Patent Publication Number: US-2021188068-A1

Title: Electric power control system

Description:
TECHNICAL FIELD 
     One embodiment of the present invention relates to a vehicle. One embodiment of the present invention relates to an automobile. One embodiment of the present invention relates to an electric power control system, an electric power control method, and a program for a vehicle, an automobile, or the like. 
     One embodiment of the present invention relates to a wheel. One embodiment of the present invention relates to a power storage device. One embodiment of the present invention relates to a secondary battery. 
     Note that one embodiment of the present invention is not limited to the above technical fields. As the technical field of one embodiment of the present invention disclosed in this specification, a semiconductor device, a display device, a light-emitting device, a power storage device, a memory device, an electronic device, a lighting device, an input device, an input/output device, a driving method thereof, or a manufacturing method thereof can be given as an example, in addition to a vehicle including an automobile. 
     BACKGROUND ART 
     In recent years, a technique for utilizing electric power of a battery as power of an automobile has been attracting attention. As such an automobile, a hybrid electric vehicle (HEV), an electric vehicle (EV), a plug-in hybrid electric vehicle (PHEV), or the like is given, for example. 
     In addition, for a battery to be provided in an automobile, development of a lithium-ion battery has been carried out. An example of a lithium-ion battery includes at least a positive electrode, a negative electrode, and an electrolytic solution (Patent Document 1). 
     PRIOR ART DOCUMENT 
     Patent Document 
     [Patent Document 1] Japanese Published Patent Application No. 2012-9418 
     SUMMARY OF THE INVENTION 
     Problems to be Solved by the Invention 
     In an automobile provided with a battery used for power, the battery is required to have a very large capacity in order to increase the mileage. However, there has been a limit to the capacity of a battery that can be provided in an automobile, because of a problem such as the volume of the battery. In particular, it has been difficult to provide a battery having a sufficient capacity in a small automobile, because of a problem of the living space being narrowed. 
     An object of one embodiment of the present invention is to achieve space-saving of a vehicle such as an automobile provided with a battery. Another object is to increase the design flexibility of a vehicle such as an automobile. 
     Another object of one embodiment of the present invention is to provide an electric power control method or an electric power control system with which electric power can be efficiently utilized in a vehicle such as an automobile provided with a battery. 
     Another object is to provide a novel electric power control method or electric power control system. Another object is to provide a novel vehicle, a novel wheel for a vehicle, a novel automobile, or a novel wheel for an automobile. Another object is to provide a novel electric power feeding system that can be used for an automobile. 
     Note that the description of these objects does not preclude the existence of other objects. In one embodiment of the present invention, there is not necessarily a need to achieve all the objects. Other objects can be derived from the descriptions of the specification, the drawings, the claims, and the like. 
     Means for Solving the Problems 
     One embodiment of the present invention is an electric power control system of a vehicle including a car body, a first battery, a second battery, and a control unit. The control unit has a function of obtaining states of charge of the first battery and the second battery. The control unit has a function of determining whether or not a difference between remaining capacities of the first battery and the second battery exceeds a predetermined value, and controlling transmission of electric power between the first battery and the second battery, in a case where the difference in the remaining capacities exceeds the predetermined value, to be made such that the remaining capacities are close to each other. 
     Furthermore, the above control unit preferably has a function of controlling transmission of electric power between the first battery and the second battery to be made, in a case where the car body is in a resting state or an idle running state. 
     Furthermore, in the above, an electric power control unit, a braking control unit, and a motor are preferably included. The braking control unit has a function of controlling the motor such that the motor generates electric power at the time of braking. The motor has a function of transmitting generated electric power to the electric power control unit. The control unit has a function of controlling the electric power control unit so as to supply electric power preferentially to either the first battery or the second battery. 
     Furthermore, in the above, an electric power control unit, a braking control unit, and a motor are preferably included. The braking control unit has a function of controlling the motor such that the motor generates electric power at the time of braking. The motor has a function of transmitting generated electric power to the electric power control unit. The control unit has a function of obtaining states of charge of the first battery and the second battery, and controlling the electric power control unit so as to supply the electric power to either the first battery or the second battery that has a smaller remaining capacity. 
     Another embodiment of the present invention is an electric power control system of an automobile including a car body, a wheel, a first battery, a second battery, and a control unit. Here, the first battery is provided in the wheel. The second battery is provided in the car body. The control unit has a function of obtaining states of charge of the first battery and the second battery. The control unit has a function of determining whether or not a difference between remaining capacities of the first battery and the second battery exceeds a predetermined value, and controlling transmission of electric power between the first battery and the second battery, in a case where the difference in the remaining capacities exceeds the predetermined value, to be made such that the remaining capacities are close to each other. 
     Furthermore, the control unit preferably has a function of controlling transmission of electric power between the first battery and the second battery to be made, in a case where the car body is in a resting state or an idle running state. 
     Furthermore, in the above, an electric power control unit, a braking control unit, and a motor are preferably included. The braking control unit has a function of controlling the motor such that the motor generates electric power at the time of braking. The motor has a function of transmitting generated electric power to the electric power control unit. The control unit has a function of controlling the electric power control unit so as to supply electric power preferentially to the first battery. 
     Furthermore, in the above, a first electric power control unit and a second electric power control unit, instead of the electric power control unit, are preferably included. The first electric power control unit has a function of controlling charge and discharge of the first battery. The second electric power control unit has a function of controlling charge and discharge of the second battery. Here, the first electric power control unit and the second electric power control unit are preferably connected so as to transmit electric power to each other. 
     Furthermore, in the above, the motor is preferably provided in the wheel. 
     Furthermore, the wheel or the automobile that can be used for the above electric power control system can use a structure described below, for example. 
     One embodiment of the present invention is a wheel including a rim portion, a disk portion, a battery, and a first electric power transmission mechanism. The battery is provided inside the rim portion or along a surface of the rim portion. The first electric power transmission mechanism is provided in the disk portion and is electrically connected to the battery. 
     Furthermore, in the above, the battery preferably is a secondary battery sealed with a film, has a belt-like shape, and is provided in a state of being wrapped around a cylindrical portion of the rim portion. In that case, the battery is preferably provided in a state of being wrapped around the cylindrical portion of the rim portion more than one lap. 
     In the above, a structure including a plurality of batteries, each of which has a cylindrical shape or a columnar shape, may also be employed. 
     Furthermore, in the above, the first electric power transmission mechanism preferably is a connector including a contact point. Alternatively, the first electric power transmission mechanism preferably has a function of wirelessly transmitting and receiving electric power. 
     Another embodiment of the present invention is a vehicle to which the above wheel can be attached, including an electric power control unit and a second electric power transmission mechanism. The second electric power transmission mechanism has a function of being electrically connected to the first electric power transmission mechanism. The electric power control unit preferably has a function of controlling charge and discharge of the battery through the second electric power transmission mechanism and the first electric power transmission mechanism. 
     Furthermore, in the above, the second electric power transmission mechanism preferably is a connector having a function of maintaining electrical connection with the first electric power transmission mechanism even when rotating. Alternatively, the second electric power transmission mechanism preferably has a function of wirelessly transmitting and receiving electric power. 
     Effects of the Invention 
     According to one embodiment of the present invention, space-saving of a vehicle such as an automobile provided with a battery can be achieved; the design flexibility of a vehicle such as an automobile can be increased; an electric power control method or an electric power control system with which electric power can be efficiently utilized in a vehicle such as an automobile provided with a battery can be provided; a novel electric power control method or a novel electric power control system can be provided; a novel vehicle, a novel wheel for a vehicle, a novel automobile, or a novel wheel for an automobile can be provided; or a novel electric power feeding system that can be used for a vehicle such as an automobile can be provided. 
     Note that the description of these effects does not preclude the existence of other effects. One embodiment of the present invention does not necessarily achieve all the effects listed above. Other effects will be derived from the description of the specification, the drawings, the claims, and the like. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  A drawing illustrating an electric power control system of an embodiment. 
         FIG. 2  A flow chart related to an operation method of an electric power control system of an embodiment. 
         FIG. 3  A flow chart related to an operation method of an electric power control system of an embodiment. 
         FIG. 4  Drawings illustrating an operation method of an electric power control system of an embodiment. 
         FIG. 5  A flow chart related to an operation method of an electric power control system of an embodiment. 
         FIG. 6  A drawing illustrating an operation method of an electric power control system of an embodiment. 
         FIG. 7  A flow chart related to an operation method of an electric power control system of an embodiment. 
         FIG. 8  Drawings illustrating an operation method of an electric power control system of an embodiment. 
         FIG. 9  Drawings each illustrating a method of antiskid control of an embodiment. 
         FIG. 10  A drawing illustrating an electric power control system of an embodiment. 
         FIG. 11  A drawing illustrating an electric power control system of an embodiment. 
         FIG. 12  A drawing illustrating an electric power control system of an embodiment. 
         FIG. 13  A drawing illustrating an electric power control system of an embodiment. 
         FIG. 14  A drawing illustrating an electric power control system of an embodiment. 
         FIG. 15  A drawing illustrating an electric power control system of an embodiment. 
         FIG. 16  Drawings each illustrating a wheel of an embodiment. 
         FIG. 17  A drawing illustrating a wheel of an embodiment. 
         FIG. 18  Drawings each illustrating a wheel of an embodiment. 
         FIG. 19  Drawings each illustrating a wheel of an embodiment. 
         FIG. 20  Drawings each illustrating a wheel of an embodiment. 
         FIG. 21  A drawing illustrating a car body and a wheel of an embodiment. 
         FIG. 22  Drawings each illustrating a car body and wheels of an embodiment. 
         FIG. 23  Drawings each illustrating a vehicle of an embodiment. 
         FIG. 24  A drawing illustrating a structure of a secondary battery of an embodiment. 
         FIG. 25  Drawings each illustrating a structure of a secondary battery of an embodiment. 
         FIG. 26  Drawings illustrating a manufacturing method of a secondary battery of an embodiment. 
         FIG. 27  Drawings illustrating a manufacturing method of a secondary battery of an embodiment. 
         FIG. 28  Drawings illustrating a manufacturing method of a secondary battery of an embodiment. 
         FIG. 29  Drawings illustrating a structure and a manufacturing method of a secondary battery of an embodiment. 
     
    
    
     EMBODIMENTS FOR CARRYING OUT THE INVENTION 
     Embodiments will be described in detail with reference to the drawings. Note that the present invention is not limited to the following description. It will be readily appreciated by those skilled in the art that modes and details of the present invention can be modified in various ways without departing from the spirit and scope of the present invention. Thus, the present invention should not be construed as being limited to the description in the following embodiments. 
     Note that in structures of the present invention described below, the same portions or portions having similar functions are denoted by the same reference numerals in different drawings, and a description thereof is not repeated. Furthermore, the same hatching pattern is applied to portions having similar functions, and the portions are not especially denoted by reference numerals in some cases. 
     Note that in each drawing described in this specification, the size, the layer thickness, or the region of each component is exaggerated for clarity in some cases. Therefore, the size, the layer thickness, or the region is not limited to the illustrated scale. 
     Note that in this specification and the like, ordinal numbers such as “first,” “second,” and the like are used in order to avoid confusion among components and do not limit the number. 
     Embodiment 1 
     In this embodiment, an electric power control method and an electric power control system of one embodiment of the present invention will be described. Furthermore, in this embodiment, a structure example of an automobile or the like which is an embodiment of a vehicle to which the electric power control method of one embodiment of the present invention can be applied will be described. 
     The electric power control system of one embodiment of the present invention is related to a vehicle having a car body and a wheel (an automobile, for example). The car body is provided with at least a control unit. The control unit can have a structure including an arithmetic device (computer), a memory device, and the like, for example. 
     Furthermore, the electric power control system of one embodiment of the present invention includes at least two or more batteries. The plurality of batteries can be provided in the car body or the wheel. It is particularly preferable that the battery be provided in each of the car body and the wheel. 
     Furthermore, the electric power control system of one embodiment of the present invention preferably has a structure where electric power can be transmitted between the plurality of batteries. 
     Furthermore, the electric power control system of one embodiment of the present invention preferably includes a motor as power. In addition, the electric power control system of one embodiment of the present invention preferably has a structure where regenerative electric power is generated by the motor and the electric power is supplied to the plurality of batteries so that the batteries can be charged. 
     The electric power control system of one embodiment of the present invention enables the regenerative electric power from the motor to be supplied preferentially to a predetermined battery, or to be supplied selectively to a battery with the smallest charge remaining (also referred to as remaining capacity) among the plurality of batteries, for example. 
     Furthermore, the system of one embodiment of the present invention is capable of transmission of electric power between batteries in such a manner that a difference in charge remaining between the batteries is decreased, in the case where there is such a difference. This operation is preferably carried out when the vehicle is in a resting state or in an idle running state. In other words, it is preferable that transmission of electric power between the batteries be carried out in a period during which the motor is not generating power, i.e., in a period during which electric power of the battery is not being supplied to the motor. 
