Patent Publication Number: US-11396225-B2

Title: Electric work vehicle, battery pack for electric work vehicle and contactless charging system

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. application Ser. No. 16/017,082 filed Jun. 25, 2018, which is a divisional of U.S. application Ser. No. 15/352,059 filed Nov. 15, 2016, now issued as U.S. Pat. No. 10,029,551, which claims priority to Japanese Patent Application Nos. 2015-224090 and 2015-224091, both filed Nov. 16, 2015, and Japanese Patent Application No. 2016-065374 filed Mar. 29, 2016. The disclosure of each of these documents is hereby incorporated in its entirety by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates to an electric work vehicle such as an electric mower, a battery pack for an electric work vehicle, and a contactless charging system. 
     Description of Related Art 
     First Related Art 
     In the field of passenger vehicles, electric vehicles that travel using the rotational force of an electric motor have started to become widespread. In such a case, a battery is mounted in the vehicle as the power source of the electric motor. The battery temperature needs to be kept at a suitable temperature in order to maintain the performance of the battery, and therefore a cooling structure is often included in the battery. For example, with a vehicle battery pack disclosed in JP 2014-075181 A, a battery cell group, which is multiple battery cells, and a cooling fan that allows air to flow to the battery cell group are accommodated in a case. In the case, an inlet through which air is guided from the outside into the case, and an outlet through which air is discharged to the outside of the case are formed. Air flow that occurs due to the cooling fan being driven so as to rotate passes through an air suction flow path that extends in the gaps between the battery cells and in a bottom portion space between the inner side bottom surface of the case and the bottom surface of the battery cell group. In other words, the cooling air guided from the outside of the vehicle to the inside of the case through the inlet is discharged from the outlet to the outside of the case via the air suction flow path, the bottom portion space, and the gaps between the battery cells. Furthermore, multiple heat radiating fins that protrude downward are provided on the lower surface of the case. 
     However, in a work vehicle that travels while performing work, as with a mower, a rice transplanter, a tractor, or the like, the surrounding environment for work travel is worse compared to the surrounding environment of an automobile or the like, and if cooling air is taken in from the surroundings, waste such as cut grass or straw tends to be mixed in with the cooling air, which causes the problem of clogging of the cooling air passage. 
     In view of the foregoing circumstances, there has been demand for a battery pack having a structure that is suitable for an electric work vehicle that performs work travel in a surrounding environment in which foreign matter such as grass or straw is floating. In such a case, if multiple battery modules are included in such a battery pack, it is also important to make the temperatures of the battery modules as uniform as possible. 
     Second Related Art 
     WO 2013/015171 A1 discloses a lawnmower in which a left and right pair of rear wheels are driven independently by a left and right pair of motors. The left and right pair of rear wheels are supported by a rear axle case extending therebetween. A gear case extends from a central portion of the rear axle case, perpendicular to the direction in which the rear axle case extends, a left-side motor is furthermore equipped on the left side so as to extend in the vehicle lateral direction from the leading end of the gear case, and a right-side motor is furthermore equipped on the right side so as to extend in the vehicle lateral direction from the leading end of the gear case. The rotational power of the motors is transmitted to an axle mounted in the rear axle case via a power transmission mechanism mounted in the gear case. As is apparent from FIG. 2 of WO 2013/015171 A1, a left and right pair of motors that extend linearly in the vehicle body lateral direction and a rear axle case that extends linearly in the vehicle body lateral direction are joined at respective central portions by a gear case that extends in the vehicle body front-rear direction. Accordingly, the case structure in which a mechanism for transmitting the power from the motors to the rear wheels is mounted has a rather complicated shape, and causes an increase in cost. Furthermore, since the left and right pair of motors are arranged inside of the vehicle body frame and the battery is arranged between the left and right pair of motors, there is a problem in that a large-sized battery cannot be used. 
     JP 2008-168869 A (or US 2009/0000839 A1 corresponding thereto) discloses a hybrid lawnmower in which a left motor that drives a left rear wheel and a right motor that drives a right rear wheel are each attached to the outside of a frame, and an engine and a battery are arranged between the left motor and the right motor. The left motor and the right motor are wheel motors, and the rear wheel axle centers match the motor rotational axis center. With this structure, the engine and the battery pack, which have large weights, are located rearward of the rear wheel axle center, as a result of which the weight balance regarding the rear wheel axle center deteriorates. However, since the center of gravity of the left motor and the right motor is substantially on the rear wheel axle center, the left motor and the right motor cannot improve the deterioration of the weight balance. 
     In view of the foregoing circumstances, there has been demand for improvement of vehicle body balance and for ensurement of sufficient battery space in an electric work vehicle including a left motor that drives a left rear wheel and a right motor that drives a right rear wheel. 
     Third Related Art 
     With a forklift disclosed in JP 2014-082339 A, a primary-side contactless power supply pad (primary-side coil) laid on the ground surface and a secondary-side contactless power supply pad (secondary-side coil) that receives power through electromagnetic coupling are provided between a front wheel and a rear wheel on one side of the lower surface of the forklift. In the case where a forklift mechanism serving as a work apparatus attached to a traveling vehicle body is arranged forward of the front wheels, as with a forklift, there is relative spatial leeway below the vehicle body between the front wheels and the rear wheels, and therefore the secondary-side coil can be arranged between the front wheels and the rear wheels. The primary-side coil is provided on the ground surface or on a support platform arranged on the ground surface. 
     With an electric mower, a mower unit serving as the work apparatus attached to the traveling vehicle body is arranged forward of the rear wheels, and therefore there is little space that can be used freely below the vehicle body frame in the region forward of the rear wheels, or in other words, in the region near the ground surface. Also, because the weight of the battery pack is large, for the stability of the vehicle body, it is preferable to arrange the battery pack at a low position on the vehicle body. Because of this, in a contactless charging system used in an electric work vehicle such as an electric mower, suitable arrangement of the battery pack, the primary-side coil, and the secondary-side coil is important. 
     SUMMARY OF THE INVENTION 
     [1] In order to solve the problem stated in the “First Related Art”, an electric work vehicle battery pack includes: a sealed battery case including a front case portion and a rear case portion; a horizontal partitioning wall that divides an interior of the front case portion into a first space and a second space in a vertical direction, and divides an interior of the rear case portion into a third space and a fourth space in the vertical direction; a battery electric unit accommodated in one of the first space, the second space, the third space and the fourth space; and a battery module accommodated in each of the remaining spaces among the first space, the second space, the third space and the fourth space. 
     Note that “sealed” above does not mean that the interior space of the battery case is kept in a completely airtight state, but is used as a term with a broader meaning that encompasses a loosely airtight state in which the flow of outside air to the interior space is suppressed and the outside air temperature and the temperature of the interior space are not easily equalized. 
     Also, the phrases “first space”, “second space”, “third space”, and “fourth space” do not limit the number of divided spaces to four, and the battery case may be further divided into a fifth space, a sixth space, and the like. 
