Patent Publication Number: US-2023143535-A1

Title: Wireless charging device, wireless charging system, and power receiving device used therein

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     The present application claims priority from Japanese application JP 2021-181480, filed on Nov. 5, 2021, the contents of which is hereby incorporated by reference into this application. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a wireless power transmission technology. 
     2. Description of the Related Art 
     For example, in small mobile electronic devices such as a mobile terminal or a game machine, it is general that charging for a secondary battery or the like that is built in the device is performed in a wired manner from an AC plug or an auxiliary power source through a charging terminal. However, recently, as a simple charging method considering usability, models using a wireless power transmitting method in which the charging is performed in a non-contact manner without using the charging terminal have increased in accordance with the spread of such electronic devices. Note that, such a demand is not limited to the small electronic devices, but extends to the field of vehicles such as an electrically assisted bicycle or an electrical kickboard. 
     Regarding a wireless power transmitting device in this technical field, for example, a wireless power transmitting device described in JP 2021-132528 A is provided. In JP 2021-132528 A, it is disclosed that transmission power of a wireless charging device is adjusted in accordance with feedback information of a device to be charged such that the wireless charging device and the device to be charged are capable of performing wireless communication, and an output voltage and/or an output current of a wireless receiving circuit in the device to be charged satisfy a current charging request of a battery. 
     In JP 2021-132528 A, in a case where the temperature of the battery is high, it is controlled such that transmission power of a wireless transmitting circuit of the wireless charging device is reduced or the operation of the wireless transmitting circuit is stopped. 
     However, in JP 2021-132528 A, a problem that when performing the charging by reducing the transmission power, a charging efficiency decreases and a problem that when frequently stopping the charging, the number of times of charging for full charging increases and the capacity of the battery decreases are not considered. 
     SUMMARY OF THE INVENTION 
     In consideration of the problems described above, an object of the invention is to provide a wireless charging device, a wireless charging system, and a power receiving device used therein that are capable of reducing a decrease in a charging efficiency and of preventing an increase in the number of times of charging. 
     According to an example of the present invention, a wireless charging device that includes a power transmitting coil and performs power transmission to a power receiving device including a power receiving coil by wireless power transmission, the device including: a communication unit receiving data from the power receiving device; a power transmitting coil exciting circuit outputting an alternating-current voltage to the power transmitting coil; and a power transmitting control unit controlling the power transmitting coil exciting circuit to control transmission power that is transmitted by the power transmitting coil, in which the power transmitting control unit is configured to acquire a temperature of a secondary battery of a charging target that is transmitted from the power receiving device through the communication unit, perform quick charging with first transmission power until the temperature of the secondary battery reaches an upper limit temperature, and perform weak charging with transmission power that is lower than transmission power for maintaining the temperature of the secondary battery not to be higher than the upper limit temperature and is lower than the first transmission power of which a charging state is uninterrupted when the temperature of the secondary battery reaches the upper limit temperature. 
     According to the invention, it is possible to provide a wireless charging device, a wireless charging system, and a power receiving device used therein that are capable of reducing a decrease in a charging efficiency and of preventing an increase in the number of times of charging. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a schematic configuration block diagram of a power transmitting/receiving system in Example 1; 
         FIG.  2    is a schematic function configuration diagram of a power receiving device main function unit in Example 1; 
         FIG.  3    is a plan view of a wireless charging device in Example 1; 
         FIG.  4    is a plan view of a power receiving device in Example 1; 
         FIG.  5    is a plan view of a state in which the power receiving device is mounted on the wireless charging device in Example 1; 
         FIG.  6    is a diagram illustrating charging control of the wireless charging device in Example 1; 
         FIG.  7    is a processing flowchart of the charging control in Example 1; 
         FIG.  8    is a processing flowchart of charging control in Example 2; 
         FIG.  9    is a processing flowchart of charging control in Example 3; 
         FIG.  10    is a schematic configuration block diagram of a power transmitting/receiving system in Example 4; 
         FIG.  11    is a diagram illustrating charging control of a wireless charging device in Example 4; 
         FIG.  12    is a diagram illustrating another charging control of the wireless charging device in Example 4; 
         FIG.  13    is a schematic external view of a wireless charging system in Example 5; 
         FIG.  14    is a schematic configuration block diagram of the wireless charging system in  FIG.  13   ; 
         FIG.  15    is a diagram illustrating a relationship between transmission power and a charging time of a wireless charging device in Example 5; 
         FIG.  16 A  is an example of a diagram illustrating a relationship between transmission power and a charging time of a wireless charging device of the related art; 
         FIG.  16 B  is an example of a diagram illustrating a relationship between a temperature and a charging time of a secondary battery of the related art; 
         FIG.  17 A  is a diagram illustrating a relationship between a charging efficiency and transmission power of a general wireless charging device; and 
         FIG.  17 B  is an example of a diagram illustrating a relationship between the transmission power of the wireless charging device of the related art and the temperature of the secondary battery. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Hereinafter, Examples of the invention will be described by using the drawings. 