     The use of the system capable of such an operation can prevent a situation in which one or more batteries are in a fully charged state or a completely discharged state (including a state with the smallest amount of charge in the usage range of the battery) among the plurality of batteries. It is known that the deterioration of a secondary battery, which can be used as a battery, is accelerated when a fully charged state or a state with an extremely small amount of charge continues. Accordingly, the system of one embodiment of the present invention can suppress the deterioration of a battery, and can provide a vehicle such as an automobile with less-frequent maintenance such as battery replacement or free from such maintenance. 
     Note that, in one embodiment of the present invention, as a form of a program that makes a control unit or a computer in the control unit execute such an operation, the program can be stored in the control unit or a memory device which is provided separately from the control unit. The control unit is capable of reading out the program from the memory device and executing the program. 
     An electric power transmission system of one embodiment of the present invention can be applied to a vehicle such as an automobile. An automobile is an embodiment of a vehicle. As an automobile, a specialized vehicle such as a civil engineering work vehicle and a crane truck is included, in addition to a car, a truck, and a bus. In addition, the electric power transmission system of one embodiment of the present invention can be installed in a one-wheeled, two-wheeled, or three-wheeled vehicle or a vehicle with five or more tires, in addition to a four-wheeled vehicle. As a two-wheeled vehicle, a structure with two wheels attached to a car body, one behind the other, like a motorcycle may be employed, or a structure with two tires provided on sides of a car body face-to-face may be employed. The system can also be applied to a bicycle, an electric bicycle, a power-assisted bicycle, a tire for an airplane, a tire for a helicopter, a tire for a vertical take-off and landing aircraft, an amphibious car, a tank, or the like. 
     In addition, the electric power transmission system of one embodiment of the present invention can be applied to a vehicle that does not use a tire. For example, it can be used for a wheel of a car that moves on a rail as a guideway. For example, it can be used for a vehicle such as a railroad (including an electric train, a steam train, a steam locomotive, and the like), a streetcar, a cable car, and the like. 
     Furthermore, one embodiment of the present invention can be applied to a toy that copies the above-mentioned vehicle. 
     Hereinafter, more detailed examples of the electric power control system, the electric power control method, or the program will be described with reference to drawings. 
     [Structure Example of System] 
       FIG. 1  shows a block diagram of a system  80  of one embodiment of the present invention. 
     The system  80  includes a car body  50 , a wheel  10   a , a wheel  10   b , wheels  70 , and the like. The car body  50  includes a control unit  61 , an electric power control unit  62   a , an electric power control unit  62   b , an electric power control unit  71 , a braking control unit  66 , a battery  65 , and the like. The wheel  10   a  includes a motor  64   a  and a battery  20   a . The wheel  10   b  includes a motor  64   b  and a battery  20   b . The wheels  70  function as follower wheels. 
     Here, an example where an automobile to which the system  80  is applied is an electric vehicle or electrical vehicle (EV), which uses electricity as power, is shown. 
     The control unit  61  has a function of performing varied electronic control in addition to power control and electric power control. Specifically, the control unit  61  can perform control of the electric power control unit  62   a , the electric power control unit  62   b , and the electric power control unit  71 , control of the braking control unit  66 , and the like. As the control unit  61 , an ECU (electric control unit, or also referred to as engine control unit) can typically be used. Furthermore, in accordance with the driving method of the automobile, an ECU with a function that is unique to an EV, an HEV (hybrid electro vehicle), or a PHEV (plug-in hybrid vehicle) is preferably used. 
     The motor  64   a  and the motor  64   b  are devices that produce power for rotating the wheel  10   a  or the wheel  10   b . The motor  64   a  can produce power in accordance with electric power supplied from the electric power control unit  62   a . Similarly, the motor  64   b  can produce power in accordance with electric power supplied from the electric power control unit  62   b.    
     The motor  64   a  and the motor  64   b  have a function of generating electric power from the rotational energy of the wheel  10   a  or the wheel  10   b  at the time of braking, and supplying the electric power to the electric power control unit  62   a  or the electric power control unit  62   b . The function like this can be referred to as an electric power regeneration function. The electric power regeneration operations of the motor  64   a  and the motor  64   b  are controlled by the control unit  61  and the braking control unit  66 . 
     The electric power control unit  62   a , the electric power control unit  62   b , and the electric power control unit  71  are each controlled by the control unit  61 . The electric power control unit  62   a , the electric power control unit  62   b , and the electric power control unit  71  have a function of controlling charge and discharge of the battery  20   a , the battery  20   b , and the battery  65 , respectively. Specifically, they each have a function of outputting electric power from the battery  20   a , the battery  20   b , or the battery  65 , and a function of supplying electric power to the battery  20   a , the battery  20   b , or the battery  65 . Furthermore, the electric power control unit  62   a , the electric power control unit  62   b , and the electric power control unit  71  preferably have a function of adjusting voltage (transforming voltage). 
     The electric power control unit  62   a  and the electric power control unit  62   b  may each have a structure including a step-up circuit (converter), a conversion circuit (inverter), and a computer that controls these, for example. The converter is a circuit that raises the voltage of electric power supplied from the battery  20   a  and the battery  20   b  to the voltage for driving the motor  64   a , the motor  64   b , or the like. The inverter is a circuit that converts a direct current voltage to an alternating current voltage for driving the motor  64   a , the motor  64   b , or the like. Furthermore, for an electric power regeneration function, a conversion circuit that converts an alternating current voltage output from the motor  64   a  or the motor  64   b  to a direct current voltage, a step-down circuit that lowers it to a voltage for charging the battery  20   a  and the battery  20   b , and the like are preferably included. 
     The electric power control unit  71  can have a structure including a step-up circuit, a step-down circuit, an inverter, a converter, or the like, and a computer that controls these, similarly to the electric power control unit  62   a  and the like. Since the electric power control unit  71  does not directly supply electric power to the motor  64   a  and the motor  64   b  here, it may include a function of converting the voltage of electric power supplied from the battery  65  to the voltage output to the electric power control unit  62   a , the electric power control unit  62   b , or another component. 
     Here, the electric power control unit  62   a , the electric power control unit  62   b , and the electric power control unit  71  are configured to be connected by an electric power transmission path to enable electric power transmission therebetween. In this way, the battery  65 , the battery  20   a , and the battery  20   b  can transmit/receive the charged electric power to/from each other. 
     The braking control unit  66  has a function of controlling braking. As a braking means, a physical brake utilizing oil pressure such as a disk brake and a drum brake (hereinafter also referred to as a physical brake), an electric brake using a load required to rotate a motor (hereinafter also referred to as an electric brake or a regenerative brake), and the like are given. En order to add the electric power regeneration function, a structure where an electric brake is used and electromotive force (also referred to as regenerative electric power) generated by rotation of a motor is utilized can be employed. Here, it is preferable that a braking system in which both of a physical brake and an electric brake are combined be used for the braking control unit  66 . 
     Here, the braking control unit  66  has a function of braking the wheel  10   a  and the wheel  10   b , with a combination of a physical brake utilizing oil pressure or the like and an electric brake. In addition, it has a function of braking the wheel  70  with a physical brake. 
     The control unit  61  calculates how much braking torque is required for each wheel, in accordance with the brake operation input by a driver and the conditions of the car body (speed, moving direction, attitude of the car body, and the like). In the case where a physical brake and an electric brake are used in combination, the control unit  61  calculates how to allocate the torque to be generated by each of the two brakes. Then, the control unit  61  controls the braking control unit  66  in accordance with the result, whereby a smooth braking operation can be performed. 
     Here, an example where the automobile to which the system  80  is applied has a structure where the motor is provided in the wheel is described. Such a structure can be referred to as an in-wheel motor. 
     A motor, a battery, and an electric power control unit that are involved in driving of one wheel can be regarded as one unit. Taking the wheel  10   a  as an example, the motor  64   a , the battery  20   a , and the electric power control unit  62   a  correspond to one unit. At this time, electric power for driving the motor  64   a  is supplied to the battery  20   a  through the electric power control unit  62   a . Furthermore, regenerative electric power generated by the motor  64   a  is supplied to the battery  20   a  through the electric power control unit  62   a . In this manner, when a unit including the motor, the battery, and the electric power control unit is provided for each wheel, the efficiency of electric power transmission is improved and an electric power loss can be reduced. 
     Note that, although an example where the wheel  10   a  includes the motor  64   a  and the battery  20   a  is described here, one embodiment is not limited to this; either one or both of the motor  64   a  and the battery  20   a  may be provided in the car body  50 , or the electric power control unit  62   a  may be provided in the wheel  10   a.    
     [Example of Operation Method] 
     An example of an operation method of the electric power control system of one embodiment of the present invention will be described below. 
     [Main Flow] 
     In  FIG. 2 , a flow chart related to an operation method of the electric power control system is shown. The following operation (control) is mainly performed by the control unit  61 . 
     First, in Step S 01 , the system starts. It corresponds to a condition where an automobile was started up or a condition where a user set and activated the system, for example. 
     Subsequently, in Step S 02 , whether or not regenerative electric power is supplied is determined. For example, whether or not a brake operation is performed and an electric brake is used is determined. In the case where regenerative electric power is supplied, the operation proceeds to Process SR 01 ; and in the case where regenerative electric power is not supplied, the operation proceeds to Step S 03 . 
     In Step S 03 , whether or not the automobile is in a resting state or an idle running state is determined. In the case where the state of the automobile is in the resting state or the idle running state, the operation proceeds to Process SR 02 ; and if not, the operation proceeds to Step S 04 . 
     Here, the resting state is a state where the automobile is stationary with respect to the ground, for example, and the motor  64   a  and the like are not producing power. The idle running state is a state where the automobile is moving with respect to the ground, for example, the motor  64   a  is not producing power, and the braking operation is not performed. In other words, in the idle running state, the automobile is moving with inertial force. 
     The resting state and the idle running state can be expressed as a state where electric power required for power is not consumed and a state where electric power is not generated by electric power regeneration. 
     Here, after Process S 12 . 01  is completed and after. Process SR 02  is completed, the operation proceeds to Step S 04 . 
     In Step S 04 , whether or not to terminate the system is determined. In the case where the system is to be terminated, the operation proceeds to Step S 05  to terminate the system. If not, the operation proceeds again to Step S 02 . 
     The above is the description of the main flow. 
     [Electric Power Regeneration Operation  1 ] 
     In  FIG. 3 , a flow chart related to Process SR 01  is shown. Process SR 01  is a process related to an electric power regeneration operation. The following operation (control) is mainly performed by the control unit  61 . 
     First, in Step S 11 , the electric power regeneration operation starts. 
     In Step S 12 , the remaining of each battery is checked. 
     In the example shown in  FIG. 1 , the state of charge of each of the battery  20   a , the battery  20   b , and the battery  65  is checked. 
     In Step S 13 , whether or not regenerative electric power can be supplied to a predetermined battery is determined. In other words, whether or not the predetermined battery among the batteries can be charged is determined. In the case where regenerative electric power can be supplied to the predetermined battery, the operation proceeds to Step S 14 . If not, the operation proceeds to Step S 15 . 
     In Step S 14 , regenerative electric power is supplied to the predetermined battery, and the operation proceeds to Step S 16 . 
     In Step S 15 , regenerative electric power is supplied to the other batteries except the predetermined battery, and the operation proceeds to Step S 16 . 
     Subsequently, in Step S 16 , whether or not supply of regenerative electric power is completed is determined. In the case where the supply of regenerative electric power is continuing, the operation returns to Step S 12 . On the other hand, in the case where the supply of regenerative electric power is completed, the operation proceeds to Step S 17 . 
     In Step S 17 , the electric power regeneration operation is completed. 
     The above is the description of the flow shown in  FIG. 3 . 
     Here, the electric power regeneration operation illustrated in  FIG. 3  will be described with reference to  FIGS. 4(A)  and (B).  FIGS. 4(A)  and (B) are schematic views selectively showing the control unit  61 , the electric power control unit  62   a , the electric power control unit  62   b , the electric power control unit  72 , the motor  64   a , the battery  20   a , the battery  20   b , and the battery  65 , from the components of the system  80  shown in  FIG. 1 . Here, the direction in which electric power is supplied is indicated by arrows. Furthermore, the state of charge of each of the batteries is schematically shown; the larger the hatched area is, the larger the amount of charge is. 
     In  FIGS. 4(A)  and (B), an example of a case where regenerative electric power is generated in the motor  64   a  is shown. Thus, the electric power generated in the motor  64   a  is first sent to the electric power control unit  62   a.    
