mechanisms  51  to  53 ), and the battery case guiding mechanism (projections  32  to  35  and guides  55  to  58 ). The projections  32  and  34  and the corresponding guides  55  and  57  are elements for positioning the battery case  20  in the front to rear direction. The projections  33  and  35  are elements for positioning the battery case  20  in the left to right direction. 
     The procedures for mounting the battery case  20  onto the electric automobile  10  will now be described. The electric automobile  10  is parked on the parking pit  12  of the battery replacement station  11 . Here, the vehicle body  10 A is positioned relative to the parking pit  12  so that the battery case holder  44  faces the opening  13 . The battery case  20  with battery cells C of which power has mostly been consumed is removed from the battery case holder  44  and the battery case holder  44  becomes empty. A battery case  20  that is to be mounted is conveyed by the conveyor  16  and placed beforehand on the platform  15  of the battery replacer  14 . Then, the lift  17  is actuated to raise the platform  15  and move the battery case  20  toward the battery case holder  44  of the vehicle body  10 A. The battery case  20  is continuously raised to accommodate the battery case  20  in the battery case holder  44 . When the battery case  20  is normally accommodated in the battery case holder  44 , the strikers  29  to  31  are engaged with the corresponding latch mechanisms  51  to  53 . Then, the drive units  51 B to  53 B are driven to lock the latches  51 A to  53 A. This holds the corresponding strikers  29  to  31  in a state inseparable from the latch mechanisms  51  to  53 . 
     The electric automobile  10  may be parked in the parking pit  12  in a state in which its tires are in contact with positioning members (not shown). However, due to the air pressure of a tire or variations in the orientation of the tires, a slight misalignment may occur between the battery case  20  and the battery case holder  44 . When misalignment of the battery case  20  and battery case holder  44  occurs, the battery case  20  and battery case holder  44  may be misaligned in the front to rear direction and the left to right direction. When such misalignment occurs, the contact between the projections  32  to  35  and the corresponding guides  55  to  58  and the guiding of the projections  32  to  35  with the guides  55  to  58  resolve the misalignment of the battery case  20  in the front to rear direction and left to right direction relative to the battery case holder  44  when the battery case  20  is raised. 
     For example, as shown in  FIG. 6 , when the battery case  20  is located slightly frontward from a position optimal for accommodation (optimal position) relative to the battery case holder  44 , as the battery case  20  rises, the upper end surface  39 B of the front projection  32  comes into point contact with the lower inner surface  61 B of the front guide  55 . As the battery case  20  continues to rise, the projection  32  rises while sliding along the lower inner surface  61 B of the front guide  55 . The front projection  32  is guided along the lower inner surface  61 B. This moves the case  20  toward the rear as it rises. In this state, even when the battery case  20  is inclined relative to the horizontal direction, there is no change in the state of contact between the upper end surface  39 B, which is a convex-curved surface, and the lower inner surface  61 B, which is an inclined planar surface. Thus, when the battery case  20  is guided to the battery case holder  44 , the sliding resistance between the front projection  32  and front guide  55  subtly varies. 
     As the battery case  20  continues to rise and the front projection  32  reaches the upper inner surface  61 A of the front guide  55 , the battery case  20  is guided to a position in which it can be accommodated in the battery case holder  44 , as shown by the double-dashed line in  FIG. 4 . In this state, the guides  55  to  58  impose restrictions with the projections  32  to  35  on the battery case  20 . This positions the battery case  20  in the front, rear, left, and right directions relative to the battery case holder  44 . As the battery case  20  further rises, the battery case  20  becomes accommodated in the battery case holder  44 . When the battery case  20  is accommodated in the battery case holder  44 , the latch mechanisms  51  to  53  hold the corresponding strikers  29  to  31 , and the battery case  20  is fastened to the vehicle body  10 A. 