     In this configuration, the interior of the sealed battery case is divided into multiple spaces by a horizontal partitioning wall, a battery electric unit is arranged in one space, and battery modules are arranged in the remaining spaces. Thus, waste such as cut grass or straw substantially does not enter the interior of the battery case. Also, because the interior of the battery case is divided vertically by the horizontal partitioning wall, the heat emitted from the multiple battery modules is prevented from being focused on the roof region of the battery case. 
     As a particularly preferable embodiment, a circulation fan configured to create a circulated air flow that circulates through the first space, the second space, the third space and the fourth space is included. In this configuration, multiple battery modules and a circulation fan are accommodated in a sealed manner in one battery case, and by circulating the air in the case internal space using the circulation fan and suppressing the entrance of outside air by sealing the battery case, the temperature of the case internal space is uniformized while foreign matter from the outside is prevented from entering. In this case, when the electric unit and the battery modules are arranged such that the cooling air created by the circulation fan passes through the electric unit and the battery modules in sequence, the temperature distribution of the battery modules can be made as uniform as possible. Accordingly, the temperature distribution of the battery module is easier to make uniform, and the battery modules electrically operate efficiently. 
     Note that the scope of the present invention extends also to an electric work vehicle in which the above-described battery pack is mounted. 
     [2] In order to solve the problem stated in the “Second Related Art”, an electric work vehicle that includes a left motor and a right motor that receive a supply of power from a battery pack, and that performs travel work by using the left motor to drive a left rear wheel and using the right motor to drive a right rear wheel includes: a left frame and a right frame that extend in a vehicle body front-rear direction with an interval therebetween in a vehicle body lateral direction; a rear wheel unit having a left rear wheel arranged outside of the left frame in the vehicle body lateral direction and a right rear wheel arranged outside of the right frame in the vehicle body lateral direction; a battery pack arranged between the left rear wheel and the right rear wheel, a front end of the battery pack being located forward of an axle center of the rear wheel unit in the vehicle body front-rear direction; a left motor that is arranged above the battery pack in the periphery of the left rear wheel and is configured to receive a supply of power from the battery pack and transmit rotational power to the left rear wheel; and a right motor that is arranged above the battery pack in the periphery of the right rear wheel and is configured to receive a supply of power from the battery pack and transmit rotational power to the right rear wheel. 
     With this configuration, due to the fact that the front end portion of the battery pack is located further forward relative to the rear wheel axle, the center of gravity of the battery pack does not deviate significantly from the rear wheel axle in a side view even if the battery pack is lengthened and the rear end portion thereof is located rearward. Therefore, adverse effects that the weight of the battery pack has on the vehicle body balance of the vehicle body are reduced. Also, by arranging the left motor and the right motor above the battery pack, the space above the battery pack is used effectively, the overall length of the vehicle body is suppressed, and an increase in compactness is possible. 
     In this case, if the battery pack is arranged such that the center of gravity of the battery pack is within a rear wheel segment defined by the left rear wheel and the right rear wheel in the vehicle body front-rear direction and the vehicle body lateral direction in plan view, adverse effects that the battery pack has on the stability of the vehicle body can mostly be ignored. The rear wheel segment in this context is a rectangle that is centered about the rear axle center, which is the axle center of the rear wheels, and is obtained by using the rear wheel radius as the length of one side in the vehicle body front-rear direction and by using the interval between the left rear wheel and the right rear wheel as the length of the other side. More preferably, the center of gravity of the battery pack is located on a center line in the vehicle body front-rear direction and is within the length of the radius of the rear wheel from the rear axle center in the vehicle body front-rear direction. Accordingly, the weight of the battery pack contributes to the stability of the vehicle body. 
     From the viewpoint of the stability of the vehicle body, it is desirable that the battery pack, which is a heavy load, is arranged at a low position. However, if the above-ground height of the battery pack is low, an inconvenience occurs in which the rear end portion of the battery pack comes into contact with the ground surface during off-road travel or uphill travel. In order to eliminate this inconvenience and ensure that the center of gravity of the battery pack is as low as possible, it is desirable to lower the above-ground height of the front portion of the battery pack and raise the above-ground height of the rear portion of the battery pack. For this reason, in a preferred embodiment of the present invention, the battery pack includes a front-side rectangular cuboid portion and a rear-side rectangular cuboid portion shifted upward relative to the front-side rectangular cuboid portion, and is formed as a level-difference three-dimensional shape having level differences on an upper surface and a lower surface of the battery pack between the front-side rectangular cuboid portion and the rear-side rectangular cuboid portion. In particular, in order to suppress contact with the ground surface during off-road travel or uphill travel, it is preferable that the rear end of the battery pack is located rearward of the rear wheel unit in the vehicle body front-rear direction, and the above-ground height of the rear end of the battery pack is configured to be higher than the above-ground height of the rear axle case. 
     [3] In order to solve the problem stated in the “Third Related Art”, a contactless charging system for an electric work vehicle includes: a primary coil unit that includes a coil power supply circuit portion and a primary coil arranged above the coil power supply circuit portion, and is arranged on a ground surface; a battery pack arranged at a rear portion of a vehicle body frame, between a left and right pair of rear wheels; a secondary coil that electromagnetically couples with the primary coil; a charging circuit portion configured to rectify power from the secondary coil and supply the rectified power to the battery pack; and a coil support member for arranging the secondary coil below the battery pack. 
     With this configuration, a primary coil unit having a coil power supply circuit portion and a primary coil is arranged on a ground surface side, and a secondary coil unit that electromagnetically couples with the primary coil is attached on the vehicle body side of the electric work vehicle. With the coil support member, the secondary coil is arranged below the battery pack, which is arranged between the left and right pair of rear wheels in the rear portion of the vehicle body frame. In order to avoid contact with an obstacle that exists on the ground surface during travel, the secondary coil is arranged at a position that is as high from the ground as possible, and therefore the position of the primary coil needs to be made higher. With this configuration, the primary coil unit has a two-stage structure, the coil power supply circuit portion being arranged on the lower stage, and the primary coil unit being arranged on the upper stage. Therefore, the primary coil unit is at a high position from the ground surface, which is structurally convenient. 
     The charging circuit portion includes a rectifier through which a large current flows, and has a relatively large shape. Therefore, the charging circuit portion requires a wide and stable installation location. For this reason, in a preferred embodiment according to the present invention, the charging circuit portion is attached to an upper portion of the battery pack. Due to the fact that the outer shape of the battery pack is relatively simple, as with a rectangular cuboid or a combination of rectangular cuboids, the upper portion of the battery pack is relatively wide and flat, and therefore the charging circuit portion is stably attached to the upper portion. 
     Furthermore, in one preferred embodiment of the present invention, a recessed portion is formed at a rear-side lower portion of the battery pack and the secondary coil is arranged in the recessed portion. Accordingly, the surroundings of the secondary coil are at least partially protected by the battery pack, which has a high rigidity, and therefore damage or the like caused by contact with an outside object is suppressed. 