       FIG. 4(A)  shows a schematic view related to the operation in Step S 14 . That is, it is an example of a case where the battery  20   a  is not fully charged and can be additionally charged. At this time, as shown in  FIG. 4(A) , the control unit  61  controls the supply of electric power so that the electric power is supplied from the electric power control unit  62   a  to the battery  20   a.    
     In contrast,  FIG. 4(B)  shows a schematic view related to the operation in Step S 15 . That is, it is a state where the battery  20   a  is fully charged and electric power cannot be supplied thereto any further. At this time, regenerative electric power is supplied from the electric power control unit  62   a  through the electric power control unit  62   b  to the battery  20   b , or through the electric power control unit  72  to the battery  65 . In  FIG. 4(B) , an example of a case where regenerative electric power is not supplied to the battery  65  since the battery  65  is fully charged, and regenerative electric power is supplied only to the battery  20   b  is shown. 
     As described above, the electric power regeneration operation of one embodiment of the present invention is capable of supplying regenerative electric power preferentially to a battery that constitutes a unit with a motor where regenerative electric power is generated. In this way, an electric power transmission loss can be reduced. 
     [Electric Power Regeneration Operation  2 ] 
     An example that is partially different from the above electric power regeneration operation  1  will be described below.  FIG. 5  is a flow chart related to the electric power regeneration operation.  FIG. 5  is different from  FIG. 3  in that Step S 23 , Step S 24 , and Step S 25  are included instead of Step S 13 , Step S 14 , and Step S 15 . 
     In Step S 23 , the states of charge of the batteries are compared, and whether or not a difference in remaining between the batteries is larger than or equal to a predetermined value is determined. Then, the operation proceeds to Step S 24  in the case where there is a difference larger than or equal to the predetermined value, and to Step S 25  if not. 
     In Step S 24 , regenerative electric power is supplied to the battery with the smallest remaining among the batteries, and the operation proceeds to Step S 16 . 
     In Step S 25 , regenerative electric power is supplied to the predetermined battery, and the program proceeds to Step S 16 . 
     The above is the description of the flow shown in  FIG. 5 . 
       FIG. 6  is a schematic view illustrating the operation of Step S 24 . In  FIG. 6 , supply of regenerative electric power is controlled such that regenerative electric power generated in the motor  64   a  is supplied to the battery  20   b  of which the amount of charge is the smallest among the battery  20   a , the battery  20   b , and the battery  65 . 
     As described above, in one embodiment of the present invention, the battery to be supplied with regenerative electric power can be switched and used in accordance with the states of charge of the batteries. In this manner, it is possible to prevent a condition where one or more of the plurality of batteries are running out of the amount of charge. 
     Here, the value to be used for determination of a difference in amount of charge of the batteries is appropriately set in accordance with a case where the capacities of the batteries are the same or a case where they are different. As an example, when a fully charged state is 100% and a discharge state is 0% in the range specified by a rated voltage range of the battery or the like, in the case where there is a difference of 10% or more, preferably 5% or more, and more preferably 2% or more between two batteries, the two batteries can be determined to have a difference larger than or equal to a predetermined value in amount of charge. Furthermore, the amount of charge may also be defined by a voltage value, a current amount, an electric power amount, or the like other than the above. 
     The above is the description of the electric power regeneration operation. 
     [Electric Power Smoothing Operation] 
     In  FIG. 7 , a flow chart related to Process SR 02  is shown; Process SR 02  is a process related to an electric power smoothing operation. Here, the electric power smoothing operation is an operation for decreasing or eliminating a difference in amount of charge of the batteries. The following operation (control) is mainly performed by the control unit  61 . 
     First, in Step S 31 , the electric power smoothing operation starts. 
     In Step S 32 , the remaining of each battery is checked. 
     In Step S 33 , the states of charge of the batteries are compared, and whether or not a difference in remaining between the batteries is larger than or equal to a predetermined value is determined. Then, the operation proceeds to Step S 34  in the case where there is a difference larger than or equal to the predetermined value, and to Step S 35  if not. 
     In Step S 34 , electric power is supplied from the battery with a large amount of remaining to the battery with a small amount of remaining among the batteries. After that, the operation returns to Step S 32 . 
     The above is the description of the flow shown in  FIG. 7 . In the flow shown in  FIG. 7 , the electric power smoothing operation is completed when a difference in remaining of the batteries becomes lower than the predetermined value. 
     Note that the process can be forcibly stopped in the case where an interrupt process is performed during the electric power smoothing operation. As the interrupt process, process related to an operation (acceleration, turn, braking, or the like) by which the state of the automobile changes from a resting state or an idle running state is given. 
       FIGS. 8(A)  and (B) are schematic views illustrating the operation of Step S 34 . 
       FIG. 8(A)  is a state right after the start of Step S 34 . In  FIG. 8(A) , supply of electric power is controlled such that electric power is supplied to the battery  20   a , which has the smallest amount of charge among the battery  20   a , the battery  20   b , and the battery  65 , from the other two batteries. 
       FIG. 8(B)  shows a state at the time when the operation of Step S 34  is completed. As shown in  FIG. 8(B) , the amounts of charge of the batteries are smoothed to be comparable. 
     Note that the operation in which electric power is supplied to the battery with the smallest amount of charge from both of the other two batteries is described here; however, one embodiment is not limited to this and an operation in which electric power is supplied from only the battery with the largest amount of charge, for example, may be employed. 
     Furthermore, in the case where electric power is supplied from two or more batteries, the supply amount may differ in accordance with the amount of charge of each battery. For example, a battery with a larger amount of charge may supply a larger amount of electric power. 
     Note that the determination criterion in the above description of the electric power regeneration operation  2  can be employed as the value used for the determination in Step S 33 . 
     In this way, the amounts of charge of the batteries can be smoothed by the electric power smoothing operation. Since deterioration of a battery in a fully charged state or a completely discharged state sometimes progresses rapidly, smoothing electric power in this manner and keeping all the batteries from a fully charged state or a discharged state can extend the life of the batteries. 
     The above is the description of the electric power smoothing operation. 
     The electric power control method and the electric power control system of one embodiment of the present invention enable a vehicle with a plurality of batteries to transmit electric power between the batteries. Accordingly, in the case where electric power regeneration is performed, it is possible to charge a predetermined battery preferentially. Furthermore, owing to the electric power smoothing operation, the amounts of charge of the batteries can be smoothed. By such a method, reduction in electric power transmission loss, extension of the battery lives, and the like become possible. 
     Note that one embodiment of the present invention may be achieved in such a manner that a program is stored in a memory portion included in the control unit  61  and read out and executed by a computer or an arithmetic device in the control unit  61 . That is, another embodiment of the present invention is the program which makes the control unit  61  execute the operations of the above flows. 
     [About Electric Power Regeneration Operation] 
     According to the system of one embodiment of the present invention, regenerative electric power can be generated by rotating the motor  64   a  or the motor  64   b . At this time, force is generated with respect to the wheel  10   a  or the wheel  10   b  in the direction stopping the rotation thereof, and this force functions as a brake. 
     Here, as a method of safety control utilizing a brake, antiskid control is given. Antiskid control is also referred to as ESC (electronic stability control) sometimes. This is a control method in which appropriate braking is operated for each wheel, when an automobile is making a turn and there is a gap between the direction that the driver intends through steering and the direction in which the automobile moves, so as to reduce the gap. 
     In one embodiment of the present invention, the braking operation at the time when such antiskid control is operated is performed by a motor, whereby regenerative electric power can be obtained. 
     The method of antiskid control is described with reference to  FIGS. 9(A)  and (B). 
     In  FIGS. 9(A)  and (B), how an automobile  90  makes a turn is shown. Here, a case where the antiskid control is operated is indicated by a solid line, and a case without the antiskid control is indicated by a dashed line. 
       FIG. 9(A)  shows an oversteering condition. In other words, it is a case where the ground friction force of the rear wheels becomes smaller than the centrifugal force and the rear wheels deviate outward at the lime of turning, so that the direction in which the car body moves deviates inward from the curve. At this time, when appropriate braking is applied to the front wheel that is outside with respect to the turning direction, as indicated by an arrow  91  in the drawing, the automobile can turn the curve with an appropriate turning radius. 
       FIG. 9(B)  shows an understeering condition. In other words, it is a case where the ground friction force of the front wheels becomes smaller than the centrifugal force and the front wheels deviate outward at the time of turning, so that the direction in which the car body moves deviates outward from the curve. At this time, when appropriate braking is applied to the rear wheel that is inside with respect to the turning direction, as indicated by an arrow  92  in the drawing, the automobile can turn the curve with an appropriate turning radius. 
     Note that, although an electric power regeneration operation in the antiskid control is described here, one embodiment is not limited to this, and regenerative electric power can be obtained by various controls using a brake. For example, regenerative electric power can be obtained also by a braking operation in an antilock brake system (ABS), a collision avoidance system, a system for reducing an impact at the time of a collision, or the like. 
     The above is the description of the electric power regeneration operation. 
     [Other Structure Examples of System] 
     Hereinafter, examples of the system having a structure different from the system  80  illustrated in  FIG. 1  will be described. Note that in the following description, description overlapping with the above description might be omitted. 
     Structure Example 1 
     A system  80   a  illustrated in  FIG. 10  is different from the system  80  illustrated in  FIG. 1  in that each of the four wheels includes a motor and a battery. 
     The system  80   a  includes a wheel  10   c , a wheel  10   d , a battery  20   c , a battery  20   d , a motor  64   c , a motor  64   d , an electric power control unit  62   c , and an electric power control unit  62   d . The electric power control unit  62   c  is connected to the battery  20   c  and the motor  64   c . The electric power control unit  62   d  is connected to the battery  20   d  and the motor  64   d.    
     Each of the electric power control unit  62   a , the electric power control unit  62   b , the electric power control unit  62   c , and the electric power control unit  62   d  is controlled by the control unit  61 , and has a structure capable of transmitting electric power to each other. 
     Structure Example 2 
     A system  80   b  illustrated in  FIG. 11  includes an electric power control unit  72  instead of the four electric power control units in  FIG. 10 . 
     The electric power control unit  72  has a function of supplying electric power selectively to the batteries, a function of outputting electric power selectively from the batteries, a function of supplying electric power selectively to the motors, a function of converting regenerative electric power output from the motors, and the like. The use of one electric power control unit  72  in which functions are integrated in this manner can increase the design flexibility of the car body  50 , in addition to reducing the number of components. 
     Furthermore, since the frequency of electric power conversion can be reduced in the case where electric power is transmitted between the batteries, the efficiency of electric power transmission between the batteries can be increased. 
     Structure Example 3 
     A system  80   c  illustrated in  FIG. 12  shows an example where the motors provided in the wheels in  FIG. 10  are placed in the car body  50  side. 
     By providing the motor  64   a , for example, in the car body  50  in this manner, the motor  64   a  and the electric power control unit  62   a  that constitute a pair can be placed close to each other; thus, the transmission efficiency of electric power can further be increased. 
     In addition, the structure of the wheel  10   a  and the like can be simplified, so that replacement of the wheel  10   a  and the like is easier. Furthermore, the wheel  10   a  and the like can be more lightweight. 
     Structure Example 4 
     A system  80   d  illustrated in  FIG. 13  is different from the system  80  illustrated in  FIG. 1  in that the motor  64   a  and the motor  64   b  are provided in the car body  50  and that one electric power control unit  72  that is connected to the batteries is included. 
     Structure Example 5 
     A system  80   e  illustrated in  FIG. 14  shows an example where the two wheels (the wheel  10   a  and the wheel  10   b ) are driven by one motor  64  provided in the car body  50 . Furthermore, the system  80   e  includes one electric power control unit  72  that is connected to the batteries. 
     Structure Example 6 
     A system  80   f  illustrated in  FIG. 15  shows an example where an engine  63 , which is an internal-combustion engine, is added to the system  80   e  illustrated in  FIG. 14 . It can be said that the automobile that can be applied to the system  80   f  is a hybrid car, which can move with two kinds of power, the engine and the motor. 
     The control unit  61  can control the operation of each of the motor  64  and the engine  63 . Accordingly, switching between a mode of driving only with the engine  63 , a mode of driving only with the motor  64 , and a mode of driving with the use of the engine  63  and the motor  64  in combination is possible. 
     Furthermore, the engine  63  also functions as a generator. Electric power generated by the engine  63  is supplied through the electric power control unit  72  to the battery  65 , the battery  20   a , or the battery  20   b , or the electric power is supplied to the control unit  61 , the braking control unit  66 , the motor  64 , or the like. 
     Note that a structure where the engine  63  is used as a generator without being used as power for driving the wheel  10   a  and the wheel  10   b  may be employed. 
     The above is the description of the other structure examples of the system. 
     At least part of this embodiment can be implemented in combination with any of the other embodiments described in this specification as appropriate. 