     When the battery case  20  is located slightly rearward from the optimal position, the contact of the rear projection  34  with the rear guide  57  and the guiding of the rear projection  34  with the rear guide  57  resolves the misalignment. When the battery case  20  is located slightly rightward or leftward from the optimal position, the contact of the right projection  33  with the right guide  56  and the guiding of the right projection  33  with the right guide  56  or the contact of the left projection  35  with the left guide  58  and the guiding of the left projection  35  with the left guide  58  resolves the misalignment. 
     The battery case  20  may be misaligned relative to the battery case holder  44  in both of the front to rear direction and the left to right direction. In this case, the misalignment in the front to rear direction and the misalignment in the left to right direction are simultaneously resolved when the projections  32  to  34  contact the corresponding guides  55  to  58  and are guided along the guides  55  to  58 . 
     In addition to misalignment of the battery case  20  and battery case holder  44  in the front to rear direction and left to right direction, misalignment may occur when one of the battery case  20  and battery case holder  44  is inclined relative to the other one. Such misalignment of the battery case  20  occurs, for example, when the vehicle body  10 A is tilted upward or downward relative to the front to rear direction or left to right direction or when the orientation of the vehicle body  10 A in a parked state is deviated from the proper orientation. In the same manner as described above, even if such misalignment occurs, when the battery case  20  rises, the projections  32  to  35  are guided while sliding along the corresponding guides  55  to  58 . This corrects the battery case  20  to the proper position. 
     For instance, in the example shown in  FIG. 7 , the battery case  20  is tilted relative to the battery case holder  44  and thereby misaligned. As a result, the rear projection  34  contacts the rear guide  57 , and the left projection  35  contacts the left guide  58 . In this case, each convex-curved surface is formed by a spherical surface in the rear projection  34  and the left projection  35 . Thus, the convex-curved surface of the rear projection  34  comes into point contact with the inclined surface of the rear guide  57 , and the convex-cured surface of the left projection  35  comes into point contact with the inclined surface of the left guide  58 . 
     In the case of  FIG. 7 , as the battery case  20  further rises, the rear projection  34  is guided while sliding along the inclined surface of the rear guide  57 , and the left projection  35  is guided while sliding along the inclined surface of the left guide  58 . The guiding of the rear guide  57  along the inclined surface of the rear projection  34  and the guiding of the guide  58  along the inclined surface of the left projection  35  corrects the tilting of the battery case  20  relative to the battery case holder  44 . Even if the position of the battery case  20  is corrected, that is, even if the position of the battery case  20  relative to the battery case holder  44  changes, the convex-curved surface of each of the projections  34  and  35  are formed by spherical surfaces. Thus, the state of point contact between the convex-curved surface of each of the projections  34  and  35  and the corresponding tapered surface basically does not change. Thus, even if the position of the battery case  20  changes when guided, the sliding resistances between the projections  34  and  35  and the corresponding guides  57  and  58  are not significantly varied. 
     Even when the battery case  20  and battery case holder  44  are misaligned due to tilting in the horizontal direction, the projections  32  to  35  are guided while sliding along the inclined surfaces of the corresponding guides  55  to  58  to correct the position of the battery case  20 . In this case, the state of contact between the convex-curved surface and inclined surface also does not basically change, and the sliding resistances between the projections  32  to  35  and the corresponding guides  55  and  58  are also subtly varied. 
     The present embodiment has the advantages described below. 
     (1) Even when misalignment occurs between the battery case  20 , which is placed on the platform  15 , and the battery case holder  44 , which is arranged in the vehicle body  10 A, the convex-curved surfaces of the projections  32  to  35  is always in point contact with the inclined surfaces (guide surfaces) of the corresponding guides  55  to  58  when the battery case  20  is being guided to the battery case holder  44 . This suppresses vibration in the sliding resistance between the convex-curved surfaces and the inclined surfaces in comparison with the prior art. Accordingly, even when a misalignment occurs between the battery case  20  and the battery case holder  44 , which is arranged in the vehicle body  10 A, the generation of vibration and noise caused by the misalignment can be suppressed. 