     Example 1 
     First, a problem of wireless power transmission of the related art will be described.  FIG.  16 A  and  FIG.  16 B  are diagrams illustrating the problem of the wireless power transmission of the related art.  FIG.  16 A  is an example of a diagram illustrating a relationship between transmission power and a charging time of a wireless charging device of the related art. In addition,  FIG.  16 B  is an example of a diagram illustrating a relationship between a temperature and a charging time of a secondary battery of a charging target. In order to prevent heat interference due to an increase in the temperature of the secondary battery, in a case where the temperature of the secondary battery is high, the wireless charging device of the related art is controlled such that the transmission power of the wireless charging device is reduced and the temperature of the secondary battery is not higher than an allowable temperature. For this reason, as illustrated in  FIG.  16 A , for example, in a case where the charging is started at 15 W, a charging efficiency is not 100%, and thus, electrical energy that is not used in the charging is consumed in the form of heat, and as illustrated in  FIG.  16 B , the temperature of the secondary battery increases. For this reason, as illustrated in  FIG.  16 A , for example, control for decreasing the transmission power to 3 W is performed such that the temperature of the secondary battery is not higher than a predetermined temperature, and the charging is continued, and for example, the secondary battery is fully charged in 5 hours, and then, the charging is ended. As described above, even in a case where quick charging is performed with high transmission power of 15 W in order for full charging in a short period of time, the transmission power is limited to 3 W due to an increase in the temperature, and thus, the charging time is approximately the same as a case of general charging at 3 W. 
     In addition,  FIG.  17 A  and  FIG.  17 B  are diagrams illustrating the charging efficiency of the wireless charging device.  FIG.  17 A  is a diagram illustrating a relationship between the charging efficiency and the transmission power of the wireless charging device. In addition,  FIG.  17 B  is an example of a diagram illustrating a relationship between the transmission power of the wireless charging device of the related art and the temperature of the secondary battery. As illustrated in  FIG.  17 A , in general, in a case where the transmission power is low, the charging efficiency decreases, and a required charging time for full charging increases. In addition, as illustrated in  FIG.  17 B , in the related art, since the transmission power of the wireless charging device is controlled such that the temperature of the secondary battery is not higher than the allowable temperature, the transmission power decreases as the temperature of the secondary battery is high, and thus, the charging efficiency decreases as the temperature of the secondary battery is high. 
     In order to reduce a decrease in the charging efficiency, it is necessary to decrease the temperature of the secondary battery. For this reason, for example, it is considered to decrease the temperature of the secondary battery by using an external device and a component such as a cooling fan or a Peltier element, which leads to component addition or an increase in cost. Therefore, in this Example, a decrease in the charging efficiency is reduced without an additional component or an increase in cost. Hereinafter, this Example will be described in detail. 
       FIG.  1    is a schematic configuration block diagram of a power transmitting/receiving system in this Example. In  FIG.  1   , the power transmitting/receiving system includes a wireless charging device  10  provided with a power transmitting coil  16  sending power (a high-frequency current) in a wireless manner, and a power receiving device  20  provided with a power receiving coil  21  receiving the power that is sent from the wireless charging device  10 . 
     In a case where the power receiving device is a small electronic device, the wireless charging device  10  may be a stationary charging stand using a general power source of AC 100 to 120 V, and may be used by being placed on a desk or a table or may be fixedly used by being embedded in a concave portion on the upper surface of such furniture. In addition, in a case where the power receiving device is a vehicle such as an electrically assisted bicycle or an electrical kickboard, the wireless charging device  10  may be a dedicated charging stand using a power source of AC 100 to 120 V on which a secondary battery detached from a vehicle body is placed, and may be fixedly used by being provided on a stand for anchoring the vehicle. 