     If the air pressure of the rear tires fluctuates or if the weight of additional freight fluctuates, the above-ground height of the secondary coil attached to the electric work vehicle side will change, and the interval between the primary coil and the secondary coil will deviate from an optimal value. This adversely affects the charging efficiency, and therefore the interval between the primary coil and the secondary coil needs to be brought near the optimal value. For this purpose, in one preferred embodiment of the present invention, the primary coil unit is provided with an elevation mechanism configured to raise and lower the primary coil. With the elevation mechanism, the above-ground height of the primary coil can be adjusted, and the interval between the primary coil and the secondary coil can be set to the optimal value. 
     In a preferred embodiment of an electric mower in which a contactless charging system is incorporated, a mower unit hangs down elevatably at a front portion of the vehicle body frame, and an electric motor unit configured to drive the rear wheels using power supplied from the battery pack via a motor power supply circuit portion is arranged forward of an axle center of the rear wheels, between the rear wheels. With this configuration, with the mower unit and the electric motor unit, it is possible to solve the problem in which the center of gravity of the vehicle body is located rearward in the vehicle body and the vehicle body balance deteriorates due to the battery pack being arranged on the rear portion of the vehicle body frame. 
     Furthermore, in one preferred embodiment of the electric mower, the electric motor unit includes a left motor that drives one rear wheel via a left transmission, and a right motor that drives the other rear wheel via a right transmission, the left motor and the left transmission being arranged between the one rear wheel and the battery pack, and the right motor and the right transmission being arranged between the other rear wheel and the battery pack. With this configuration, stability of the vehicle body and compactness of the travel power transmission system are achieved by consolidating heavy loads such as the electric motor unit and the transmission, which constitute the travel power transmission system, near the axle center of the rear wheel. 
     The mower unit is near the ground surface during mowing work travel, and is pulled up to its highest position away from the ground during non-work travel. The mower unit is located forward in the vehicle body relative to the secondary coil, and therefore fulfills the role of a guard for the secondary coil during travel. For this reason, it is preferable that the lower surface of the mower unit at the highest position is set to be lower than the lower surface of the secondary coil. 
     Other features and advantages will become apparent upon reading the embodiments described below with reference to the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram showing a first embodiment (same through to  FIG. 9 ), and is a partially cut-away perspective view showing a basic structure of a battery pack. 
         FIG. 2  is a side view schematically showing a relationship between the battery pack mounted in an electric work vehicle and rear wheels. 
         FIG. 3  is a plan view schematically showing a relationship between the battery pack mounted in the electric work vehicle and the rear wheels. 
         FIG. 4  is a side view of a riding electric mower, which is an example of an electric work vehicle. 
         FIG. 5  is a plan view of the riding electric mower. 
         FIG. 6  is a schematic view showing a vehicle body frame, the battery pack, and a driving mechanism for a rear wheel unit. 
         FIG. 7  is a schematic view of the battery pack. 
         FIG. 8  is a view in vertical section of the battery pack. 
         FIG. 9  is an exploded view of a battery case. 
         FIG. 10  is a diagram showing a second embodiment (same through to  FIG. 16 ), and is a side view schematically showing a basic configuration of an electric work vehicle in which a contactless charging system is incorporated. 
         FIG. 11  is a plan view schematically showing a basic configuration of the electric work vehicle in which the contactless charging system is incorporated. 
         FIG. 12  is a functional block diagram showing a basic configuration of an electric circuit of the contactless charging system. 
         FIG. 13  is a side view of an electric mower. 
         FIG. 14  is a plan view of the electric mower. 
         FIG. 15  is a perspective view showing a support structure for the battery pack, an electric motor unit, and a transmission in the vehicle body frame. 
         FIG. 16  is a view in vertical section showing a region of the battery pack and the contactless charging system. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     In the description hereinafter, unless explicitly described otherwise, a “vehicle body front-rear direction” is a direction of a vehicle body central axis (also referred to as “vehicle body longitudinal axis”) that extends in a horizontal direction along a travel direction of the vehicle body on which a battery pack is mounted. A “vehicle body lateral direction” (also referred to simply as “lateral direction”) is a direction that extends in the horizontal direction, orthogonal to the vehicle body central axis. “Front (forward)” means on the forward side in the vehicle body front-rear direction, and “rear (rearward)” means on the reverse side in the vehicle body front-rear direction. “Left (leftward)” means left when facing the vehicle body forward direction, and “right (rightward)” means right when facing the vehicle body forward direction. 
     First Embodiment 
     Prior to describing specific embodiments of the battery pack relating to the present invention, a basic structure of the battery pack mounted in the electric work vehicle will be described with reference to  FIG. 1 , and a basic arrangement of the battery pack at the time of mounting such a battery pack in the electric work vehicle will be described with reference to  FIGS. 2 and 3 . 
     A battery pack  6  as shown in  FIG. 1  includes a battery case  60 , a horizontal partitioning wall  63 , a battery electric unit  68  (a group of electric devices/accessories related to a battery; the collective nomination will be simply referred to as “electric unit  68 ” also) and multiple battery modules  6 A. The battery case  60  is a sealed case including a front case portion  61  and a rear case portion  62 . Note that in  FIG. 1 , the left side is defined as the front side (forward), and the right side is defined as the rear side (rearward). The front case portion  61  and the rear case portion  62  are not individual members, but are respective nominations of a front-side portion and a rear-side portion when considering the battery case  60  as being divided into a front-side portion and a rear-side portion. The battery case  60  is formed continuously by the front case portion  61  and the rear case portion  62 . In the example as shown in  FIG. 1 , a region of transitioning from the front case portion  61  to the rear case portion  62  is inclined upward. Accordingly, the rear case portion  62  is shifted to the upper side relative to the front case portion  61  in the vertical direction, and the battery case  60  is a three-dimensional object having an upward level difference in the middle. This level difference is required due to restrictions on site such as an installation space or the like. Therefore, the battery case  60  may have a downward level difference, or there may be no level difference. 
     The horizontal partitioning wall  63  is a plate member that is provided so as to approximately divide the interior of the battery case  60  into two parts, namely an upper and a lower part. The interior of the front case portion  61  is divided in the up-down direction into a first space S 1  and a second space S 2  by the horizontal partitioning wall  63 , and the interior of the rear case portion is divided in the up-down direction into a third space S 3  and a fourth space S 4  by the horizontal partitioning wall  63 . The first space S 1  and the third space S 3  are in communication and form an upper portion space of the battery case. Similarly, the second space S 2  and the fourth space S 4  are in communication and form a lower portion space of the battery case. A front end gap G 1  that allows air flow between the first space S 1  and the second space S 2  is formed between a front end  63   a  of the horizontal partitioning wall  63  and a wall surface in front of the horizontal partitioning wall  63 . A rear end gap G 2  ( FIG. 8 ) that allows air flow between the third space S 3  and the fourth place S 4  is formed between a rear end  63   b  of the horizontal partitioning wall  63  and a wall surface behind the horizontal partitioning wall  63 . Accordingly, an air circulation path is formed within the battery case  60  that extends from the first space S 1 , passes through the second space S 2 , the fourth space S 4  and the third space S 3 , and returns to the first space S 1 . 