     (2) The front projection  32  and the right projection  33  are located at portions of the battery case  20  near the right front corner  25 . Thus, the front projection  32  and the right projection  33 , which are orthogonal to each other at the right front corner  25 , are proximal to each other. 
     Further, the rear projection  34  and the left projection  35  are located at portions of the battery case  20  near the left rear corner  28 . Thus, the rear projection  34  and the left projection  35 , which are orthogonal to each other at the left rear corner  28 , are proximal to each other. In addition to the right front corner  25  and left rear corner  28  being located at diagonal positions, the projections  32  to  35  are guided by the corresponding guides  55  to  58 . This positions the battery case  20  relative to the battery case holder  44  with high accuracy. 
     (3) The convex-curved surfaces of the projections  32  to  35  and the inclined surfaces of the guides  55  to  58  are in a state of point contact. Thus, variation in the sliding resistance between the convex-curved surface and the inclined surface is decreased in comparison with the prior art regardless of the direction the battery case  20  is tilted in and misaligned relative to the vehicle body  10 A. 
     (4) The four projections  32  to  35  of the battery case  20  and the four guides  55  to  58 , which correspond to the projections  32  to  35 , form the battery case guiding mechanism. This reduces the number of components of the battery case guiding mechanism. The reduction in the number of components of the battery case guiding mechanism reduces the manufacturing cost and weight of the battery mounting structure. 
     (5) The projections  32  to  35  and the corresponding guides  55  to  58  are each formed from a hard resin. Thus, the sliding is satisfactory between the projections  32  to  35  and the guides  55  to  58 , and the battery case  20  can stably be guided to the battery case holder  44 . Further, during sliding of the projections  32  to  35  and the guides  55  to  58 , the generation of vibration and noise can be suppressed. 
     A battery mounting structure according to a second embodiment of the present invention will now be described. The present embodiment differs from the first embodiment in that the guides  55  to  58  are arranged as case side members on the battery case  20  and the projections  32  to  35  are arranged as vehicle side members on the tetragonal frame  45 . In the present embodiment, same reference numerals are given to those components that are the same as the corresponding components of the first embodiment. Such components will not be described. 
     The battery case  20  of the battery mounting structure in the present embodiment includes four side surfaces in the front, rear, left, and right directions and four corners (right front corner, left front corner, right rear corner, and left rear corner). The battery case  20  includes strikers (not shown) having the same structure as the first embodiment. As shown in  FIG. 8 , which is a cutaway view showing a main part of a battery guiding mechanism in the second embodiment, a front guide  71  is arranged on the front surface  21  of the battery case  20 , and a front projection  75  is arranged to project rearward from the inner surface  46 A of the front frame segment  46 . 
     The front guide  71 , which serves as a case side member of the battery case guiding mechanism, is arranged at a portion near the right front corner  25  of the front surface  21  of the battery case  20 . As shown in  FIG. 8 , the front guide  71  is fixed by a bolt (not shown) to a support plate  72 , which is arranged on the front surface  21  of the battery case  20 . The front guide  71  is formed from a hard resin. 
     An outer surface  73 , which is the front surface of the front guide  71 , includes a lower outer surface  73 A and an upper outer surface  73 B. The lower outer surface  73 A is a vertical surface. The upper outer surface  73 B is defined by an inclined surface, which is inclined toward the battery case  20 . The front guide  71  includes an upper end extending upward from an upper surface of the battery case  20 , and the inclined surface extends rearward from the front surface  21  of the battery case  20 . Since the upper end of the front guide  71  projects upward from the upper surface of the battery case  20 , when raising the battery case  20 , direct interference between the battery case  20  and the vehicle body  10 A is suppressed. The inclined surface  73 B and vertical surface  73 A of the front guide  71  contact the front projection  75  of the vehicle body  10 A when replacing the battery case  20 . Although not shown in the drawings, in the battery case  20 , a right guide is arranged on a portion of the right surface  23  near the right front corner  25 , a rear guide is arranged on a portion of the rear surface  22  near the left rear corner  28 , and a left guide is arranged on a portion of the left surface  24  near the left rear corner  28 . These guides, which serve as case side members, have the same structure as the front guide  71 . 