     In  FIG.  1   , the wireless charging device  10  includes a power transmitting coil  16 , a power source  11 , a rectifying/smoothing circuit  12 , a DC/DC converter  13 , a power transmitting control unit  14 , a power transmitting coil exciting circuit  15 , and a communication unit  31 . 
     The power source  11 , for example, includes a switch IC for switching on/off of a power source cable or power source supply inputting an alternating-current voltage (AC 100 V) from a power source plug, and the like, and supplies the alternating-current voltage that is transmitted through the power source cable to the rectifying/smoothing circuit  12 . 
     The rectifying/smoothing circuit  12 , for example, is a circuit using a semiconductor diode and a capacitor, converts the input alternating-current voltage into a direct-current voltage of a constant voltage by performing rectifying (direct-current converting) and smoothing processing of the alternating-current voltage, and supplies the converted power to the DC/DC converter  13 . Note that, an AC adapter may be used instead of the power source  11  and the rectifying/smoothing circuit  12 , or may be a so-called mobile battery that is a high-capacity secondary battery. 
     The DC/DC converter  13  converts (steps down) the input direct-current voltage to a voltage required for the excitation of the power transmitting coil  16 , and supplies the power after being stepped down to the power transmitting control unit  14 . 
     The power transmitting control unit  14  supplies the direct-current voltage that is supplied from the DC/DC converter  13  to the power transmitting coil exciting circuit  15  or stops the supply. Note that, the power transmitting control unit  14  is a processor such as a CPU or a MPU, and comprehensively controls the entire wireless charging device  10  by software processing in which the processor executes a basic program stored in a memory device. 
     The power transmitting coil exciting circuit  15  includes an inverter circuit converting a direct-current voltage into an alternating-current voltage in order to excite the power transmitting coil  16 . In addition, the power transmitting coil exciting circuit  15  converts the direct-current voltage that is supplied from the power transmitting control unit  14  into an alternating-current voltage of a predetermined voltage and a predetermined frequency, and outputs the alternating-current voltage to the power transmitting coil  16 . 
     The power transmitting coil  16 , for example, is a spiral type circular coil in which an electrical wire such as a litz wire is wound approximately in the shape of a ring in plane. 
     The communication unit  31  performs data transmission between the wireless charging device  10  and the power receiving device  20 . 
     Next, the power receiving device  20 , for example, is a mobile terminal device such as a smart phone, and the power receiving coil  21  configuring a power receiving unit is disposed in the housing. In  FIG.  1   , the power receiving device  20  includes the power receiving coil  21 , a rectifying/smoothing circuit  22 , a charging control unit  23 , a secondary battery  24 , a temperature sensor  26 , and a communication unit  27 , as the power receiving unit, and in addition, includes a power receiving device main function unit  25 . In a case where the power receiving device  20  is a smart phone, the power receiving device main function unit  25 , for example, includes a touch panel type manipulation input unit having both of a manipulation input function and an image display function, an image processing unit, a voice processing unit, a sensor unit, a communication unit, and the like, as a main function unit. The details will be described below. 
     In  FIG.  1   , the power receiving coil  21  is a spiral type circular coil having the same configuration as that of the power transmitting coil  16  described above. The rectifying/smoothing circuit  22 , for example, is a circuit including a diode or a capacitor, and generates a direct-current voltage of a stable voltage by rectifying (pulsating) and smoothing an induced current (alternating current) generated in the power receiving coil  21 . The charging control unit  23  supplies the direct-current voltage that is input from the rectifying/smoothing circuit  22  to the secondary battery  24 . In addition, the charging control unit  23  performs charging control of the secondary battery  24 , on the basis of temperature information of the secondary battery  24  that is measured by the temperature sensor  26 . Note that, the charging control unit  23  is a processor such as a CPU or a MPU, and comprehensively controls the entire power receiving unit of the power receiving device  20  by software processing in which the processor executes a basic program stored in a memory device. The details of the charging control unit  23  will be described below. Note that, the temperature sensor  26  may be built in the secondary battery  24 . 