     Embodiment 2 
     In this embodiment, a structure example of a wheel that can be used in the electric power transmission system or the like described in Embodiment 1 as an example, and a vehicle (e.g., an automobile) to which the wheel can be attached will be described. 
     [Structure Example of Wheel] 
     In  FIG. 16(A) , a schematic perspective view of a wheel  10  of one embodiment of the present invention is shown. The wheel  10  includes a rim portion  11 , a disk portion  12 , a battery  20 , and a connector  21 . 
     The wheel  10  can be installed in a vehicle such as an automobile using a tire, or the like. An automobile is an embodiment of a vehicle. As an automobile, a specialized vehicle such as a civil engineering work vehicle and a crane truck is included, in addition to a car, a truck, and a bus. In addition, the wheel  10  of one embodiment of the present invention can be installed in a one-wheeled, two-wheeled, or three-wheeled vehicle or a vehicle with five or more tires, in addition to a four-wheeled vehicle. As a two-wheeled vehicle, a structure with two wheels attached to a car body, one behind the other, like a motorcycle may be employed, or a structure with two tires provided on sides of a car body face-to-face may be employed. The system can also be applied to a bicycle, an electric bicycle, a power-assisted bicycle, a tire for an airplane, a tire for a helicopter, a tire for a vertical take-off and landing aircraft, an amphibious car, a tank, or the like. 
     In addition, the wheel can be applied to a vehicle that does not use a tire. For example, it can be used for a wheel of a car that moves on a rail as a guideway. For example, it can be used for a vehicle such as a railroad (including an electric train, a steam train, a steam locomotive, and the like), a streetcar, a cable car, and the like. 
     Furthermore, one embodiment of the present invention can be applied to a toy that copies the above-mentioned vehicle. 
     In the rim portion  11 , as it gets closer to the outside in a width direction, the radius of curvature gets larger. Furthermore, the rim portion  11  has a cylindrical portion  15  at the central part in the width direction. The battery  20  is provided in a state of being bent along the portion  15  of the rim portion  11 . In  FIG. 16(A) , an example where the battery  20  is provided inside the portion  15  of the rim portion  11  is shown. 
     The disk portion  12  includes a plurality of bolt holes  13  for attachment to the car body  50  described later. Furthermore, the connector  21  is provided in the disk portion  12 . The connector  21  is electrically connected to the battery  20 . In addition, the connector  21  includes a contact point to be electrically connected to a connector of the car body of the automobile or the like. 
       FIG. 16(B)  is a schematic cross-sectional view in a circumferential direction of the rim portion  11 . In  FIG. 16(B) , the disk portion  12  and the like are indicated by dashed lines to show the position relation. 
     The rim portion  11  has a two-layered structure at the cylindrical portion  15 , and has a space therein. The battery  20  is placed in the space inside the rim portion  11 . The battery  20  is placed in a state of being curved along the curvature of the rim portion  11 . In other words, the battery  20  is placed so as to be wrapped around part of the cylindrical portion  15  of the rim portion  11 . Such a structure is preferable because unbalance of the center of gravity of the wheel  10  can be inhibited. 
     The battery  20  preferably has a function of being bent along a curved surface. In particular, the battery  20  preferably is a secondary battery sealed with a film. The radius of curvature with which the battery  20  can be bent is a radius of curvature that is at least smaller than the inner diameter of the rim portion  11 . The detail of a secondary battery that is suitable for the battery  20  will be described later. 
     The battery  20  is preferably fixed to the rim portion  11  with an adhesive or a pressure sensitive adhesive. At this time, it is preferable to use an adhesive or a pressure sensitive adhesive from which the battery  20  can be detached without being damaged, in which case the replacement of the battery  20  when deteriorating is easy. 
     Note that although a structure where the battery  20  is fixed to the rim portion  11  is described here, a structure where the battery  20  is not fixed to the rim portion  11  may also be employed. For example, with a structure where the wheel  10  has a support portion to which the battery  20  is fixed and the support portion idles or does not rotate with respect to the car body when the rim portion  11  rotates with respect to the car body, the weight of a portion of the wheel  10  that rotates can be made lighter and the motion performance of the automobile can be improved. 
     The battery  20  includes a terminal  22 . The terminal  22  of the battery  20  and the connector  21  provided on the disk portion  12  are electrically connected to each other through a cable  23 . The cable  23  is provided inside the disk portion  12 . It is preferable that the terminal  22  and the cable  23 , or the cable  23  and the connector  21  have a detachable mechanism, in which case replacement of the battery  20  is easy. 
     Note that although a structure where the battery  20  includes only the terminal is employed here, a structure where the battery  20  includes a BMU (battely management unit) may also be employed. The BMU can monitor overcharge and overdischarge of the battery  20 , monitor overcurrent, control a cell balancer, manage the deterioration condition of the battery, calculate the battery remaining ((charging rate) state of charge: SOC), control a cooling fan of a driving secondary battery, and control detection of failure, for example. Furthermore, when the BMU is provided in the battery  20 , it is preferable to have a function of outputting data of the battery  20  obtained by the BMU to the electric power control unit  62  included in the car body  50  described later. 
     In  FIG. 16(C) , a schematic cross-sectional view of the rim portion  11  in a width direction, in a state of being attached to the car body  50  of an automobile is shown. 
       FIG. 16(C)  shows a case where the disk portion  12  is shaped such that part thereof has a space. The cable  23  is placed in the space of the disk portion  12  and electrically connected to the connector  21  located at the center of the disk portion  12 . 
     The car body  50  includes a fixing portion  51  and a connector  52 . The fixing portion  51  has a function of transmitting power from a device (engine device, motor) that generate power, such as an engine or a motor included in the car body  50 , to the wheel  10 . With the rotation of the fixing portion  51 , the wheel  10  that is fixed to the fixing portion  51  can be rotated. The fixing portion  51  can be fixed to the disk portion  12  with a bolt in a region not shown in the drawing. The connector  52  has a contact point at the end to be electrically connected to the connector  21 , and has a mechanism to be engaged with the connector  21 . 
     The connector  52  preferably has a mechanism with which electrical connection is not broken by the rotation, for example. A rotatable connector (rotary connector) formed using liquid metal, such as mercury or gallium, a slip ring having a brush, or the like can be used, for example. Use of the rotatable connector can prevent problems caused by wear and thus is preferable. 
     Since the connector  21  and the connector  52  are electrically connected to each other, electric power charged in the battery  20  can be supplied to the car body  50 . Furthermore, it is also possible to charge the battery  20  with electric power input from the car body  50 . Furthermore, a structure where data from the above-mentioned BMU can be transmitted between the connector  21  and the connector  52  may be employed. 
     Thus, the wheel  10  of one embodiment of the present invention can be used as an auxiliary power source of an automobile, for example. Furthermore, in the case where the battery  20  having a sufficient capacity is mounted on the wheel  10 , it becomes possible to use the battery  20  as a main power source of the automobile, without a power source being mounted on the automobile. With the use of the wheel  10  like this, it becomes possible to reduce the volume of a battery mounted on the automobile, so that space-saving in the automobile can be achieved. In addition, the design flexibility of the automobile can be improved. For example, the living space or the space of the trunk can be expanded. 
     Although a case where the connector  21  and the connector  52  are used as electric power transmission mechanisms respectively provided in the wheel  10  and the car body  50  is described above, a structure utilizing transmission and reception of electric power using an electromagnetic induction method, a magnetic resonance method, an electric wave method, or the like (also referred to as contactless electric power transmission, non-contact electric power transmission, wireless power feeding, or the like) requires no physical contact point and thus is preferable. In  FIG. 17 , an example where contactless electric power transmission is performed between the wheel  10  and the car body  50  is shown. 
     In  FIG. 17 , the wheel  10  includes a circuit  25  and an antenna  26 , instead of the above connector  21 . Here, a structure including the circuit  25  and the antenna  26  may be referred to as a wireless module. The circuit  25  is electrically connected to the battery  20  through the cable  23 . Furthermore, the antenna  26  is electrically connected to the circuit  25 . 
     The circuit  25  has a function of transmitting electric power of the battery  20  through the antenna  26  to an antenna  53  attached to the car body  50 . In addition, the circuit  25  has a function of converting electric power received by the antenna  26  to electric power supplied to the battery  20 . 
     The car body  50  includes the antenna  53 , a cable  54 , and an antenna support portion  55 , instead of the connector  52 . The antenna  53  is attached to a position that faces the antenna  26  when the wheel  10  is attached to the car body  50 . The antenna support portion  55  has a function of supporting the antenna  53 . The antenna  53  and the antenna support portion  55  may have a hole or a notch portion so as not to physically interfere with the bolt for fixing the fixing portion  51  and the wheel  10 . The cable  54  has a function of electrically connecting a circuit (not shown) provided inside the car body  50  and the antenna  53  to each other. As the circuit, one having a similar function to that of the above circuit  25  can be used. 
     In  FIG. 17 , an example where a window portion  27  is provided in part of the disk portion  12  that is located between the antenna  26  and the antenna  53  is shown. For the window portion  27 , a material that does not inhibit the propagation of signals between the antenna  26  and the antenna  53  can be used. The material of the window portion  27  can be appropriately selected in accordance with the method of contactless electric power transmission; for example, a material with a higher insulating property, higher permittivity, or less likelihood of shielding wireless signals, electric waves, electromagnetic waves, or the like, than a material used for the disk portion  12  can be used. 
     With such a structure, transmission and reception of electric power can be easily performed even when the wheel  10  rotates. Furthermore, the structure is preferable because it does not have a physical contact point and problems of wear and damage do not occur. 
     In  FIGS. 18(A)  and (B), examples of modes of the rim portion  11  different from the above are shown. 
     Although an example where the rim portion  11  has a hollow structure and the battery  20  is provided inside the rim portion  11  is described above, one embodiment is not limited to this; a structure where the battery  20  is wrapped around or pasted on the surface of the rim portion  11  may be employed. In  FIG. 18(A) , an example where the battery  20  is wrapped around the outer circumference of the rim portion  11  is shown. Here, when a tire (not shown) is attached to the wheel  10 , the surface of the outer circumference of the rim portion  11  is covered with the tire. Accordingly, the battery  20  would not be exposed even when the battery  20  is wrapped around the outer circumference of the rim portion  11  in this manner, which is preferable. Furthermore, in the case where the exterior of the battery  20  has sufficient weatherability, a structure where the battery  20  is provided along the inner circumference of the rim portion  11  as shown in  FIG. 18(B)  may be employed. 
     In  FIGS. 19(A)  and (B), examples of different modes of the battery  20  are shown. 
     Although examples where the battery  20  covers a range shorter than one lap of the circumference of the cylindrical portion  15  of the rim portion  11  are described above, a structure where the battery  20  is wrapped to cover more than one lap may be employed. In  FIG. 19(A) , an example where the battery  20  is wrapped about two laps with respect to the rim portion  11  is shown. The longer the length of the battery  20  is, the larger the capacity of the battery  20  can be, which is preferable. 
     Furthermore, as shown in  FIG. 19(B) , a structure where the battery  20  includes a plurality of belt-like secondary batteries having the terminal  22  in common may be employed. For example, a structure where a plurality of the batteries  20  shown in  FIG. 19(B)  or the like is stacked and used can be employed. With such a structure, a resistance component of the battery  20  can be reduced. Although a larger number of members such as a film of the battery  20  are required, as compared with the structure shown in  FIG. 19(A) , the size of one secondary battery can be made comparatively small, so that large-sized equipment need not be introduced for the manufacture, which is preferable. 
     In  FIGS. 20(A) , (B), and (C), examples of using batteries with different modes are shown. 
     Although the case where the belt-like battery  20  sealed with a film is used is described above, batteries with different shapes can be used. 
     In  FIG. 20 (A 1 ), an example where cylindrical batteries  41  are used is shown. In addition, the external appearance of the battery  41  is shown in  FIG. 20 (A 2 ). The battery  41  is sealed with a cylindrical exterior member, and includes a pair of terminals  45 . With the use of a plurality of cylindrical batteries  41 , the batteries  41  can be placed at high density inside the rim portion  11 . 
     In  FIG. 20 (B 1 ), an example where prism-like batteries  42  are used is shown. In addition, the external appearance of the battery  42  is shown in  FIG. 20 (B 2 ). With the use of a plurality of prism-like batteries  42 , capacity per volume of one battery  42  can be increased, and the number of batteries to be provided in one wheel  10  can be reduced as compared with the case where the cylindrical batteries  41  are used. 