     The bottom surface  46 C of the front frame segment  46  is fixed to a support plate  76 , which is bent to be crank-shaped. The front projection  75  is fixed by a bolt  77  and nut  78  to the support plate  76 . In the support plate  76 , the portion contacting the bottom surface  56 C of the front frame segment  46  and the portion contacting the front projection  75  are separated in the front to rear direction so as not to face each other. The front projection  75  is formed from a hard resin. The front projection  75  includes a distal surface  79 , which is the surface that faces the battery case  20 . The distal surface  79  includes an upper end surface  79 A and a lower end surface  79 B. The upper end surface  79 A is a curved vertical surface, and the lower end surface  79 B is defined by a convex-curved surface. The convex-curved surface is formed by a spherical surface. Thus, when cutting the convex-curved surface in the vertical direction or horizontal direction, the cut convex-curved surface is always arcuate. Although not shown in the drawings, projections respectively corresponding to the right guide, rear guide, and left guide are provided as vehicle side members. These projections have the same structure as the projections  75 . 
     In the present embodiment, even if the battery case  20  is misaligned with the battery case holder  44  in the front, rear, left, or right directions or misaligned with the battery case holder  44  due to tilting, when the guides of the battery case  20  slide along the inclined surfaces of the projections of the vehicle body  10 A and are guided, the state of contact between the convex-curved surfaces and the inclined surfaces basically does not change. Further, the sliding resistance between the projections and the corresponding guides does not significantly vary. Accordingly, the present invention has substantially the same advantages as the first embodiment. 
     A battery mounting structure according to a third embodiment of the present invention will now be described. The third embodiment differs from the first and second embodiments in that a battery case  80  includes a cutout portion  85  and in that the battery case  80  is not tetragonal when viewed from above. In the present invention, same reference numerals are given to those components that are the same as the corresponding components of the first embodiment. Such components will not be described. 
       FIG. 9  is a plan view showing the battery case  80  of the present embodiment. The battery case  80  shown in  FIG. 9  includes the tetragonal cutout portion  85  in the right front corner. In addition to a front surface  81 , a rear surface  82 , a right surface  83 , and a left surface  84 , the battery case  80  includes a cutout right surface  89  and a cutout front surface  90 . The cutout right surface  89  and cutout front surface  90  define the cutout portion  85 . The cutout right surface  89  and right surface  83  are parallel planes, and the cutout front surface  90  and front surface  81  are parallel surfaces. Thus, the cutout front surface  90  and the cutout right surface  89  intersect each other at a right angle. 
     The battery case  80  includes a left front corner  86 , which is formed where the front surface  81  and the left surface  84  intersect each other at a right angle, a right rear corner  87 , which is formed where the rear surface  82  and the right surface  83  intersect each other at a right angle, and a left rear corner  88 , which is formed where the rear surface  82  and the left surface  84  intersect each other. Since the battery case  80  includes the cutout portion  85 , the right front corner of the battery case  80  includes a cutout front corner  91 , which is defined by the front surface  81  and the cutout right surface  89 , a cutout right corner  92 , which is defined by the right surface  83  and the cutout front surface  90 , and an inner corner  93 , which is defined by the cutout right surface  89  and the cutout front surface  90 . In the present embodiment, the cutout front corner  91 , which is formed where the front surface  81  and the cutout right surface  89  intersect each other at a right angle, corresponds to a first corner, and the left rear corner  88 , which is formed where the remaining surfaces of the rear surface  82  and the left surface  84  intersect each other at a right angle, corresponds to a second corner. The cutout front corner  91  and left rear corner  88  are substantially located at diagonal positions of the battery case  80 . The battery case  80  of the present embodiment includes a total of six side surfaces facing the front, rear, left, and right directions. 