     The secondary battery  24  is a battery that can be repeatedly charged and discharged, and for example, is a lithium-ion battery. 
     The communication unit  27  performs data transmission between the wireless charging device  10  and the power receiving device  20  through the communication unit  31 . Note that, in a case where the power receiving device main function unit  25  does not include a communication unit, the communication unit  27  may function as the communication unit. In addition, the data transmission of the communication units  31  and  27  may be performed by using the power transmitting coil  16  and the power receiving coil  21  that are a coil for power transmission, or may be performed by using another wireless method, for example, Bluetooth (Registered Trademark), near field communication (NFC), and the like. 
       FIG.  2    is a schematic function configuration diagram of the power receiving device main function unit  25  in a case where the power receiving device  20  is a smart phone. As illustrated in  FIG.  2   , the power receiving device main function unit  25  includes a main control unit  251 , a memory unit  253 , a manipulation input unit  254 , an image processing unit  255 , a voice processing unit  256 , a sensor unit  257 , a communication unit  258 , an extended interface (I/F)  259 , and the like, which are electrically connected through a system bus  252 . 
     The main control unit  251  is a processor such as a CPU or a MPU, and controls each of the function units of the entire power receiving device main function unit  25  by software processing in which the processor executes a basic program stored in a memory unit  253 . Note that, the main control unit  251  may have the function of the charging control unit  23 , and may control not only the power receiving device main function unit  25  but also the entire power receiving device  20  including the power receiving unit. 
     Note that, each function of the power receiving device main function unit  25  in  FIG.  2    is the same as the function of a generally known smart phone, and the details thereof will be omitted, but the function will be simply described below. 
     The manipulation input unit  254  is a user manipulation interface accepting manipulation input of a user with respect to the power receiving device  20 . Specifically, the manipulation input unit  220  includes a manipulation key such as a power source key, a volume key, and a home key, a touch panel, and the like. The touch panel is a touch screen that is superimposed and integrally disposed on a display unit. 
     The image processing unit  255  includes the display unit, an image signal processing unit, and an imaging unit, generates an electrical signal imaged by the imaging unit as digital image data, and displays the generated image data. In addition, the image data read out from the memory unit  253  is displayed on the display unit. 
     The voice processing unit  256  includes a voice output unit, a voice signal processing unit, and a voice input unit, outputs a voice that is processed by the voice signal processing unit, and inputs the voice of the user from the voice input unit. 
     The sensor unit  257  includes an acceleration sensor detecting movement, vibration, impact, or the like, a gyroscope sensor detecting an angular velocity in a rotation direction to grasp the state of a vertical, horizontal, or oblique posture, and the like. 
     The communication unit  258  is connected to a network by a wireless communication method, transmits and receives data with respect to a management server on the network, and performs near-field wireless communication or the like. 
     The extended I/F  259  is an interface group for extending the function of the power receiving device  20 . 
       FIG.  3    is a plan view of the wireless charging device  10  in this Example. In  FIG.  3   , the wireless charging device  10  includes a flat placement surface on which the power receiving device  20  is placed, and the power transmitting coil  16  is disposed in the lower portion of the placement surface approximately in parallel. 
       FIG.  4    is a plan view of the power receiving device  20  in this Example. In  FIG.  4   , the surface of the display panel  28  is a flat surface in the image of a smart phone, and the power receiving coil  21  is disposed in the lower portion of the display panel  28  approximately in parallel to the lower surface of the housing. 
       FIG.  5    is a plan view of a state in which the power receiving device  20  illustrated in  FIG.  4    is placed on the wireless charging device  10  illustrated in  FIG.  3    in this Example. As illustrated in  FIG.  5   , the power receiving device  20  is placed on the placement surface of the wireless charging device  10 , and power is transmitted to the power receiving coil  21  from the power transmitting coil  16 , and thus, the secondary battery  24  of the power receiving device  20  is charged. 
     Note that, in a case where the power receiving device is a vehicle such as an electrically assisted bicycle or an electrical kickboard, and the wireless charging device  10  charges the secondary battery that is detached from the vehicle body, as illustrated in  FIG.  5   , the secondary battery is charged by being placed on the placement surface of the wireless charging device  10 . In addition, in a case where the wireless charging device  10  is fixedly used by being provided on a stand for anchoring the vehicle, the secondary battery  24  of the vehicle is charged by setting the power transmitting coil  16  of the wireless charging device  10  and the power receiving coil  21  of the vehicle to face each other to be approximately a parallel surface. 