     In the example as shown in  FIG. 1 , the electric unit  68  is arranged in the first space S 1 , and one battery module  6 A is arranged in each one of the second space S 2 , the third space S 3  and the fourth space S 4 . The battery module  6 A is formed by a number of battery cells  6   a . Furthermore, a circulation fan  69  is arranged in the first space S 1  so as to send air from the first space S 1  to the third space S 3 . Accordingly, a flow path for circulated air is created, on which air passes through the first space S 1 , the third space S 3 , the fourth space S 4 , the second space S 2  and back to the first space S 1  in the stated order. The flow of the circulated air not only cools the electric unit  68 , but also uniformizes the temperatures of the second space S 2 , the third space S 3  and the fourth space S 4 , in which the battery modules  6 A are arranged. Accordingly, malfunction of the battery module  6 A caused by non-uniformity in the temperature distribution is prevented. 
     The battery case  60  is configured to be at least partially removable or has an opening formed therein to be closed by a lid, for the purpose of maintenance inspection of the electric unit  68  or the battery modules  6 A. 
       FIGS. 2 and 3  are a side view and a plan view, respectively, for illustrating a basic example of mounting the battery pack  6  as shown in  FIG. 1  in the electric work vehicle. The electric work vehicle includes a vehicle body frame  20  that includes a left frame  21  and a right frame  22  that extend in the vehicle body front-rear direction with an interval therebetween in the vehicle body lateral direction, and at least one crossbeam  23  that connects the left frame  21  and the right frame  22 . A left front wheel  11 L and a right front wheel  11 R that constitute a front wheel unit are arranged at a front portion of the vehicle body frame  20 . A left rear wheel  12 L and a right rear wheel  12 R that constitute a rear wheel unit are arranged on a rear side relative to the center of the vehicle body frame  20 . Hereinafter, if there is no particular need to make a distinction, the left front wheel  11 L and the right front wheel  11 R will be referred to collectively as “front wheel unit  11 ”, and the left rear wheel  12 L and the right rear wheel  12 R will be referred to collectively as “rear wheel unit  12 ”. A rear axle center Pr extends in a vehicle body lateral direction. 
     The battery pack  6  is arranged in a rear half region of the vehicle body frame  20  between the left frame  21  and the right frame  22 . In a lateral side view, the front end of the battery pack  6  is located forward of the rear axle center Pr and approximately near the front end of the rear wheel unit  12 . The rear end of the battery pack  6  is located approximately at the rear end of the vehicle body frame  20 . As is apparent from  FIG. 3 , the battery pack  6  has a width sufficient to snugly fit the battery pack  6  between the left frame  21  and the right frame  22 , and extends with an equal width in the vehicle body front-rear direction. Also, as is apparent from  FIG. 2 , the front half portion of the battery pack  6  is in the axle space of the rear wheel unit  12 . As a more preferable mode of arranging the battery pack  6 , the center of gravity of the battery pack  6  is in the region between the front end and the rear end of the rear wheel unit  12  on a center line in the vehicle body front-rear direction. In other words, the center of gravity of the battery pack  6  is within the distance of the rear wheel unit  12  from the rear axle center Pr in the vehicle body front-rear direction. Accordingly, the weight of the battery pack  6  contributes to the stability (favorable balance) of the vehicle body. 
     Furthermore, the battery pack  6  has a shape in which the rear case portion  62  is shifted to the upper side relative to the front case portion  61  in the up-down direction. In other words, the battery pack  6  has a three-dimensional shape constituted by a front-side rectangular cuboid portion  6 F and a rear-side rectangular cuboid portion  6 B, and the rear-side rectangular cuboid portion  6 B protrudes upward relative to the front-side cuboid portion  6 F. In other words, an upward level difference is formed on the upper surface and the lower surface of the battery pack  6 . 
     Accordingly, an above-ground height Hb of the rear case portion  62  of the battery pack  6  is higher than an above-ground height Hf of the front case portion  61  of the battery pack  6 . In the example as shown in  FIG. 2 , the above-ground height Hf of the front case portion  61  is approximately the same as the above-ground height of the lower end of the rear axle case  31  of the rear wheel unit  12 . The above-ground height Hb of the rear case portion  62  of the battery pack  6  is higher than the above-ground height of the rear axle case  31  and is approximately the same as the above-ground height of the rear axle center Pr. The lowest above-ground height of the vehicle body frame  20  at the portion supporting the battery pack  6  approximately coincides with the lowest above-ground height of the battery pack  6 . Due to the above-ground height Hf of the front case portion  61  being set to be lower, the center of gravity of the battery pack  6  is lower, and travel stability is more preferable. Also, due to the above-ground height Hb of the rear case portion  62  being set to be higher, the rear end of the battery pack  6  or the vehicle body frame  20  is less likely to collide with the ground surface or stones during uphill travel or off-road travel. 
     The left motor  4 L that transmits the rotational power to the left rear wheel  12 L is arranged near the front case portion  61  of the battery pack in the periphery of the left rear wheel  12 L. The right motor  4 R that transmits the rotational power to the right rear wheel  12 R is arranged near the rear case portion  62  in the periphery of the right rear wheel  12 R. The left motor  4 L and the right motor  4 R are arranged at left-right symmetrical positions. In order to create spaces for arranging the left motor  4 L and the right motor  4 R, the portion covering the first space S 1  of the front case portion  61  may be recessed. A driver seat  16  is arranged above the left motor  4 L and the right motor  4 R, and thus space is used effectively. 
     For further effective use of space, in the battery pack  6  illustrated in  FIG. 1 , the front-side rectangular cuboid portion  6 F of the battery pack  6  has a protruding central portion  6 Fc that extends in the vehicle body front-rear direction, with an upper right portion and an upper left portion of the front-side rectangular cuboid portion  6 F being removed. Due to this, spaces are created on the left and right of the central portion  6 Fc. The left motor  4 L enters the space on the left side of the central portion  6 Fc of the front-side rectangular cuboid portion  6 F, and the right motor  4 R enters the space on the right side of the central portion  6 Fc. 
     Next, a specific example of an electric work vehicle on which a battery pack is mounted will be described.  FIG. 4  is a side view of a riding electric mower, which is an example of an electric work vehicle, and  FIG. 5  is a plan view of the riding electric mower. 
     As shown in  FIGS. 4 and 5 , the riding electric mower (hereinafter referred to as simply “mower”) includes a vehicle body  10 . The vehicle body  10  is supported on the ground by a caster-type front wheel unit  11  having a left front wheel  11 L and a right front wheel  11 R, and by a rear wheel unit  12  including a left rear wheel  12 L and a right rear wheel  12 R which are rotatably driven. The vehicle body  10  has a vehicle body frame  20  as a base frame. The vehicle body frame  20  includes a left frame  21 , a right frame  22 , and a crossbeam  23  that joins the left frame  21  and the right frame  22 . A mower unit  13  hangs down from the vehicle body frame  20  via a link mechanism  14  between the front wheel unit  11  and the rear wheel unit  12 . The mower unit  13  includes a blade transmission mechanism  131  and a blade  132  rotated by the blade transmission mechanism  131 . A driver seat  16  is arranged in a central region in the vehicle body front-rear direction of the vehicle body  10 . The basic structure of the battery pack  6  and the basic arrangement of the vehicle body frame  20  employ those described with reference to  FIGS. 1 and 2 . 