     A front striker  29 , which projects in an orthogonal direction (frontward) from the front surface  81 , is arranged at the left front corner  86  of the front surface  81 . Another front striker  29 , which projects in an orthogonal direction (frontward) from the cutout front surface  90 , is arranged on the cutout front surface  90 . Further, a right striker  30  is arranged on a portion of the right surface  83  near the right rear corner  87 . A left striker  31  is arranged on a portion of the left surface near the left rear corner  88 . The strikers  29  to  31  have the same structure as the strikers  29  to  31  of the first embodiment. The strikers  29  form part of a battery fastening mechanism that fastens the battery case  80  to the electric automobile  10 . 
     Although not shown, the latch mechanisms  51  to  53 , which hold the strikers  29  to  31 , are arranged in the vehicle body  10 A. The strikers  29  to  31  and the latch mechanisms  51  to  53  correspond to the battery fastening mechanism. 
     A front projection  32 , which serves as a case side member of a battery case guiding mechanism, is arranged on the front surface  81  near the cutout front corner  91 . A right projection  33 , which serves as a case side member of the battery case guiding mechanism, is arranged on a portion of the cutout right surface  89  near the cutout front corner  91 . The front projection  32  and right projection  33  are projections arranged at portions near the cutout front corner  91  on the side surfaces  81  and  89 . Further, a rear projection  34  is arranged on a portion of the rear surface  82  near the left rear corner  88 , and a left projection  35  is arranged on a portion of the left surface near the left rear corner  88 . The rear projection  34  and left projection  35  are case side members of the battery case guiding mechanism and are projections arranged at portions near the left rear corner  88  on the side surfaces  82  and  84 . 
     Although not shown in the drawings, the guides  55  to  58 , which respectively correspond to the front projection  32 , right projection  33 , rear projection  34 , and left projection  35 , are each arranged in the vehicle body  10 A. The guides  55  to  58 , which correspond to the projections  32  to  35 , have the same structure as the guides  55  to  58  of the first embodiment. In the present embodiment, the battery case  80  is not tetragonal when viewed from above. However, the cutout front corner  91  and the left rear corner  88  are substantially located at the most separated diagonal positions in the battery case  80 . Thus, the third embodiment has substantially the same advantages as the first embodiment. 
     A battery mounting structure according to a fourth embodiment of the present invention will now be described. The present embodiment differs from the first embodiment in that the convex-curved surface of a projection serving as a vehicle body side member is formed by an arcuate surface. In the present invention, same reference numerals are given to those components that are the same as the corresponding components of the first embodiment. Such components will not be described in detail.  FIG. 10(   a ), which is a partial cutaway view showing a main part of a battery guiding mechanism in a fourth embodiment of the present invention, shows a front projection  101  and a front guide  55 , which is arranged on the inner surface of the front frame segment  46 .  FIG. 10(   b ) is a perspective view showing the front projection  101  of the present embodiment. 
     The front projection  101  shown in  FIG. 10(   a ) is fixed by a bolt  37  and nut  38  to a support plate  36 , which is fixed to the front surface  21  of the battery case  20  and has an L-shaped cross-section. The front projection  32  is formed from a resin. As shown in  FIGS. 10(   a ) and  10 ( b ), the front projection  101  includes a distal surface  102 . The distal surface  102  includes a lower end surface  102 A and an upper end surface  102 B. The lower end surface  102 A is a vertical surface, and the upper end surface  102 B is formed by a convex-curved surface. The convex-curved surface is formed by an arcuate surface, and the arcuate surface has a gradient relative to the horizontal direction that becomes gradual from the lower side toward the upper side. 