       FIG.  6    is a diagram illustrating charging control of the wireless charging device in this Example. In  FIG.  6   , the upper diagram is a diagram illustrating a relationship between transmission power and a charging time of the wireless charging device, and the lower diagram is a diagram illustrating a relationship between a temperature and a charging time of the secondary battery of the charging target. 
     As illustrated in the upper diagram of  FIG.  6   , for example, in a case where the quick charging is started with high transmission power of 15 W, as illustrated in the lower diagram, electrical energy that is not used in the charging is consumed in the form of heat, and the temperature of the secondary battery increases. Then, when the temperature of the secondary battery reaches an upper limit temperature (in the drawing, 45°), the temperature of the secondary battery is decreased, and thus, as illustrated in the upper diagram, for example, control is performed such that the transmission power is decreased to 1 W, and weak charging is performed with slight transmission power of which a charging state is uninterrupted. Accordingly, as illustrated in the lower diagram, a temperature decrease due to natural cooling is superior to a temperature increase due to the electrical energy that is not used in the charging, and the temperature of the secondary battery decreases. When the temperature of the secondary battery reaches a lower limit temperature, the quick charging is restarted with high transmission power of 15 W, and subsequently, the weak charging and the quick charging are repeated when the temperature of the secondary battery is within a range of the upper limit temperature and the lower limit temperature. Then, the charging is ended at a time point when the secondary battery is fully charged. Accordingly, the quick charging is performed with high transmission power of 15 W, and thus, a decrease in the charging efficiency due to low transmission power is suppressed, and the natural cooling of the secondary battery can be accelerated by the weak charging. In addition, since the secondary battery has a characteristic problem that the capacity decreases in a case where the number of times of charging increases, an increase in the number of times of charging can be prevented by continuing the charging with the weak charging, and thus, the problem can be solved. 
     That is, in this Example, the quick charging is performed with high transmission power until the temperature of the secondary battery reaches the upper limit temperature, and as illustrated in  FIG.  16 A  of the related art, weak transmission power that is lower than transmission power for maintaining the temperature of the secondary battery not to be higher than upper limit temperature and has a value in which a charging state is uninterrupted is set when the temperature of the secondary battery reaches the upper limit temperature, and thus, a natural temperature reduction of the secondary battery can be accelerated, and an increase in the number of times of charging can be prevented. Accordingly, it is possible to provide a wireless charging device and a power receiving device used therein that are capable of reducing a decrease in the charging efficiency and of preventing an increase in the number of times of charging. 
     Note that, trickle charging in which charging is constantly performed with a small current to maintain full charging to compensate for natural discharging of the secondary battery may be performed after the full charging. For example, in this Example, as an example of a charging mode, Quick Charging (Constant-Current Charging): 15 W, Usual Charging (Constant-Voltage Charging): 3 W, Weak Charging: 1 W, and Trickle Charging: 0.1 W may be set. 
       FIG.  7    is a processing flowchart of the charging control in this Example.  FIG.  7    is an example in which a charging control program is applied to the power transmitting control unit of the wireless charging device, and the power transmitting control unit acquires the temperature of the secondary battery of the charging target, performs determination on a power transmission side, and controls the transmission power. 
     In  FIG.  7   , first, in step S 101 , as initial setting, a weak charging flag indicating whether or not it is in the middle of the weak charging is set to 0. Next, in step S 102 , the power transmitting control unit  14  determines whether or not it is a state in which the power receiving device  20  is placed on the placement surface of the wireless charging device  10  and power is transmitted to the power receiving coil  21  from the power transmitting coil  16 . Then, in a case where there is no power receiving device  20  on the placement surface of the wireless charging device  10 , the power transmitting control unit  14  waits until the power receiving device  20  is placed on the placement surface. 
     In a case where there is the power receiving device  20  on the placement surface of the wireless charging device  10 , the process proceeds to step S 103 , and the power transmitting control unit  14  acquires a remaining battery level of the secondary battery  24 . That is, the power transmitting control unit  14  acquires the remaining battery level of the secondary battery from the charging control unit  23  of the power receiving device  20  through the communication unit  27  and the communication unit  31 . 