       FIG. 6  is a perspective view showing the vehicle body frame  20 , the battery pack  6  and the drive mechanism for the rear wheel unit  12 . The drive mechanism that supplies the rotational power to the rear wheel unit  12  includes a motor  4 , a transmission case  30  and the rear axle case  31 . As shown in  FIG. 6 , the left frame  21  and the right frame  22  each branch vertically from an intermediate region and re-join in a rear region. In other words, in the rear half region, the left frame  21  includes an upper frame portion  21   a  and a lower frame portion  21   b . Similarly, in the rear half region, the right frame  22  includes an upper frame portion  22   a  and a lower frame portion  22   b . In a side view, a left transmission case  30 L is arranged in the region between the upper frame portion  21   a  and the lower frame portion  21   b  of the left frame  21 , and a right transmission case  30 R is arranged in the region between the upper frame portion  22   a  and the lower frame portion  22   b  of the right frame  22 . A left rear axle case  31 L is connected to the left transmission case  30 L, and the left rear wheel  12 L is supported by the left rear axle case  31 L. A right rear axle case  31 R is connected to the right transmission case  30 R, and the right rear wheel  12 R is supported by the right rear axle case  31 R. The left rear axle case  31 L is a tubular member in which the left rear axle  41  of the left rear wheel  12 L is mounted, and which performs bearing support, and the outer shape thereof is a truncated cone shape. Similarly, the right rear axle case  31 R is a tubular member in which the right rear axle  42  of the right rear wheel  12 R is mounted, and which performs bearing support, and the outer shape thereof is a truncated cone shape. The battery pack  6  is arranged such that, in plan view, the center of gravity of the battery pack  6  is approximately located on a center line in the vehicle body front-rear direction and is within the length of the rear wheel radius on the front side and rear side from the rear axle center Pr. 
     The left transmission case  30 L is a hollow member that extends forward in the vehicle body front-rear direction from the left rear axle case  31 L orthogonally to the rear axle center Pr which acts also as the central axis center of the left rear axle case  31 L, and the left transmission having a gear transmission mechanism is housed within the left transmission case  30 L. A surface for mounting the left motor  4 L is formed on a power input portion  32   f  of the left transmission case  30 L. Similarly, the right transmission case  30 R is a hollow member that extends forward in the vehicle body front-rear direction from the right rear axle case  31 R orthogonally to the rear axle center Pr which acts also as the central axis center of the right rear axle case  31 R, and the right transmission having a gear transmission mechanism is housed within the right transmission case  30 R. The gear transmission mechanism generally reduces the input power, but may increase the input power. Still alternatively, there may be provided a gear transmission mechanism that does not increase and reduce the input power. A surface for mounting the right motor  4 R is formed on the power input portion  32   f  of the right transmission. In this embodiment, the left transmission case  30 L and the left rear axle case  31 L are formed integral, and the right transmission case  30 R and the right rear axle case  31 R are formed integral. Note that instead of the gear transmission mechanism, a chain transmission mechanism, a transmission shaft mechanism or the like may be used as the left transmission and the right transmission. 
     Speed changing operations on the left motor  4 L and the right motor  4 R are performed using a left and right pair of speed changing levers  18  (see  FIGS. 4 and 5 ), which are arranged on both sides of the driver seat  16 . When a speed changing lever  18  is held at a front-rear neutral position, the corresponding motor  4  enters a stopped state; and by operating the speed changing lever  18  forward from the neutral position, the corresponding motor  4  performs driving so as to rotate normally and realizes a forward speed change, and by operating the speed changing lever  18  rearward, the corresponding motor  4  performs driving so as to rotate in reverse, thereby realizing a reverse speed change. By independently operating the left and right pair of speed changing levers  18 , the left motor  4 L and the right motor  4 R are independently subjected to variable speed driving control. For example, when the left and right pair of speed changing levers  18  are operated by approximately the same operation amount in the forward direction from the neutral position, both the left rear wheel  12 L and the right rear wheel  12 R are driven at approximately the same speed, and the vehicle body  10  travels forward linearly. When the left and right pair of speed changing levers  18  are operated by approximately the same operation amount in the rearward direction from the neutral position, the left rear wheel  12 L and the right rear wheel  12 R are driven in the reverse direction at approximately the same speed, and the vehicle body  10  travels in reverse linearly. Furthermore, when the left and right pair of speed changing levers  18  are operated by mutually different operation amounts, the left rear wheel  12 L and the right rear wheel  12 R are driven at different speeds, and the vehicle body  10  turns. In such a case, a turn with a small radius can be performed by setting one of the left rear wheel  12 L and the right rear wheel  12 R to a low speed close to zero and operating the other to the forward side or the reverse side at a high speed. Furthermore, by driving the left rear wheel  12 L and the right rear wheel  12 R in mutually opposite directions, it is possible to cause the vehicle body  10  to perform a spin turn using the approximate central portion of the rear wheel unit  12  as the turn center. 
     With reference to  FIGS. 6, 7, 8 and 9 , a detailed configuration of the battery pack  6  will be described next. The battery pack  6  includes multiple battery modules  6 A accommodated in the battery case  60 . In the description of the basic configuration of the battery pack  6  with reference to  FIGS. 1, 2 and 3 , the battery pack  6  is described as an integral unit including the battery case  60  and the battery module  6 A. The front half of the battery pack  6  is defined as the front-side rectangular cuboid portion  6 F, and the rear half of the battery pack  6  is defined as the rear-side rectangular cuboid portion  6 B. Furthermore, the protruding central portion  6 Fc is formed on the upper half of the front-side rectangular cuboid portion  6 F, the left motor  4 L is arranged in the space on the left side of the central portion  6 Fc, and the right motor  4 R is in the space on the right side of the central portion  6 Fc. Such an arrangement structure is employed also in the present embodiment. In the description of the battery pack  6  hereinafter, however, the structure of the battery case  60  and the structure of the battery modules  6 A, which are originally separate members, will be described separately. 
     The battery case  60  is a molded product divided into an upper portion and a lower portion, forming an upper case  60   a  and a lower case  60   b , respectively. However, in order to facilitate the description of the internal structure, the battery case  60  will be described as being divided into a front case portion  61  and a rear case portion  62 . 