     In  FIG. 20 (C 1 ), an example where columnar batteries  43  each having a curved surface in part thereof are used is shown. In addition, the external appearance of the battery  43  is shown in  FIG. 20 (C 2 ). It is preferable that the radius of curvature of the curved surface of the battery  43  be substantially the same as the radius of curvature of the inner wall of the rim portion  11 . In that case, as shown in  FIG. 20 (C 1 ), the space between the battery  43  and the rim portion  11  can be reduced and the batteries  43  can be placed at high density. The battery  43  may have a columnar shape whose section has roughly a fan-like shape, for example. 
     The battery  41 , the battery  42 , and the battery  43  described here as examples need not have a bendable function; a battery that is sealed with an exterior member with high hardness such as a metal may be used therefor. Furthermore, as each of the battery  41 , the battery  42 , and the battery  43 , a winding or stacked secondary battery can be used. 
     The above is the description of the structure examples of the wheel. 
     Application Example 
     Structure examples of the wheel of one embodiment of the present invention and an automobile to which the wheel can be attached will be described below. 
     In  FIG. 21 , a block diagram that describes major structures of the car body  50  of the automobile and the wheel  10  is shown. Here, a structure of a hybrid car that uses both an engine and a motor as devices (engine device, motor) generating power will be described as an example. 
     The wheel  10  includes the battery  20  and an electric power transmission mechanism  30 . 
     The above-mentioned connector  21  and the wireless module including the circuit  25  and the antenna  26  correspond to the electric power transmission mechanism  30 . 
     The car body  50  is provided with an electric power transmission mechanism  60 , the control unit  61 , the electric power control unit  62 , the engine  63 , the motor  64 , the battery  65 , and the like. 
     The above-mentioned connector  52 , a wireless module including the antenna  53 , the cable  54 , the circuit, and the like correspond to the electric power transmission mechanism  60 . 
     The electric power transmission mechanism  30  and the electric power transmission mechanism  60  have structures that can transmit/receive electric power to/from each other; the above structures are just examples and one embodiment is not limited thereto. 
     The engine  63  and the motor  64  are devices that produce power for rotating the wheel  10 . The motion of the engine  63  is controlled by the control unit  61 . The motor  64  is driven by electric power supplied from the electric power control unit  62 . 
     The control unit  61  has a function of controlling power of the automobile. Specifically, it can control driving of the engine  63 , the electric power control unit  62 , and the like. In addition to that, the control unit  61  may have a function of comprehensibly controlling a variety of electronically-controlled auxiliary devices. As a typical example of the control unit  61 , an ECU (engine control unit) can be used. Furthermore, an ECU having a function that is unique to an EV, an HEV, or a PHEV is preferably used in accordance with the driving method of the automobile. 
     The electric power control unit  62  controls the amount of electric power supply to the motor  64 , in response to orders from the control unit  61 . The electric power control unit  62  can be referred to as a PCU (power control unit). 
     Furthermore, the electric power control unit  62  preferably has a function of switching the battery  65  included in the car body  50  and the battery  20  included in the wheel  10 , in accordance with the states of charge of the batteries. In the case where the battery  65  is used as a main power source, for example, when the charging rate of the battery  65  decreases to a certain level or lower, the motor  64  and the like can be driven with the use of electric power from the battery  20 . With such operation, the battery  20  can be used as an auxiliary power source. 
     In the case of managing the states of charge of the battery  65  and the battery  20 , the electric power control unit  62  has the function of the above-mentioned BMU; alternatively, the battery  65  and the battery  20  have structures with the BMU, and the electric power control unit  62  may take control in accordance with data supplied from the BMUs. 
     Furthermore, it is preferable that the electric power control unit  62  have a function of charging the battery  65  and the battery  20  by using electric power generated by the motor  64  at the time of deceleration (also referred to as an electric power regeneration function). 
     As the electric power control unit  62 , a structure including a step-up circuit (converter) that increases a voltage output from the battery  65  or the battery  20  to a voltage driving the motor  64 , a conversion circuit (inverter) that converts a direct current voltage to an alternating current voltage for driving the motor  64 , and a computer that controls these may be employed. In addition, in the case where the electric power regeneration function is added, a conversion circuit that converts an alternating current voltage output from the motor  64  to a direct current voltage, a step-down circuit that lowers it to a voltage for charging the battery  65  and the battery  20 , and the like are preferably provided. 
     Note that although the structure of an HEV provided with both the engine  63  and the motor  64  is described as an example here, a structure in which the engine  63  is not provided is employed for an EV. Furthermore, for a PHEV, a structure further provided with a socket is employed, the structure in which the electric power control unit  62  has a function of controlling charging of the battery  65  and the battery  20 , using electric power supplied from the outside through the socket. 
     The above is the description of an application example. 
     Modification Examples 
     Application examples which are partly different from the above application example will be described below. 
     In  FIG. 22(A) , a schematic view of the car body  50  to which the wheels  10  are attached is shown. The car body  50  shown in  FIG. 22(A)  includes one control unit  61 , four motors  64 , and four electric power control units  62 . One motor  64  and one electric power control unit  62  are placed near a portion to which one wheel  10  is attached. 
     The control unit  61  can control the four electric power control units  62 . Electric power of the battery  20  included in one wheel  10  is converted to electric power for driving the motor  64  by the electric power control unit  62  placed nearby. With the rotation of the motor  64 , the wheel  10  connected thereto rotates. 
     In this manner, when a structure where each of the four wheels  10  is provided with a motor such as the motor  64  is employed, each of the four wheels  10  can rotate independently. In addition, the rotation directions of the four wheels  10  can be controlled separately from each other. Accordingly, move of the car body  50  in a direction that was impossible for a conventional automobile becomes possible, such as move of the car body  50  in a lateral direction, rotation of the car body  50  at the place, or the like. 
     In this manner, placing the electric power control unit  62  and the motor  64  near the wheel  10  can reduce loss of electric power from the battery  20  to the electric power control unit  62 . 
     In  FIG. 22(B) , an example where the wheel  10  includes the electric power control unit  62  and the motor  64  in addition to the battery  20  is shown. A structure where the wheel  10  is provided with the motor  64  can be referred to as an in-wheel motor. With such a structure, the motor  64  provided in each wheel  10  can be driven by electric power of the battery  20 . With such a structure, move of the car body  50  is possible even when the car body  50  is heavily damaged in an accident, for example. 
     In addition, since the electric power control unit  62  and the motor  64  can be provided in the wheel  10 , only mechanisms such as the control unit  61  that controls the four wheels  10  need to be placed in the car body  50 , and the structure thereof can be significantly simplified. Accordingly, the design flexibility of the car body  50  can be improved, and space-saving can be achieved. 
     Note that in  FIGS. 22(A)  and (B), a structure including the above-mentioned battery  65 , engine  63 , and the like may be employed. 
     The above is the description of the modification examples. 
     Although the structure where the battery  20  is included in the wheel  10  to which a tire is attached is described above as one embodiment of the present invention, one embodiment is not limited thereto. A structure where the battery  20  is provided in a wheel having a function of rotating may be employed, for example. When such a structure is applied to a wheel of the car body, the battery  20  included in the wheel can be used as an auxiliary power source or a main power source. As an example, the battery  20  can be applied to a flywheel. In particular, a structure where the battery  20  is provided in a position close to the outer circumference of a flywheel can increase the moment of inertia generated by the flywheel. 
     Here, a semiconductor device such as a transistor in which silicon carbide, gallium nitride, or an oxide semiconductor is used is preferably used in an electronic component included in the electric power transmission mechanism  60 , the control unit  61 , the electric power control unit  62 , or other devices in the car body, or an electronic component used for the electric power transmission mechanism  30 , the BMU, or the like in the wheel  10 . 
     In particular, an oxide semiconductor having a wider band gap than silicon is preferably used. The use of a semiconductor material having a wider band gap and a lower carrier density than silicon is preferable because off-state current of the transistor can be reduced. 
     For example, the oxide semiconductor preferably contains at least at least indium (In) or zinc (Zn), More preferably, the oxide semiconductor contains In-M-Zn-based oxide (M is a metal such as Al, Ti, Ga, Ge, Y, Zr, Sn, La, Ce, or Hf). 
     As the semiconductor layer, it is particularly preferable to use an oxide semiconductor film including a plurality of crystal parts whose c-axes are aligned perpendicular to a surface on which the semiconductor layer is formed or the top surface of the semiconductor layer and in which a grain boundary is not observed between adjacent crystal parts. 
     Such an oxide semiconductor does not have a grain boundary and thus is excellent in stability of electrical characteristics. 
     The use of such materials for the semiconductor layer makes it possible to provide a highly reliable transistor in which a change in the electrical characteristics is suppressed. 
     Furthermore, its low off-state current enables retention of charges accumulated in a capacitor that is series-connected to the transistor over a long period of time. The use of such a transistor enables fabrication of an electronic component in which power consumption is considerably reduced. 
     Note that one embodiment of the present invention is not limited thereto. The example where one embodiment of the present invention is applied to a wheel is described; however, one embodiment is not limited thereto, for example. Various embodiments of the invention are described in this embodiment and the other embodiments, and one embodiment of the present invention is not limited to a particular embodiment. For example, one embodiment of the present invention can be applied to a wheel, a thing equivalent to a wheel, or a thing other than a wheel. 
     This embodiment can be implemented in combination with any of the other embodiments described in this specification as appropriate. 
     Embodiment 3 
     The electric power control system described as an example in Embodiment 1 or the wheel described as an example in Embodiment 2 can be applied to an automobile using a tire. An automobile is an embodiment of a vehicle. As an automobile, a specialized vehicle such as a civil engineering work vehicle and a crane truck is included, in addition to a car, a truck, and a bus. In addition, the wheel  10  of one embodiment of the present invention can be installed in a one-wheeled, two-wheeled, or three-wheeled vehicle or a vehicle with five or more tires, in addition to a four-wheeled vehicle. As a two-wheeled vehicle, a structure with two wheels attached to a car body, one behind the other, like a motorcycle may be employed, or a structure with two tires provided on sides of a car body face-to-face may be employed. The system can also be applied to a bicycle, an electric bicycle, a power-assisted bicycle, a tire for an airplane, a tire for a helicopter, a tire for a vertical take-off and landing aircraft, an amphibious car, a tank, or the like. 
       FIG. 23  illustrates examples of a vehicle using one embodiment of the present invention. An automobile  8400  illustrated in  FIG. 23(A)  is an electric vehicle that uses an electric motor as a power source for driving. Alternatively, it is a hybrid vehicle capable of selecting and using either an electric motor or an engine as a power source for driving as appropriate. The use of one embodiment of the present invention allows fabrication of a high-mileage vehicle. Furthermore, the automobile  8400  includes a secondary battery. The secondary battery is capable of supplying electric power to a light-emitting device such as a headlight  8401  or a room light (not illustrated), in addition to driving the electric motor. 
     The secondary battery can also supply electric power to a display device included in the automobile  8400 , such as a speedometer or a tachometer. Furthermore, the secondary battery can supply electric power to a semiconductor device included in the automobile  8400 , such as a navigation system. 
     An automobile  8500  illustrated in  FIG. 23(B)  can be charged when the secondary battery (not shown) of the automobile  8500  is supplied with electric power from external charging equipment by a plug-in system, a contactless power feeding system, or the like. In  FIG. 23(B) , a state in which the secondary battery included in the automobile  8500  is charged from a ground-based charging apparatus  8021  through a cable  8022  is shown. In charging, a given method such as CHAdeMO (registered trademark) or Combined Charging System may be employed as a charging method, the standard of a connector, or the like as appropriate. The charging apparatus  8021  may be a charging station provided in a commerce facility or a household power source. For example, with the use of a plug-in technique, the secondary battery (not shown) included in the automobile  8500  can be charged by being supplied with electric power from the outside. The charge can be performed by converting alternating current electric power into direct current electric power through a converter such as an AC-DC converter. 
     Furthermore, although not shown, the vehicle may include a power receiving device so that it can be charged by being supplied with electric power from an above-ground power transmitting device in a contactless manner. In the case of the contactless power feeding system, by fitting a power transmitting device in a road or an exterior wall, charge can be performed not only when the vehicle is stopped but also when driven. In addition, the contactless power feeding system may be utilized to perform transmission and reception of electric power between vehicles. Furthermore, a solar cell may be provided in the exterior of the vehicle to charge the secondary battery when the vehicle stops or moves. To supply electric power in such a contactless manner, an electromagnetic induction method or a magnetic resonance method can be used. 
     In  FIG. 23(C) , an electric two-wheeled vehicle  8600  is shown. The electric two-wheeled vehicle  8600  includes a car body  8601 , a wheel  8602 , a tire  8603 , a handlebar  8604 , an operation lever  8605 , and the like. 
     A passenger can ride on the car body  8601  standing up. The electric two-wheeled vehicle  8600  includes a gyroscope sensor and a computer in the car body  8601 , and the moving direction and speed can be controlled in accordance with a change in position of the center of gravity. For example, it can go forward when the passenger leans forward and the center of gravity moves forward, and stop or go backward when the passenger leans backward and the center of gravity moves backward. In addition, it can make a turn when the passenger moves the center of gravity rightward or leftward. 