     Then, in step S 104 , the power transmitting control unit  14  determines whether or not the secondary battery is fully charged. In a case where the secondary battery is fully charged, in step S 105 , the weak charging flag is set to 0, the process returns to step S 103 , the remaining battery level is acquired again, and the processing of steps S 103 , S 104 , and S 105  is repeated until the secondary battery is not fully charged. 
     In a case where the secondary battery is not fully charged, the process proceeds to step S 106 , and the power transmitting control unit  14  acquires the temperature of the secondary battery. That is, the power transmitting control unit  14  acquires the temperature of the secondary battery from the temperature sensor  26  of the power receiving device  20  through the charging control unit  23 , the communication unit  27 , and the communication unit  31 . 
     Then, in step S 107 , the power transmitting control unit  14  determines whether or not the weak charging flag is 1. In a case where the weak charging flag is not 1, the process proceeds to step S 108 , and the power transmitting control unit  14  determines whether or not the temperature of the secondary battery is within a temperature specification range. That is, the power transmitting control unit  14  determines whether or not the temperature of the secondary battery is within the range of the upper limit temperature and the lower limit temperature, and in a case where the temperature is within the range, the process proceeds to step S 110 , the quick charging is performed, the weak charging flag is set to 0 to indicate that it is in the middle of the quick charging, and the process proceeds to step S 102 . 
     In step S 108 , in a case where the temperature of the secondary battery is not within the range of the upper limit temperature and the lower limit temperature, that is, when the temperature reaches the upper limit temperature, the process proceeds to step S 111 , the weak charging is performed, the weak charging flag is set to 1 to indicate that it is in the middle of the weak charging, and the process proceeds to step S 102 . 
     In addition, in step S 107 , in a case where the weak charging flag is 1, the process proceeds to step S 109 , and the power transmitting control unit  14  determines whether or not the temperature of the secondary battery is within the temperature specification range. That is, the power transmitting control unit  14  determines whether or not the temperature of the secondary battery is within the range of the upper limit temperature and the lower limit temperature, and in a case where the temperature is within the range, the process proceeds to step S 111 , the weak charging is performed, the weak charging flag is set to 1, and the process proceeds to step S 102 . In step S 109 , in a case where the temperature of the secondary battery is not within the range of the upper limit temperature and the lower limit temperature, that is, when the temperature reaches the lower limit temperature, the process proceeds to step S 110 , the quick charging is performed, the weak charging flag is set to 0, and the process proceeds to step S 102 . As described above, it is controlled such that the weak charging and the quick charging are repeated when the temperature of the secondary battery is within the range of the upper limit temperature and the lower limit temperature. 
     Note that, in the subsequent step S 104 , in a case where the secondary battery is fully charged, the charging may be ended or the trickle charging may be performed. In addition, the acquisition of the temperature of the secondary battery in step S 106  may not be the temperature itself but a digital value from which the temperature is calculated. In addition, in a case where the temperature can be acquired, the weak charging and the quick charging may be switched in a predetermined time. 
     As described above, according to this Example, the quick charging is performed with high transmission power until the temperature of the secondary battery reaches the upper limit temperature, and a decrease in the charging efficiency is reduced, and weak transmission power that is lower than the transmission power for maintaining the temperature of the secondary battery not to be higher than the upper limit temperature and has the value in which the charging state is uninterrupted is set when the temperature of the secondary battery reaches the upper limit temperature, and thus, a natural temperature reduction of the secondary battery can be accelerated, and an increase in the number of times of charging can be prevented. Accordingly, it is possible to provide a wireless charging device and a power receiving device used therein that are capable of reducing a decrease in the charging efficiency and of preventing an increase in the number of times of charging. 
     Example 2 
     In Example 1, the charging control program is applied to the power transmitting control unit of the wireless charging device, and the power transmitting control unit acquires the temperature of the secondary battery of the charging target, performs the determination on the power transmission side, and controls the transmission power. In contrast, in this Example, an example will be described in which the charging control program is applied to the charging control unit of the power receiving device, and the power receiving device acquires the temperature of the secondary battery of the charging target, determines the charging mode, and controls the transmission power through communication. 