     The front case portion  61  is a three-dimensional member with an upper portion on which the protruding central portion  611  is formed by recessing the upper left portion and upper right portion of a rectangular cuboid. In other words, on both sides in the vehicle body lateral direction of the central portion  611 , the length in the vehicle body lateral direction is shorter to form motor-accommodating spaces that allow the left motor  4 L and the right motor  4 R to enter, respectively. The rear case portion  62  is a rectangular cuboid connected to the front case portion  61  in the front-rear direction. The front case portion  61  and the rear case portion  62  are connected in an orientation in which the rear case portion  62  is shifted upward, and the battery case  60  has a three-dimensional shape with a vertical level difference between the front side and the rear side. Note that the central portion  611  has an enlarged portion  612  that is enlarged on the left and right and upward in the region connecting the front case portion  61  and the rear case portion  62 , and the internal space of the enlarged portion  612  is expanded. 
     The rear case portion  62  has a rectangular cuboid shape with the same lateral width (length in the vehicle body lateral direction) as the front case portion  61 . The rear case portion  62  is shifted upward relative to the front case portion  61 , whereby the battery case  60  has a level-difference three-dimensional shape with a level difference between the front case portion  61  and the rear case portion  62 . 
     As shown in  FIGS. 8 and 9 , the interior of the battery case  60  divided into the upper case  60   a  and the lower case  60   b  is provided with a horizontal partitioning wall  63 , which divides the interior into upper and lower spanning across the front case portion  61  and the rear case portion  62 . The interior of the front case portion  61  is divided into the first space S 1  on the upper side and the second space S 2  on the lower side by the horizontal partitioning wall  63 , and the interior of the rear case portion  62  is divided into the third space S 3  on the upper side and the fourth space S 4  on the lower side by the horizontal partitioning wall  63 . The horizontal partitioning wall  63  is a plate member that is formed through a bending process so as to have a vertical level difference similar to that of the battery case  60 ; and the second space S 2 , the third space S 3  and the fourth space S 4  have approximately the same shape and volume. The width and height of the first space S 1  are smaller than those of the other spaces. A vertical partitioning wall  64  is provided between the first space S 1  and the third space S 3 . 
     Battery modules  6 A of the same specification are accommodated in the second space S 2 , the third space S 3  and the fourth space S 4 . As shown in  FIG. 9 , the battery module  6 A is a rectangular cuboid whose height is lower compared to its longitudinal dimension and lateral dimension. A multiple of battery cells  6   a  are accommodated in the interior of the battery module  6 A. 
     The first space S 1  accommodates an electric unit  68  including a relay, a fuse and the like which are provided on an electrical wire for interconnecting the battery modules  6 A and an external device. A circulation fan  69 , which suctions the air in the first space S 1  and sends it to the third space S 3 , is mounted on the vertical partitioning wall  64 . The electrical system for the circulation fan  69  is incorporated in the electric unit  68 . A front end gap G 1  is formed between a front end  63   a  of the horizontal partitioning wall  63  and the front case portion  61 , and a rear end gap G 2  is formed between a rear end  63   b  of the horizontal partitioning wall  63  and the rear case portion  62 . Furthermore, the battery case  60  has a sealed construction such that grass and waste are not taken into the interior space of the battery case  60  from the outside, or such that air flow between the interior space and the outside is suppressed. Accordingly, in the interior of the battery case  60 , a circulated air flow path (indicated by the arrows in  FIG. 8 ) is formed which starts from the circulation fan  69 , passes through the third space S 3 , the fourth space S 4  and the second space S 2  to reach the first space S 1 , and returns to the circulation fan  69 . The temperatures of the four spaces in the battery case  60  are equalized by the circulated air that flows on the circulated air flow path. Also, the electric unit  68  is cooled by such circulated air. In order to improve the cooling effect, fins  60   f  are provided on the wall surface of the battery case  60 , although this is shown only partially in the drawing. The fins  60   f  can be formed on one or both of the inner wall surface and the outer wall surface of the battery case  60 . 
     The upper wall of the front case portion  61  is provided with an opening for maintenance inspection of the electric unit  68 , and is usually closed by a lid  613 . 
     As shown in  FIG. 4 , in the present embodiment, a work motor  4 W that provides power to the mower unit  13  serving as the work apparatus is arranged on the rear portion of the mower unit  13 , and the power from the work motor  4 W is transmitted to the blade transmission mechanism  131  via a belt. Instead of this, the work motor  4 W can be arranged on the front side of the battery pack  6 . In such a case, a PTO shaft (power takeoff shaft), including an output shaft and a relay shaft from the work motor  4 W, extends forward in the vehicle body front-rear direction, whereby the power from the work motor  4 W is transmitted to the blade transmission mechanism  131  of the mower unit  13  via the PTO shaft. 
     Other Embodiments in the First Embodiment 
     (1) In the above-described embodiment, the battery electric unit  68  is accommodated in the first space S 1 , but this is not limitative. Instead thereof, the battery electric unit  68  may be accommodated in one of the second space S 2 , the third space S 3  and the fourth space S 4 ; and the battery modules  6 A may be accommodated each within the remainder (i.e. each of the others of the four spaces S 1 -S 4 ). 
     (2) In the above-described embodiment, a riding electric mower is illustrated as an example of the electric work vehicle in which the battery pack is mounted, but this is not limitative. Instead thereof, the battery pack can also be mounted in another electric work vehicle, such as an electric rice transplanter, an electric tractor, etc. 
     Second Embodiment 
     A basic configuration of an electric work vehicle in which a contactless charging system is incorporated will be described with reference to  FIGS. 10 and 11 . 
     The electric work vehicle includes a vehicle body frame  20  that includes a left frame  21  and a right frame  22  that extend in the vehicle body front-rear direction with an interval therebetween in the vehicle body lateral direction, and at least one crossbeam  23  that connects the left frame  21  and the right frame  22 . A left front wheel  11 L and a right front wheel  11 R that constitute a front wheel unit  11  are arranged at the front portion of the vehicle body frame  20 . A left rear wheel  12 L and a right rear wheel  12 R that constitute a rear wheel unit  12  are arranged rearward of the center of the vehicle body frame  20 . Hereinafter, if there is no particular need to make a distinction, the left front wheel  11 L and the right front wheel  11 R will be referred to collectively as “front wheel unit  11 ”, and the left rear wheel  12 L and the right rear wheel  12 R will be referred to collectively as “rear wheel unit  12 ”. A rear axle center Pr extends in a vehicle body lateral direction. 
     The battery pack  6  is arranged in a region in the rear half of the vehicle body frame  20  between the left frame  21  and the right frame  22 , and in a lateral side view. The front end portion of the battery pack  6  is located forward of the rear axle center Pr and approximately near the front end of the rear wheel unit  12 . The rear end of the battery pack  6  is located approximately at the rear end of the vehicle body frame  20 . As is apparent from  FIG. 11 , the battery pack  6  has a width sufficient to snugly fit the battery pack  6  between the left frame  21  and the right frame  22  and extends with an equal width in the vehicle body front-rear direction. Also, the front half of the battery pack  6  enters the axle space of the rear wheel unit  12 . The battery pack  6  has a shape obtained by cutting away a front-side upper portion region and a rear-side lower portion region from an approximate rectangular cuboid, the cutaway rear-side lower portion region creates a flat rectangular cuboid-shaped rear recessed portion BS, and the cut away front-side upper portion region creates a flat rectangular cuboid-shaped front recessed portion FS. 