     In the electric two-wheeled vehicle  8600 , a motor, a battery, the other control device and the like are provided in the car body  8601  or the wheel  8602 . 
     A light-emitting device is provided at the edge of the handlebar  8604  and functions as a winker that indicates the turning direction to other people around. 
     The operation lever  8605  is provided for performing a brake operation, for example. En addition, the operation lever  8605  can perform various operations such as power on/off operations, a winker operation, and a lock operation, in addition to a brake operation. 
     According to one embodiment of the present invention, an automobile provided with a battery in which more space-saving is achieved than in a conventional one can be fabricated. In addition, an automobile with improved design flexibility can be fabricated. An automobile capable of utilizing electric power at high efficiency can be fabricated. 
     Furthermore, the cycle characteristics of a secondary battery become better and the reliability can be improved. Furthermore, according to one embodiment of the present invention, characteristics of a secondary battery can be improved, and thus the secondary battery itself can be made compact and lightweight. The compact and lightweight secondary battery contributes to a reduction in the weight of a vehicle, and thus increases the mileage. Furthermore, the secondary battery included in the vehicle can be used as a source for supplying electric power to other products than the vehicle. In such a case, the use of a commercial power source can be avoided at peak time of electric power demand. 
     Embodiment 4 
     A structure example of a secondary battery that can be used for the battery  20  of one embodiment of the present invention and an example of a manufacturing method thereof will be described below with reference to drawings. An example of a bendable secondary battery will be described below. 
     Structure Example 
       FIG. 24  is a perspective view showing an external appearance of a secondary battery  102 .  FIG. 25(A)  is a cross-sectional view of a portion indicated by dashed-dotted line A 1 -A 2  in  FIG. 24 . In addition,  FIG. 25(B)  is a cross-sectional view of a portion indicated by dashed-dotted line B 1 -B 2  in  FIG. 24 . 
     The secondary battery  102  of one embodiment of the present invention includes, in an exterior body  507 , a positive electrode  511  covered with a separator  503 , a negative electrode  515 , and an electrolyte solution  504 . Note that in  FIG. 24  and  FIG. 25  is shown an example of the secondary battery that includes one positive electrode including a positive electrode active material layer  502  on one side of a positive electrode current collector  501 , one positive electrode including a positive electrode active material layer  502  on each side of a positive electrode current collector  501 , one negative electrode including a negative electrode active material layer  506  on one side of a negative electrode current collector  505 , and one positive electrode including a negative electrode active material layer  506  on each side of a negative electrode current collector  505 . The positive electrode  111  is electrically connected to a positive electrode lead  121 , and the negative electrode  115  is electrically connected to a negative electrode lead  125 . The positive electrode lead  121  and the negative electrode lead  125  are also referred to as lead electrodes or lead terminals. Parts of the positive electrode lead  121  and the negative electrode lead  125  are positioned outside the exterior body. Furthermore, the secondary battery  102  is charged and discharged through the positive electrode lead  121  and the negative electrode lead  125 . 
     Note that, although in  FIG. 25  the positive electrode  111  is covered with the separator  503 , one embodiment of the present invention is not limited thereto. The positive electrode  111  need not necessarily covered with the separator  503 , for example. The negative electrode  115 , instead of the positive electrode  111 , may be covered with the separator  503 , for example. 
     [Positive Electrode] 
     The positive electrode  511  is made up of the positive electrode current collector  501 , the positive electrode active material layer  502  formed over the positive electrode current collector  501 , and the like. Although in  FIG. 25  is shown the example including one positive electrode  511  including the positive electrode active material layer  502  on one side of the positive electrode current collector  501  with a sheet shape (or a band-like shape) and one positive electrode  511  including the positive electrode active material layer  502  on each side of the positive electrode current collector  501 , one embodiment of the present invention is not limited thereto. Only the positive electrodes  511  each including the positive electrode active material layer  502  on one side of the positive electrode current collector  501  may be used. Only the positive electrodes  511  each including the positive electrode active material layer  502  on each side may also be used. The use of the positive electrodes  511  including the positive electrode active material layer  502  on each side allows increase in the capacity of the secondary battery  102 . In addition, the secondary battery  102  including three or more positive electrodes  511  may be employed. An increase in the number of positive electrodes  511  in the secondary battery  102  allows an increase in the capacity of the secondary battery  102 . 
     For the positive electrode current collector  501 , a material that has high conductivity and does not dissolve at the potential of the positive electrode, such as a metal such as stainless steel, gold, platinum, aluminum, or titanium, or an alloy thereof can be used. An aluminum alloy to which an element that improves heat resistance, such as silicon, titanium, neodymium, scandium, or molybdenum, is added may also be used. Furthermore, it may be formed using a metal element that forms silicide by reacting with silicon. As the metal element that forms silicide by reacting with silicon, zirconium, titanium, hafnium, vanadium, niobium, tantalum, chromium, molybdenum, tungsten, cobalt, nickel, and the like are given. As the shape of the positive electrode current collector  501 , a foil-like shape, a plate-like shape (a sheet-like shape), a net-like shape, a punching-metal shape, an expanded-metal shape, or the like can be used as appropriate. The positive electrode current collector  501  preferably has a thickness of greater than or equal to 5 μm and less than or equal to 30 μm. Furthermore, the surface of the positive electrode current collector  501  may be provided with an undercoat layer using graphite or the like. 
     The positive electrode active material layer  502  may include, in addition to a positive electrode active material, a binder for increasing adhesion of the positive electrode active material, a conductive additive for increasing the conductivity of the positive electrode active material layer  502 , and the like. 
     As the positive electrode active material used for the positive electrode active material layer  502 , a composite oxide having an olivine crystal structure, a layered rock-salt crystal structure, or a spinel crystal structure, and the like are given. As the positive electrode active material, a compound such as LiFeO 2 , LiCoO 2 , LiNiO 2 , LiMn 2 O 4 , V 2 O 5 , Cr 2 O 5 , or MnO 2  is used, for example. 
     In particular, LiCoO 2  is preferable because it has advantages such as high capacity, higher stability in the air than LiNiO 2 , and higher thermal stability than LiNiO 2 . 
     Furthermore, it is preferable to add a small amount of lithium nickel oxide (LiNiO 2  or LiNi 1-x M x O 2  (0&lt;x&lt;1) (M=Co, Al, or the like)) to a lithium-containing material having a spinel crystal structure which contains manganese such as LiMn 2 O 4  because characteristics of the secondary battery using such a material can be improved. 
     A complex material (general formula LiMPO 4  (M is one or more of Fe(II), Mn(II), Co(II), and Ni(II))) can also be used. Typical examples of the general formula LiMPO 4  which can be used as a material are lithium compounds such as LiFePO 4 , LiNiPO 4 , LiCoPO 4 , LiMnPO 4 , LiFe a Ni b PO 4 , LiFe a Co b PO 4 , LiFe a Mn b PO 4 , LiNi a Co b PO 4 , LiNi a Mn b PO 4  (a+b is less than or equal to 1, 0&lt;a&lt;1, and 0&lt;b&lt;1), LiFe c Ni d Co e PO 4 , LiFe c Ni d Mn e PO 4 , LiNi c Co d Mn e PO 4  (c+d+e is less than or equal to 1, 0&lt;c&lt;1, 0&lt;d&lt;1, and 0&lt;e&lt;1), and LiFe f Ni g Co h Mn i PO 4  (f+g+h+i is less than or equal to 1, 0&lt;f&lt;1, 0&lt;g&lt;1, 0&lt;h&lt;1, and 0&lt;i&lt;1). 
     In particular, LiFePO 4  is preferable because it meets requirements with balance for the positive electrode active material, such as safety, stability, high capacity density, and the existence of lithium ions that can be extracted in initial oxidation (charging). 
     A complex material such as general formula Li( 2-j )MSiO 4  (M is one or more of Fe(II), Mn(II), Co(II), and Ni(II), 0≤j≤2) can also be used. Typical examples of the general formula Li( 2-j )MSiO 4  which can be used as a material are lithium compounds such as Li( 2-j )FeSiO 4 , Li( 2-j )NiSiO 4 , Li( 2-j )CoSiO 4 , Li( 2-j )MnSiO 4 , Li( 2-j )Fe k Ni l SiO 4 , Li( 2-j )Fe k Co l SiO 4 , Li( 2-j )Fe k Mn l SiO 4 , Li (2-j) Ni k Co l SiO 4 , Li( 2-j )Ni k Mn l SiO 4  (k+l is less than or equal to 1, 0&lt;k&lt;1, 0&lt;l&lt;1), Li( 2-j )Fe m Ni n Co q SiO 4 , Li( 2-j )Fe m Ni n Mn q SiO 4 , Li( 2-j )Ni m Co n Mn q SiO 4  (m+n+q is less than or equal to 1, 0&lt;m&lt;1, 0&lt;n&lt;1, 0&lt;q&lt;1), and Li( 2-j )Fe r Ni s Co t Mn u SiO 4  (r+s+t+u is less than or equal to 1, 0&lt;r&lt;1, 0&lt;s&lt;1, 0&lt;t&lt;1, 0&lt;u&lt;1). 
     A nasicon compound expressed by general formula A x M 2 (XO 4 ) 3  (A=Li, Na, or Mg, M=Fe, Mn, Ti, V, or Nb, X=S, P, Mo, W, As, or Si) can also be used as the positive electrode active material. As the nasicon compound, Fe 2 (MnO 4 ) 3 , Fe 2 (SO 4 ) 3 , and Li 3 Fe 2 (PO 4 ) 3 , and the like are given. As the positive electrode active material, a compound expressed by general formula Li 2 MPO 4 F, Li 2 MP 2 O 7 , or Li 5 MO 4  (M=Fe or Mn), a perovskite fluoride such as NaFeF 3  or FeF 3 , a metal chalcogenide (a sulfide, a selenide, or a telluride) such as TiS 2  or MoS 2 , an oxide with an inverse spinel crystal structure such as LiMVO 4 , a vanadium oxide (V 2 O 5 , V 6 O 13 , LiV 3 O 8 , or the like), a manganese oxide, an organic sulfur compound, or the like can also be used. 
     Note that, in the case where carrier ions are alkali metal ions other than lithium ions, or alkaline-earth metal ions, an alkali metal (e.g., sodium or potassium) or an alkaline-earth metal (e.g., calcium, strontium, barium, beryllium, or magnesium) instead of lithium may be used as the positive electrode active material. For example, a layered oxide containing sodium such as NaFeO 2  or Na 2/3 [Fe 1/2 Mn 1/2 ]O 2  may be used as the positive electrode active material. 
     Furthermore, a material in which two or more of the above materials are combined may be used as the positive electrode active material. For example, a solid solution in which two or more of the above materials are combined can be used as the positive electrode active material. For example, a solid solution of LiCo 1/3 Mn 1/3 Ni 1/3 O 2  and Li 2 MnO 3  can be used as the positive electrode active material. 
     Note that although not illustrated, a conductive material such as a carbon layer may be provided on a surface of the positive electrode active material layer  502 . With the provision of the conductive material such as the carbon layer, conductivity of the electrode can be increased. For example, a carbon layer coating on the positive electrode active material layer  502  can be formed by mixing a carbohydrate such as glucose at the time of baking the positive electrode active material. 
     The average particle diameter of the primary particle of the granular positive electrode active material layer  502  to be used is preferably greater than or equal to 50 nm and less than or equal to 100 mm. 
     As the conductive additive, acetylene black (AB), graphite (black lead) particles, carbon nanotubes, graphene, fullerene, or the like can be used. 
     A network for electron conduction can be formed in the positive electrode  511  by the conductive additive. The conductive additive allows maintaining of a path for electric conduction between the positive electrode active material layers  502 . The addition of the conductive additive to the positive electrode active material layer  502  leads to the positive electrode active material layer  502  having high electron conductivity. 
     As the binder, instead of polyvinylidene fluoride (PVDF) which is a typical one, polyimide, polytetrafluoroethylene, polyvinyl chloride, ethylene-propylene-diene polymer, styrene-butadiene rubber, acrylonitrile-butadiene rubber, fluorine rubber, polyvinyl acetate, polymethyl methacrylate, polyethylene, nitrocellulose or the like can be used. 
     The content of the binder with respect to the total amount of the positive electrode active material layer  502  is preferably greater than or equal to 1 wt % and less than or equal to 10 wt %, further preferably greater than or equal to 2 wt % and less than or equal to 8 wt %, and still further preferably greater than or equal to 3 wt % and less than or equal to 5 wt %. The content of the conductive additive with respect to the total amount of the positive electrode active material layer  502  is preferably greater than or equal to 1 wt % and less than or equal to 10 wt %, further preferably greater than or equal to 1 wt % and less than or equal to 5 wt %. 