       FIG.  8    is a processing flowchart of charging control in this Example. In  FIG.  8   , the charging control program is applied to the charging control unit of the power receiving device. 
     In  FIG.  8   , first, in step S 201 , as initial setting, the weak charging flag indicating whether or not it is in the middle of the weak charging is set to 0. Next, in step S 202 , the charging control unit  23  determines whether or not it is a state in which the power receiving device  20  is placed on the placement surface of the wireless charging device  10  and power is transmitted to the power receiving coil  21  from the power transmitting coil  16 . Then, in a case where the power receiving device  20  is not placed on the placement surface of the wireless charging device  10 , the charging control unit  23  waits until the power receiving device  20  is placed on the placement surface. 
     In a case where there is the power receiving device  20  on the placement surface of the wireless charging device  10 , the process proceeds to step S 203 , and the charging control unit  23  acquires the remaining battery level of the secondary battery  24 . 
     Then, in step S 204 , the charging control unit  23  determines whether or not the secondary battery is fully charged. In a case where the secondary battery is fully charged, in step S 205 , the weak charging flag is set to 0, the process returns to step S 203 , the remaining battery level is acquired again, and the processing of steps S 203 , S 204 , and S 205  is repeated until the secondary battery is not fully charged. 
     In a case where the secondary battery is not fully charged, the process proceeds to step S 206 , the charging control unit  23  acquires the temperature of the secondary battery from the temperature sensor  26 . 
     Then, in step S 207 , the charging control unit  23  determines whether or not the weak charging flag is 1. In a case where the weak charging flag is not 1, the process proceeds to step S 208 , and the charging control unit  23  determines whether or not the temperature of the secondary battery is within the temperature specification range. That is, in step S 208 , in a case where the temperature of the secondary battery is within the range of the upper limit temperature and the lower limit temperature, the process proceeds to step S 210 , and a request for the quick charging is transmitted to the power transmitting control unit  14  through the communication unit  27 , and the communication unit  31  of the wireless charging device  10 . The power transmitting control unit  14  receives the request for the quick charging and performs the quick charging. Further, in step S 210 , the weak charging flag is set to 0 to indicate that it is in the middle of the quick charging, and the process proceeds to step S 202 . In step S 208 , in a case where the temperature of the secondary battery is not within the range of the upper limit temperature and the lower limit temperature, that is, when the temperature reaches the upper limit temperature, the process proceeds to step S 211 , and a request for the weak charging is transmitted to the power transmitting control unit  14  through the communication unit  27 , and the communication unit  31  of the wireless charging device  10 . The power transmitting control unit  14  receives the request for the weak charging, and performs the weak charging. Further, in step S 211 , the weak charging flag is set to 1 to indicate that it is in the middle of the weak charging, and the process proceeds to step S 202 . 
     In addition, in step S 207 , in a case where the weak charging flag is 1, the process proceeds to step S 209 , and the charging control unit  23  determines whether or not the temperature of the secondary battery is within the temperature specification range. That is, in S 209 , in a case where the temperature of the secondary battery is within the range of the upper limit temperature and the lower limit temperature, the process proceeds to step S 211 , the request for the weak charging is performed, the weak charging flag is set to 1, and the process proceeds to step S 202 . In step S 209 , in a case where the temperature of the secondary battery is not within the range of the upper limit temperature and the lower limit temperature, that is, when the temperature reaches the lower limit temperature, the process proceeds to step S 210 , the request for the quick charging is performed, the weak charging flag is set to 0, and the process proceeds to step S 202 . As described above, it is controlled such that the weak charging and the quick charging are repeated when the temperature of the secondary battery is within the range of the upper limit temperature and the lower limit temperature. 
     Note that, in the subsequent step S 204 , in a case where the secondary battery is fully charged, a request may be performed such that the charging is ended or a request for the trickle charging may be performed. In addition, a request may be performed such that the weak charging and the quick charging are switched in a predetermined time instead of the upper limit temperature and the lower limit temperature. In addition, the charging control program may be pre-installed before the power receiving device is shipped, or may be downloaded by updating an application or an OS after the shipment. 