     In the present embodiment, for example, the battery case  20  may be tilted in the front to rear direction relative to the horizontal direction. As a result, the battery case  20  is misaligned from the battery case holder  44  (as shown by the double-dashed line in  FIG. 10(   a )). Even if such misalignment occurs, when the battery case  20  is being raised, the convex-curved surface  102 B of the front projection  101  is in line contact with the inclined surface  61 B of the lower inner surface  61 B of the front guide  55 . When the front projection  101  slides along the inclined surface of the front guide  55  and is guided, the state of line contact between the convex-curved surface and the inclined surface basically does not change, and the sliding resistance between the front projection  101  and the front guide  55  is subtly varied. Accordingly, in the range in which the contact state of the convex-curved surface and the inclined surface does not change, the fourth embodiment has substantially the same advantages as the first embodiment. 
     The present invention is not limited to the above embodiment and various changes may be made with the scope of the invention. 
     In the first to fourth embodiments, the material forming the projections, which serve as the case members, and the guides, which serve as the vehicle body members, are each formed from a hard resin. However, the material forming the case side members and vehicle body side members is not limited to resin and may be any material satisfying the required sliding capability, wear resistance, and rust resistance. For example, the material forming the projections and guides may be a predetermined metal material having a surface coated with a fluorocarbon resin (e.g., polytetrafluoroethylene). Alternatively, the material forming the projections and guides may be a predetermined metal material having a plated surface. 
     In the first to fourth embodiments, each battery fastening mechanism includes strikers and latch mechanisms. However, the mechanism or means for fastening a battery to a vehicle body is not limited to strikers and latch mechanisms and only need to at least fasten and unfasten the battery to and from the vehicle body. 
     In the first to fourth embodiment, the four sides of the battery case extends parallel to the front, rear, left, and right sides of the electric automobile but are not necessarily limited in such a manner. That is, the side surfaces of the battery case may be inclined relative to the front, rear, left, and right sides of the electric automobile. 
     In the first to fourth embodiments, the distal surface of a projection includes a lower portion (or upper portion) having a vertical surface and an upper portion (or lower portion) having a convex-curved surface. However, the distal surface of the projection may include only a convex-curved surface. 
     In the first to fourth embodiments, the distal surface of a projection is a combination of a convex-curved surface and a curved vertical surface (or vertical surface). However, the distal surface may be formed by only a convex-curved surface. 
     In the first to fourth embodiments, the battery case includes four side surfaces in the front, rear, left, and right directions. However, the battery case only needs to include at least four side surfaces in the front, rear, left, and right direction. For example, in addition to the four side surfaces, the battery case may include a side surface inclined relative to the front to rear direction and the left to right direction or a side surface having an irregular shape. 
     DESCRIPTION OF THE REFERENCE NUMERALS 
     
         
         
           
               10 : electric automobile,  10 A: vehicle body,  11 : battery replacement station,  13 : opening,  14 : battery replacer,  20  and  80 : battery case,  21  and  81 : front surface,  22  and  82 : rear surface,  23  and  83 : right surface,  24  and  84 , left surface,  25 : right front corner serving as first corner,  26  and  86 : left front corner,  27  and  87 : right rear corner,  28  and  88 : left rear corner serving as second corner,  32  and  101 : front projection serving as case side member,  33 : right projection serving as case side member,  34 : rear projection serving as case side member,  35 : left projection serving as case side member,  39 : distal surface,  39 A: lower end surface,  39 B: upper end surface (convex-curved surface),  40 : upper portion,  44 : battery case holder,  55 : front guide serving as vehicle body side member,  56 : right guide serving as vehicle side member,  57 : rear guide serving as vehicle body member,  58 : left guide serving as vehicle body side member,  61 : inner surface,  61 A: upper inner surface,  61 B: lower inner surface (tapered surface),  71 : front guide serving as case side member,  75 : projection serving as vehicle body side member,  85 : cutout portion,  89 : cutout right surface,  90 : cutout front surface,  91 : cutout front corner,  92 : cutout right corner,  93 : inner corner.