     In the case where the positive electrode active material layer  502  is formed by a coating method, the positive electrode active material, the binder, and the conductive additive are mixed to form a positive electrode paste (slurry), which is applied onto the positive electrode current collector  501  and dried. 
     [Negative Electrode] 
     The negative electrode  515  is made up of the negative electrode current collector  505 , the negative electrode active material layer  506  formed over the negative electrode current collector  505 , and the like. Although in  FIG. 25  is shown the example including one negative electrode  515  including the negative electrode active material layer  506  on one side of the negative electrode current collector  505  with a sheet shape (or a band-like shape) and one negative electrode  515  including the negative electrode active material layer  506  on each side of the negative electrode current collector  505 , one embodiment of the present invention is not limited thereto. Only the negative electrodes  515  each including the negative electrode active material layer  506  on one side of the negative electrode current collector  505  may be used. In this case, the sides of the negative electrode current collectors  505 , each of which is not provided with the negative electrode active material layer  506 , are preferably placed to be in contact with each other, because the contacting sides with less friction can be made and stress generated when the secondary battery  102  is curved can be easily released. Only the negative electrodes  515  each including the negative electrode active material layer  506  on each side of the negative electrode current collector  505  may also be used. The use of the negative electrode  515  including the negative electrode active material layer  506  on each side allows increase in the capacity of the secondary battery  102 . In addition, the secondary battery  102  including three or more negative electrodes  515  may be employed. An increase in the number of negative electrodes  515  in the secondary battery  102  allows an increase in the capacity of the secondary battery  102 . 
     For the negative electrode current collector  505 , a material that has high conductivity and is not alloyed with a carrier ion of lithium or the like, such as stainless steel, gold, platinum, iron, copper, titanium, or an alloy thereof can be used. An aluminum alloy to which an element that improves heat resistance, such as silicon, titanium, neodymium, scandium, or molybdenum, is added can also be used. As the shape of the negative electrode current collector  505 , a foil-like shape, a plate-like shape (a sheet-like shape), a net-like shape, a punching-metal shape, an expanded-metal shape, or the like can be used as appropriate. The negative electrode current collector  505  preferably has a thickness greater than or equal to 5 μm and less than or equal to 30 μm. Furthermore, the surface of the negative electrode current collector  505  may be provided with an undercoat layer using graphite or the like. 
     The negative electrode active material layer  506  may include, in addition to a negative electrode active material, a binder for increasing adhesion of the negative electrode active material, a conductive additive for increasing the conductivity of the negative electrode active material layer  506 , and the like. 
     There is no particular limitation on the negative electrode active material as long as it is a material with which lithium can be dissolved and precipitated or a material into/from which lithium ions can be inserted and extracted. Other than a lithium metal or lithium titanate, a carbon-based material generally used in the field of power storage, an alloy-based material, or the like can also be used as the material of the negative electrode active material layer  506 . 
     A lithium metal is preferable because of its low oxidation-reduction potential (3.045 V lower than that of a standard hydrogen electrode) and high specific capacity per unit weight and per unit volume (3860 mAh/g and 2062 mAh/cm 3 , respectively). 
     As a carbon-based material, graphite, graphitizing carbon (soft carbon), non-graphitizing carbon (hard carbon), a carbon nanotube, graphene, carbon black, and the like are given. 
     As graphite, artificial graphite such as meso-carbon microbeads (MCMB), coke-based artificial graphite, or pitch-based artificial graphite, natural graphite such as spherical natural graphite can be given. 
     Graphite has a low potential substantially equal to that of a lithium metal (0.1 V to 0.3 V vs. Li/Li + ) when lithium ions are inserted between layers (when a lithium-graphite intercalation compound is formed). For this reason, a lithium ion battery can have a high operating voltage. En addition, graphite is preferable because of its advantages such as relatively high capacity per unit volume, small volume expansion, low cost, and safety greater than that of a lithium metal. 
     As the negative electrode active material, an alloy-based material or an oxide which enables charge-discharge reaction by an alloying reaction and a dealloying reaction with lithium can also be used. In the case where lithium ions are carrier ions, as an alloy-based material, a material containing at least one of Mg, Ca, Al, Si, Ge, Sn, Pb, Sb, Bi, Ag, Au, Zn, Cd, Hg, In, and the like, can be given, for example. Such elements have higher capacity than carbon; in particular, silicon has a significantly high theoretical capacity of 4200 mAh/g. For this reason, silicon is preferably used as the negative electrode active material. As the alloy-based material using such elements, Mg 2 Si, Mg 2 Ge, Mg 2 Sn, SnS 2 , V 2 Sn 3 , FeSn 2 , CoSn 2 , Ni 3 Sn 2 , Cu 6 Sn 5 , Ag 3 Sn, Ag 3 Sb, Ni 2 MnSb, CeSb 3 , LaSn 3 , La 3 Co 2 Sn 7 , CoSb 3 , InSb, SbSn, and the like are given, for example. 
     As the negative electrode active material, an oxide such as SiO, SnO, SnO 2 , titanium oxide (TiO 2 ), lithium titanium oxide (Li 4 Ti 5 O 12 ), lithium-graphite intercalation compound (Li x C 6 ), niobium oxide (Nb 2 O 5 ), tungsten oxide (WO 2 ), or molybdenum oxide (MoO 2 ) can also be used. 
     As the negative electrode active material, Li 3-x M x N (M=Co, Ni, or Cu) with a Li 3 N type structure, which is a double nitride of lithium and a transition metal, can also be used. For example, Li 2.6 Co 0.4 N 3  is preferable because of high charge and discharge capacity (900 mAh/g and 1890 mAh/cm 3 ). 
     A double nitride of lithium and a transition metal is preferably used, in which case lithium ions are contained in the negative electrode active material and thus it can be used in combination with a material for a positive electrode active material which does not contain lithium ions, such as V 2 O 5  or Cr 3 O 8 . Note that in the case of using a material containing lithium ions as a positive electrode active material, the double nitride of lithium and a transition metal can be used as the negative electrode active material by extracting the lithium ions contained in the positive electrode active material in advance. 
     A material which causes a conversion reaction can also be used as the negative electrode active material. For example, a transition metal oxide with which an alloying reaction with lithium is not caused, such as cobalt oxide (CoO), nickel oxide (NiO), or iron oxide (FeO), may be used for the negative electrode active material. As the material that causes a conversion reaction, in addition, the reaction is caused by oxides such as Fe 2 O 3 , CuO, Cu 2 O, RuO 2 , and Cr 2 O 3 , sulfides such as CoS 0.89 , NiS, and CuS, nitrides such as Zn 3 N 2 , Cu 3 N, and Ge 3 N 4 , phosphides such as NiP 2 , FeP 2 , and CoP 3 , and fluorides such as FeF 3  and BiF 3 . Note that the above fluorides can be used as positive electrode active materials since their potentials are high. 
     In the case where the negative electrode active material layer  506  is formed by a coating method, the negative electrode active material and the binder are mixed to form a negative electrode paste (slurry), which is applied onto the negative electrode current collector  505  and dried. Note that a conductive additive may be added to the negative electrode paste. 
     Graphene may be formed on a surface of the negative electrode active material layer  506 . In the case where the negative electrode active material is silicon, the volume is greatly changed due to occlusion and release of carrier ions in charge-discharge cycles; therefore, adhesion between the negative electrode current collector  505  and the negative electrode active material layer  506  is decreased, resulting in degradation of battery characteristics caused by charge and discharge. Thus, graphene is preferably formed on a surface of the negative electrode active material layer  506  containing silicon because even when the volume of silicon is changed in charge-discharge cycles, decrease in the adhesion between the negative electrode current collector  505  and the negative electrode active material layer  506  can be inhibited, which reduces degradation of battery characteristics. 
     Furthermore, a coating film of an oxide or the like may be formed on the surface of the negative electrode active material layer  506 . A coating film formed by decomposition or the like of an electrolyte solution in charging cannot release electric charges used at the formation, and therefore forms irreversible capacity. In contrast, the coating film of an oxide or the like provided on the surface of the negative electrode active material layer  506  in advance can reduce or prevent generation of irreversible capacity. 
     As the coating film coating the negative electrode active material layer  506 , an oxide film of any one of niobium, titanium, vanadium, tantalum, tungsten, zirconium, molybdenum, hafnium, chromium, aluminum, or silicon or an oxide film containing any one of these elements and lithium can be used. Such a coating film is sufficiently dense, compared with a conventional coating film formed on a surface of a negative electrode due to a decomposition product of an electrolyte solution. 
     For example, niobium oxide (Nb 2 O 5 ) has a low electric conductivity of 10 −9  S/cm and a high insulating property. For this reason, a niobium oxide film inhibits electrochemical decomposition reaction between the negative electrode active material and the electrolyte solution. On the other hand, niobium oxide has a lithium diffusion coefficient of 10 −9  cm 2 /sec and high lithium ion conductivity. Therefore, niobium oxide can transmit lithium ions. Silicon oxide or aluminum oxide may also be used. 
     For the formation of a coating film that coats the negative electrode active material layer  506 , a sol-gel method can be used, for example. A sol-gel method is a method for forming a thin film in such a manner that a solution of metal alkoxide, a metal salt, or the like is changed into a gel, which has lost its fluidity, by hydrolysis reaction and polycondensation reaction and the gel is baked. Since a sol-gel method is a method of forming a thin film from a liquid phase, raw materials can be mixed uniformly on the molecular level. For this reason, by adding a negative electrode active material such as graphite to a raw material of the metal oxide film which is a solvent, the active material can be easily dispersed into the gel. In such a manner, the coating film can be formed on the surface of the negative electrode active material layer  506 . A decrease in the capacity of the power storage unit can be prevented by using the coating film. 
     [Separator] 
     As a material for forming the separator  503 , a porous insulator such as cellulose, polypropylene (PP), polyethylene (PE), polybutene, nylon, polyester, polysulfone, polyacrylonitrile, polyvinylidene fluoride, tetrafluoroethylene, or polyphenylene sulfide can be used. Furthermore, nonwoven fabric of a glass fiber or the like, or a diaphragm in which a glass fiber and a polymer fiber are mixed may also be used. 
     [Electrolyte Solution] 
     As an electrolyte in the electrolyte solution  504 , a material having carrier ion mobility and containing lithium ions serving as carrier ions is used. Typical examples of the electrolyte are lithium salts such as LiPF 6 , LiClO 4 , LiAsF 6 , LiBF 4 , LiCF 3 SO 3 , Li(CF 3 SO 2 ) 2 N, Li(C 2 F 5 SO 2 ) 2 N, and Li(SO 2 F) 2 N. One of these electrolytes may be used alone, or two or more of them may be used in an appropriate combination and in an appropriate ratio. 
     As a solvent of the electrolyte solution  504 , a material having carrier ion mobility is used. As the solvent of the electrolyte solution, an aprotic organic solvent is preferable. Typical examples of an aprotic organic solvent include ethylene carbonate (EC), propylene carbonate, dimethyl carbonate, diethyl carbonate (DEC), ethylmethyl carbonate (EMC), γ-butyrolactone, acetonitrile, dimethoxyethane, tetrahydrofuran, and the like, and one or more of these can be used. Furthermore, when a gelled high-molecular material is used as the solvent of the electrolytic solution or a high-molecular material for gelling is added to the electrolytic solution, for example, safety against liquid leakage and the like is improved. Furthermore, a thinner and lighter storage battery can be provided. Typical examples of a gelled high-molecular material include a silicone gel, an acrylic gel, an acrylonitrile gel, a polyethylene oxide-based gel, a polypropylene oxide-based gel, a gel of a fluorine-based polymer, and the like. Furthermore, the use of one or more kinds of ionic liquids (room temperature molten salts) which have features of non-flammability and non-volatility as the solvent of the electrolyte solution can prevent the storage battery from exploding or catching fire even when the storage battery internally shorts out or the internal temperature increases owing to overcharging or the like. Note that an ionic liquid is a salt in the fluid state and has high ion mobility (conductivity). In addition, an ionic liquid contains a cation and an anion. As an ionic liquid, an ionic liquid containing an ethylmethylimidazolium (EMI) cation, an ionic liquid containing an N-methyl-N-propylpiperidinium (PP 13 ) cation, or the like is given. 
     [Exterior Body] 
     There are a variety of structures of a secondary battery, and a film is used for formation of the exterior body  507  in this embodiment. Note that as the film for forming the exterior body  507 , a single-layer film selected from a metal film (aluminum, stainless steel, nickel steel, or the like), a plastic film made of an organic material, a hybrid material film including an organic material (an organic resin, fiber, or the like) and an inorganic material (ceramic or the like), and a carbon-containing inorganic film (a carbon film, a graphite film, or the like); or a stacked-layer film including two or more of the above films is used. Forming depressions or projections on a metal film, to which embossing is easily performed, by embossing increases the surface area of the exterior body  507  exposed to outside air, achieving efficient heat dissipation. 