     The motor  4 , that drives the rear wheel unit  12 , i.e. the driving wheels, includes a left motor  4 L that transmits rotational power to the left rear wheel  12 L, and a right motor  4 R that transmits rotational power to the right rear wheel  12 R. As shown in  FIG. 11 , the left motor  4 L enters the left side region of the front recessed portion FS. The right motor  4 R enters the right side region of the front recessed portion FS. The left motor  4 L and the right motor  4 R are arranged at positions that are left-right symmetrical. The left motor  4 L and the right rear axle case  31 L are joined by a left transmission  130 L, and the right motor  4 R and the right rear axle case  31 R are joined by a right transmission  130 R. The left transmission  130 L is arranged on the outside of the left frame  21 , and the right transmission  130 R is arranged on the outside of the right frame  22 . The left motor  4 L and the right motor  4 R are collectively referred to as an “electric motor unit” as well. The left transmission  130 L and the right transmission  130 R are collectively referred to as a “transmission  130 ”. The left rear axle case  31 L and the right rear axle case  31 R are collectively referred to as a “rear axle case  31 ”. A driver seat  16  is arranged above the electric motor unit (motor  4 ), and thus the space is used effectively. 
     A contactless charging system, that charges the battery pack  6  without coming into contact therewith, includes a primary coil unit  8  arranged and movable on a ground surface, and a secondary coil unit  7  attached to the electric work vehicle. In the example as shown in  FIG. 10 , the primary coil unit  8  has a two-stage structure, a coil power supply circuit portion  81  is arranged on the lower stage, a primary coil  80  is arranged on the upper stage, and an elevation mechanism  8   a  is provided between the coil power supply circuit portion  81  and the primary coil  80 . The above-ground height of the primary coil  80  can be adjusted using the elevation mechanism  8   a . A box-shaped housing  8   b  accommodates the coil power supply circuit portion  81 , the primary coil  80  and the elevation mechanism  8   a ; and the housing  8   b  is provided with wheels  85  for moving the primary coil unit  8 , and stoppers  86  for holding a stopped position of the primary coil unit  8 . The secondary coil unit  7  includes a secondary coil  70  that electromagnetically couples with the primary coil  80 , and a charging circuit portion  71  that rectifies the power from the secondary coil  70  and supplies the rectified power to the battery pack  6 . In the example as shown in  FIG. 10 , the secondary coil  70  and the charging circuit portion  71  that constitute the secondary coil unit  7  are not formed integral with each other, but the secondary coil  70  is arranged below the battery pack  6 , and the charging circuit portion  71  is arranged on the upper part of the battery pack  6 . The secondary coil  70  enters a rear recessed portion BS, which is a recessed portion formed in the lower portion of the battery pack  6 , and the secondary coil  70  is supported by one or both of the battery pack  6  and the vehicle body frame  20  using a coil support member  79 . 
     As shown in  FIG. 12 , the coil power supply circuit portion  81  of the primary coil unit  8  includes a rectifying circuit  82 , a communication control unit  83  and an inverter  84 . The charging circuit portion  71  of the secondary coil unit  7  includes a rectifying circuit  72  and a communication control unit  73 . The rectifying circuit  82  is connected to a commercial power source via a power source cable to generate a DC voltage, and provides the inverter  84  with the generated DC voltage. The inverter  84  generates an alternating voltage using the input DC voltage and causes the alternating voltage to flow through the primary coil  80 . The alternating voltage generated by the secondary coil  70  that is electromagnetically coupled to the primary coil  80  is allowed to flow through the rectifying circuit  72  of the charging circuit portion  71 , whereby a charging DC voltage with a predetermined voltage value is outputted from the rectifying circuit  72 . The rectifying circuit  72  is connected to the battery pack  6 , and a charging current is supplied to the battery pack  6  using the charging DC voltage. 
     The communication control unit  83  of the coil power supply circuit portion  81  and the communication control unit  73  of the charging circuit portion  71  can exchange information through wireless communication. During the charging of the battery pack  6 , charging information is exchanged to control the coil power supply circuit portion  81  and the charging circuit portion  71 . A notifying device (a buzzer  91 , a lamp  92 , or the like, see  FIG. 10 ), that notifies a driver or an operator that the primary coil  80  and the secondary coil  70  are at appropriate positions, is included in one or both of the coil power supply circuit portion  81  and the charging circuit portion  71 . 
     Note that the battery pack  6  is connected to a power drive unit  5  serving as an output destination. A motor control unit  50  and an inverter  51  are included in the power drive unit  5 , and a motor drive current, which is outputted from the inverter  51  based on a control signal from the motor control unit  50 , is sent to the left motor  4 L and the right motor  4 R. 
     One specific embodiment of the electric work vehicle will be described next with reference to the drawings.  FIG. 13  is a side view of a mid-mount electric mower, which is an example of an electric work vehicle, and  FIG. 14  is a plan view of the mid-mount electric mower. The electric mower (hereinafter referred to simply as “mower”) employs the contactless charging system as described above with reference to  FIGS. 10, 11 and 12 . 
     As shown in  FIGS. 13 and 14 , the mower includes the vehicle body  10 . The vehicle body  10  is supported on the ground by a caster-type front wheel unit  11  including a left front wheel  11 L and a right front wheel  11 R, and by a rear wheel unit  12  including a left rear wheel  12 L and a right rear wheel  12 R which are rotatably driven. The vehicle body  10  has a vehicle body frame  20  as a base frame. The vehicle body frame  20  includes the left frame  21 , the right frame  22 , and the crossbeam  23  that joins the left frame  21  and the right frame  22 . A mower unit  13  hangs down from the vehicle body frame  20  via a link mechanism  14  between the front wheel unit  11  and the rear wheel unit  12 . The mower unit  13  includes a blade transmission mechanism  131  and blades  132  that are rotated by the blade transmission mechanism  131 . A driver seat  16  is arranged in a central region in the vehicle body front-rear direction of the vehicle body  10 . 
     As shown in  FIG. 15 , the left frame  21  and the right frame  22  each branch vertically from a front/rear central region and rejoin in a rear region in the front-rear direction. In more particular, the left frame  21  includes an upper frame portion  21   a  and a lower frame portion  21   b  in the rear half region thereof. Similarly, the right frame  22  includes an upper frame portion  22   a  and a lower frame portion  22   b  in the rear half region thereof. In a lateral side view, a left transmission  130 L is arranged in the region between the upper frame portion  21   a  and the lower frame portion  21   b  of the left frame  21 , and a right transmission  130 R is arranged in the region between the upper frame portion  22   a  and the lower frame portion  22   b  of the right frame  22 . A left rear axle case  31 L is connected to the left transmission  130 L, and the left rear wheel  12 L is supported by the left rear axle case  31 L. A right rear axle case  31 R is connected to the right transmission  130 R, and the right rear wheel  12 R is supported by the right rear axle case  31 R. The left rear axle case  31 L is a tubular member in which the left rear axle  41  of the left rear wheel  12 L is mounted, and which performs bearing support, and the outer shape thereof is a truncated cone shape. Similarly, the right rear axle case  31 R is a tubular member in which the right rear axle  42  of the right rear wheel  12 R is mounted, and which performs bearing support, and the outer shape thereof is a truncated cone shape. The battery pack  6  is arranged such that, in plan view, the center of gravity of the battery pack  6  is approximately located on the center line in the vehicle body front-rear direction and falls within the length of the rear wheel radius on the front side and rear side from the rear axle center Pr. 