     Furthermore, in the case where the secondary battery  102  is changed in form by externally applying force, bending stress is externally applied to the exterior body  507  of the secondary battery  102 , and this might partly deform or damage the exterior body  507 . Projections or depressions formed on the exterior body  507  can relieve a strain caused by stress applied to the exterior body  507 . Therefore, the reliability of the secondary battery  102  can be increased. Note that a “strain” is the scale of change in form indicating the displacement of a point of an object relative to the reference (initial) length of the object. Formation of depressions or projections on the exterior body  507  can reduce the influence of a strain caused by application of external force to the power storage unit to an acceptable range. Thus, the power storage unit having high reliability can be provided. 
     The above is the description of the structure example. 
     [Fabrication Method Example] 
     An example of a fabrication method of the secondary battery  102  will be described below. 
     (Preparing Positive Electrode and Covering it with Separator) 
     First, the positive electrode  511  in which the positive electrode active material layer  502  is formed is placed on the separator  503  (see  FIG. 26(A) ). Note that  FIG. 26(A)  illustrates an example where the positive electrode active material layer  502  is provided on each side of the positive electrode current collector  501  having a meandering shape in which a slit is formed. 
     The slit formed in the positive electrode current collectors  501  can suppress the difference between the positions of end portions of the plurality of current collectors when the secondary battery  102  is curved. The slit can also relieve tension applied to the current collector far from the curvature center. 
     Furthermore, the positive electrode active material layer  502  is not provided in a region  511   a , which overlaps with a slit of the negative electrode  515 , when overlapping with the negative electrode  515  in a later step. If the positive electrode active material layer  502  is provided in the region  511   a  overlapping with the slit of the negative electrode  515 , there is no negative electrode active material layer  506  in a region overlapping with the positive electrode active material layer  502 , which might cause a problem in a battery reaction. Specifically, carrier ions released from the positive electrode active material layer  502  might concentrate in the negative electrode active material layer  506  closest to the slit, and the carrier ions might be deposited on the negative electrode active material layer  506 . Thus, the deposition of the carrier ions on the negative electrode active material layer  506  can be suppressed when there is no positive electrode active material layer  502  provided in the region  511   a , which overlaps with the slit of the negative electrode  515 . 
     Then, the separator  503  is folded along the dotted line in  FIG. 26(A)  so that the positive electrode  511  is sandwiched by the separator  503 . Next, the outer edges of the separator  503 , which is outside of the positive electrode  511 , are bonded to form the bag-like separator  503  (see  FIG. 26(B) ). The bonding of the outer edges of the separator  503  can be performed with the use of an adhesive or the like, by ultrasonic welding, or by thermal fusion bonding. 
     In this embodiment, polypropylene is used as the separator  503 , and the outer edges of the separator  503  are bonded to each other by heating. Bonding portions  503   a  are illustrated in  FIG. 26(B) . In such a manner, the positive electrode  511  can be covered with the separator  503 . The separator  503  is formed so as to cover the positive electrode active material layer  502  and need not necessarily cover the whole positive electrode  511 . 
     Note that although the separator  503  is folded in  FIG. 26 , one embodiment of the present invention is not limited thereto. For example, the positive electrode  511  may be sandwiched between two separators. In that case, the bonding portion  503   a  may be formed to surround almost all of the four sides. 
     The outer edges of the separator  503  may be bonded intermittently or may be bonded at dot-like bonding portions provided at regular intervals. 
     Alternatively, bonding may be performed along only one side of the outer edges. Alternatively, bonding may be performed along only two sides of the outer edges. Alternatively, bonding may be performed on four sides of the outer edges. Accordingly, the four sides can be in an even state. 
     Although the case where the positive electrode  511  is covered with the separator  503  is shown in  FIG. 26  and the like, one embodiment of the present invention is not limited thereto. The positive electrode  511  need not necessarily be covered with the separator  503 , for example. The negative electrode  515 , instead of the positive electrode  511 , may be covered with the separator  503 , for example. 
     (Preparing Negative Electrode) 
     Next, the negative electrode  515  is prepared (see  FIG. 26(C) ). In  FIG. 26(C) , an example where the negative electrode active material layer  506  is provided on each side of the negative electrode current collector  505  having a meandering shape in which a slit is formed is shown. 
     The slit formed in the negative electrode current collectors  505  can suppress the difference between the positions of end portions of the plurality of current collectors when the secondary battery  102  is curved. The slit can also relieve tension applied to the current collector far from the curvature center. 
     (Making Positive Electrodes and Negative Electrodes Overlap with Each Other and Connecting Leads) 
     Next, the positive electrodes  511  and the negative electrodes  515  are stacked (see  FIG. 27(A) ). In this embodiment, an example where two positive electrodes  511  and two negative electrodes  515  are used is shown. 
     Next, the positive electrode lead  521  including a sealing layer  520  is electrically connected to positive electrode tabs of the plurality of positive electrode current collectors  501  by ultrasonic wave irradiation with pressure applied (ultrasonic welding). 
     The lead electrode is likely to be cracked or cut by stress due to external force applied after fabrication of the power storage unit. 
     Thus, when subjected to ultrasonic welding, the positive electrode lead  521  are placed between bonding dies provided with projections, whereby a connection region and a curved portion can be formed in the positive electrode tab ( FIG. 27(B) ). 
     The provision of this curved portion can relieve stress due to external force applied after fabrication of the secondary battery  102 . Therefore, the reliability of the secondary battery  102  can be improved. 
     Furthermore, without limiting to the formation of the curved portion in the positive electrode tab, forming the positive electrode current collector using a high-strength material such as stainless steel to a thickness of 10 μm or less so as to make a structure that easily relieves stress due to external force applied after fabrication of a secondary battery may be employed. 
     It is needless to say that two or more of the above examples may be combined to relieve concentration of stress in the positive electrode tab. 
     Then, in a manner similar to that of the positive electrode current collector  501 , the negative electrode lead  525  including the sealing layer  520  is electrically connected to the negative electrode tab of the negative electrode current collector  505  by ultrasonic welding. 
     (Preparing Exterior Body and Covering Positive Electrodes and Negative Electrodes) 
     A film used as an exterior body is folded, and thermocompression bonding is performed along one side of the folded exterior body. A portion where thermocompression bonding is performed along one side of the folded exterior body  507  is indicated as a bonding portion  507   a  in  FIG. 27(B) . With this exterior body  507 , the positive electrodes  511  and the negative electrodes  515  are covered. 
     (Injecting Electrolyte Solution) 
     Next, thermocompression bonding is also performed along one side of the exterior body  507 , which overlaps with the sealing layer  520  of the positive electrode lead  521  and the sealing layer  520  including the negative electrode lead  525  ( FIG. 28(A) ). After that, the electrolyte solution  504  is injected from an unsealed side  507   b  of the exterior body  507 , which is indicated in  FIG. 28(A) , into a region covered with the exterior body  507 . 
     Then, the remaining open side of the exterior body  507  is sealed under vacuum, heating, and pressing, whereby the secondary battery  102  is obtained ( FIG. 28(B) ). Injecting the electrolyte solution and sealing are performed in an environment from which oxygen is eliminated, for example, with the use of a glove box. The evacuation may be performed with a vacuum sealer, a liquid pouring sealer, or the like. Heat and pressure application can be performed by setting the exterior body  110  between two heatable bars included in the sealer. An example of the conditions is as follows: the degree of vacuum is 60 kPa, the heating temperature is 190° C., the pressure is 0.1 MPa, and 3 seconds. Here, pressure may be applied to a unit through the exterior body  507 . By the pressure application, bubbles which enter between the positive electrode and the negative electrode at the time of injection can be removed. 
     Modification Example 
     As a modification example of the secondary battery  102 , a secondary battery  102  is illustrated in  FIG. 29(A) . The secondary battery  102  illustrated in  FIG. 29(A)  is different from the secondary battery  102  in  FIG. 24  in the arrangement of the positive electrode lead  521  and the negative electrode lead  525 . Specifically, the positive electrode lead  521  and the negative electrode lead  525  in the secondary battery  102  in  FIG. 24  are provided on the same side of the exterior body  507 , whereas the positive electrode lead  521  and the negative electrode lead  525  in the secondary battery  102  in  FIG. 29  are provided on different sides of the exterior body  507 . In this way, the leads of the secondary battery of one embodiment of the present invention can be freely positioned, and accordingly the design flexibility is high. Thus, the design flexibility of a product using the secondary battery of one embodiment of the present invention can also be improved. Furthermore, the yield of products each including the secondary battery of one embodiment of the present invention can be increased. 
       FIG. 29(B)  illustrates a fabrication process of the secondary battery  102  in  FIG. 29(A) . The fabrication method of the secondary battery  102  in  FIG. 24  can be referred to for the details. Note that in  FIG. 29(B) , the electrolyte solution  504  is not illustrated. 
     Pressing, e.g., embossing may be performed to form unevenness in advance on a surface of a film used as the exterior body  507 . The unevenness on the surface of the film increases flexibility of a secondary battery and further relieves stress. The depressions or projections of a surface (or a rear surface) of the film formed by embossing form an obstructed space that is sealed by the film serving as a part of a wall of the sealing structure and whose inner volume is variable. It can be said that the depressions or projections of the film form an accordion structure or bellows structure in this obstructed space. Note that embossing, which is a kind of pressing, is not necessarily employed and any method that allows formation of a relief on part of the film is employed. 
     Note that one embodiment of the present invention is not limited thereto. Various embodiments of the invention are described in this embodiment and other embodiments, and one embodiment of the present invention is not limited to a particular embodiment. For example, although an example of application of one embodiment of the present invention to a lithium-ion secondary battery is described, one embodiment of the present invention is not limited thereto. One embodiment of the present invention can be applied to a variety of secondary batteries, a lead storage battery, a lithium-ion polymer secondary battery, a nickel-hydrogen storage battery, a nickel-cadmium storage battery, a nickel-iron storage battery, a nickel-zinc storage battery, a silver oxide-zinc storage battery, a solid-state battery, an air battery, a primary battery, a capacitor or a lithium ion capacitor, and the like. One embodiment of the present invention is not necessarily applied to a lithium-ion secondary battery. 
     The above is the description of fabrication method example. 
     At least part of this embodiment can be implemented in combination with any of the other embodiments described in this specification as appropriate. 
     REFERENCE NUMERALS 
     
         
           10  wheel 
           10   a  wheel 
           10   b  wheel 
           10   c  wheel 
           10   d  wheel 
           11  rim portion 
           12  disk portion 
           13  bolt hole 
           15  portion 
           20  battery 
           20   a  battery 
           20   b  battery 
           20   c  battery 
           20   d  battery 
           21  connector 
           22  terminal 
           23  cable 
           25  circuit 
           26  antenna 
           27  window portion 
           30  electric power transmission mechanism 
           41  battery 
           42  battery 
           43  battery 
           45  terminal 
           50  car body 
           51  fixing portion 
           52  connector 
           53  antenna 
           54  cable 
           55  antenna support portion 
           60  electric power transmission mechanism 
           61  control unit 
           62  electric power control unit 
           62   a  electric power control unit 
           62   b  electric power control unit 
           62   c  electric power control unit 
           62   d  electric power control unit 
           63  engine 
           64  motor 
           64   a  motor 
           64   b  motor 
           64   c  motor 
           64   d  motor 
           65  battery 
           66  braking control portion 
           70  wheel 
           71  electric power control unit 
           72  electric power control unit 
           80  system 
           80   a  system 
           80   b  system 
           80   c  system 
           80   d  system 
           80   e  system 
           80   f  system 
           90  automobile 
           91  arrow 
           92  arrow 
           102  secondary battery 
           111  positive electrode 
           115  negative electrode 
           121  positive electrode lead 
           125  negative electrode lead 
           501  positive electrode current collector 
           502  positive electrode active material layer 
           503  separator 
           503   a  bonding portion 
           504  electrolyte solution 
           505  negative electrode current collector 
           506  negative electrode active material layer 
           507  exterior body 
           507   a  bonding portion 
           507   b  side 
           511  positive electrode 
           511   a  region 
           515  negative electrode 
           520  sealing layer 
           521  positive electrode lead 
           525  negative electrode lead 
           8021  charging apparatus 
           8022  cable 
           8400  automobile 
           8401  headlight 
           8500  automobile 
           8600  electric two-wheeled vehicle 
           8601  car body 
           8602  wheel 
           8603  tire 
           8604  handlebar 
           8605  operation lever