     The left transmission  130 L includes a gear transmission mechanism and extends forward in the vehicle body front-rear direction orthogonally to the rear axle center Pr, which acts also as the central axle center of the left rear axle case  31 L. The left transmission  130 L and the left rear axle case  31 L are formed integral, and the left motor  4 L is joined to the input portion of the left transmission  130 L. Similarly, the right transmission  130 R includes a gear transmission mechanism, and extends forward in the vehicle body front-rear direction so as to be orthogonal to the rear axle center Pr, which acts also as the central axis center of the right rear axle case  31 R. The right transmission  130 R and the right rear axle case  31 R are formed integral, and the right motor  4 R is joined to the input portion of the right transmission  130 R. 
     Speed changing operations on the left motor  4 L and the right motor  4 R are performed using a left and right pair of speed changing levers  18  (see  FIGS. 12 and 13 ), which are arranged on both sides of the driver seat  16 . When a speed changing lever  18  is held at a front-rear neutral position, the corresponding left motor  4 L or right motor  4 R enters the stopped state. By operating the speed changing lever  18  forward from the neutral position, the left motor  4 L or right motor  4 R performs forward driving and forward speed change is realized, and by operating the speed changing lever  18  rearward, the left motor  4 L or right motor  4 R performs reverse driving and reverse speed change is realized. By independently operating the left and right pair of speed changing levers  18 , the left motor  4 L and the right motor  4 R can independently perform variable speed driving. 
     As shown in  FIGS. 15 and 16 , the battery pack  6  includes multiple battery modules  6 A accommodated in the battery case  60 . The battery case  60  is a molded product divided into an upper and lower portion, and includes an upper case  60   a  and a lower case  60   b . Hereinafter, the front half of the battery case  60  is referred to as a front case portion  61 , and the rear half of the battery case  60  is referred to as a rear case portion  62 . 
     The rear case portion  62  is shifted upward relative to the front case portion  61 , whereby the battery case  60  has a level-difference cubic shape with a level difference between the front case portion  61  and the rear case portion  62 . 
     As shown in  FIG. 16 , the interior of the battery case  60  divided into the upper case  60   a  and the lower case  60   b  is provided with the horizontal partitioning wall  63 , which vertically divides the interior into two portions spanning across the front case portion  61  and the rear case portion  62 . The interior of the front case portion  61  is divided into the first space S 1  on the upper side and the second space S 2  on the lower side by the horizontal partitioning wall  63 , and the interior of the rear case portion  62  is divided into the third space S 3  on the upper side and the fourth space S 4  on the lower side by the horizontal partitioning wall  63 . The horizontal partitioning wall  63  is a plate member that is formed through a bending process so as to have a vertical level difference similar to that of the battery case  60 , and the second space S 2 , the third space S 3  and the fourth space S 4  have approximately the same shape and volume. The width and height of the first space S 1  are smaller than those of the other spaces. A vertical partitioning wall  64  is provided between the first space S 1  and the third space S 3 . 
     The electric unit  68  is accommodated in the first space S 1 . Battery modules  6 A of the same specification are accommodated in the second space S 2 , the third space S 3 , and the fourth space S 4 . A multiple of battery cells  6   a  are accommodated in the interior of the battery module  6 A. 
     The first space S 1  accommodates an electric unit  68  having a relay, a fuse and the like provided on an electrical wire for connecting the battery module  6 A with an external device. The circulation fan  69 , which suctions the air in the first space S 1  and sends it to the third space S 3 , is mounted on the vertical partitioning wall  64 . The electrical system for the circulation fan  69  is incorporated in the electric unit  68 . Accordingly, in the interior of the battery case  60 , a circulated air flow path (indicated by the arrows in  FIG. 16 ) is formed which starts from the circulation fan  69 , passes through the third space S 3 , the fourth space S 4  and the second space S 2  to reach the first space S 1 , and returns to the circulation fan  69 . The temperatures of the four spaces in the battery case  60  are equalized by the circulated air that flows on the circulated air flow path. 
     As shown in  FIG. 16 , the secondary coil  70  is held by the coil support member  79  so as to oppose the lower surface of the lower case  60   b  of the battery pack  6 . The coil support member  79  is fixed to the vehicle body frame  20  via a crossbar that joins the lower frame portion  21   b  of the left frame  21  and the lower frame portion  22   b  of the right frame  22 . The secondary coil  70  is arranged between the lower frame portion  21   b  on the left side and the lower frame portion  22   b  on the right side in an orientation in which the magnetic force lines thereof are oriented in the vertical direction, and therefore the lower frame portion  21   b  and the lower frame portion  22   b  on the right side form a protection fence in the vehicle body lateral direction. 
     The primary coil unit  8  is moved so that the primary coil  80  is located directly below the secondary coil  70 . In such a case, adjustment of the position in the vehicle body lateral direction can be performed by moving the primary coil unit  8 , and adjustment of the position in the vehicle body front-rear direction can be performed by slightly moving the mower forward or in reverse. The buzzer  91  and lamp  92  serving as the notifying devices that perform reporting when the primary coil  80  and the secondary coil  70  are in the correct positional relationship (when the correct power is transmitted) are attached to the coil support member  79 . A similar notifying device may be provided on the primary coil unit  8 . Furthermore, in order to align the primary coil  80  with the secondary coil  70  by operating the mower, a lamp on an operation panel may be used for a notifying device as well. 
     Other Embodiments in the Second Embodiment 
     (1) In the above-described embodiment, the primary coil  80  and the coil power supply circuit portion  81  of the primary coil unit  8  are arranged in two, upper and lower stages. Instead thereof, the primary coil  80  and the coil power supply circuit portion  81  may be arranged laterally side by side. 
     (2) In the above-described embodiment, the secondary coil unit  7  is divided into the secondary coil  70  and the charging circuit portion  71  and arranged at the lower portion and the upper portion of the battery pack  6 , respectively. Instead thereof, the secondary coil  70  and the charging circuit portion  71  may be formed integral with each other and arranged below the battery pack  6 . 
     (3) In the above-described embodiment, a mid-mount electric mower is given as an example of an electric work vehicle in which a contactless charging system of the present invention is incorporated. Instead thereof, a contactless charging system of the present invention can also be applied to a front-mount electric mower, an agricultural work machine such as a rice transplanter, a combine-harvester and tractor, a construction machine such as a backhoe and a bucket loader or the like.