     As described above, according to this Example, as with Example 1, it is possible to provide a wireless charging device and a power receiving device used therein that are capable of reducing a decrease in the charging efficiency and of preventing an increase in the number of times of charging. Further, in addition to this, it is not necessary to transmit a remaining secondary battery level or temperature information to the wireless charging device, and thus, a load on the communication unit can be reduced. 
     Example 3 
     In Examples 1 and 2, it has been described that when the temperature of the secondary battery reaches the upper limit temperature, the weak charging with the weak transmission power having the value in which the charging state is uninterrupted is set. However, in a case where the power receiving device, for example, is a smart phone, the power consumption increases when an application with high power consumption is activated, and the consumption of the secondary battery also increases, and thus, an example will be described in which control is performed such that the transmission power in the weak charging increases in accordance with the state of an application to be used. 
       FIG.  9    is a processing flowchart of charging control in this Example.  FIG.  9    illustrates a case in which the charging control program is applied to the charging control unit of the power receiving device. In  FIG.  9   , the same reference numerals will be applied to the same functions as those in  FIG.  8   , and the description thereof will be omitted.  FIG.  9    is different from  FIG.  8    in that steps S 301  and S 302  are added when performing the weak charging. 
     That is, in  FIG.  9   , in step S 208 , in a case where the temperature of the secondary battery is not within the range of the upper limit temperature and the lower limit temperature (No), or in step S 209 , in a case where the temperature of the secondary battery is within the range of the upper limit temperature and the lower limit temperature (Yes), the weak charging is performed, but in this case, in step S 301 , for example, as the application with high power consumption, it is determined whether or not there is a request for lighting a backlight, and in a case where there is no request for lighting the backlight, as with  FIG.  8   , the process proceeds to step S 211 , the request for the weak charging is transmitted to the power transmitting control unit  14 , the weak charging flag is set to 1, and the process proceeds to step S 202 . In step S 301 , in a case where there is the request for lighting the backlight, the process proceeds to step S 302 , a request for power to be added due to the lighting of the backlight is performed together with the request for the weak charging, the weak charging flag is set to 1, and the process proceeds to step S 202 . The power transmitting control unit  14  is transmission power obtained by adding power corresponding to the power to be added due to the lighting of the backlight to weak power of which a charging state is uninterrupted when receiving the request for the weak charging and performing the weak charging, and performs the weak charging that is lower than the transmission power for maintaining the temperature of the secondary battery not to be higher than the upper limit temperature. Accordingly, even in a case where an application requiring high power consumption is activated, an increase in the number of times of charging can be prevented without interrupting the charging state. In addition, a natural temperature reduction of the secondary battery can be accelerated. 
     Note that,  FIG.  9    is an example of a case of lighting the backlight when the smart phone receives an e-mail, and it may be instructed to variably increase the transmission power of the weak charging in accordance with application information such that the charging state is uninterrupted even when executing another application requiring high power consumption. 
     In addition, as with Example 1, this Example can also be applied to a case in which the charging control program is applied to the power transmitting control unit of the wireless charging device, and the power transmitting control unit acquires the temperature of the secondary battery of the charging target, performs the determination on the power transmission side, and controls the transmission power. 
     As described above, according to this Example, the quick charging is performed with high transmission power and a decrease in the charging efficiency is reduced until the temperature of the secondary battery reaches the upper limit temperature, and transmission power that is lower than the transmission power for maintaining the temperature of the secondary battery not to be higher than the upper limit temperature and is weak power of which a charging state is uninterrupted even in a case of executing the application requiring high power consumption of the power receiving device is set when the temperature of the secondary battery reaches the upper limit temperature, and thus, a natural temperature reduction of the secondary battery can be accelerated, and an increase in the number of times of charging can be prevented even when executing the application. 
     Example 4 
     In Examples 1 to 3, as the wireless charging device that is capable of reducing a decrease in the charging efficiency and of preventing an increase in the number of times of charging, the wireless charging device charging one power receiving device has been described. In contrast, in this Example, an example will be described in which the wireless charging device includes a plurality of power transmitting coils, and is capable of charging a plurality of power receiving devices. 
       FIG.  10    is a schematic configuration block diagram of a power transmitting/receiving system in this Example. In  FIG.  10   , the same reference numerals will be applied to the same functions as those in  FIG.  1   , and the description thereof will be omitted.  FIG.  10    is different from  FIG.  1    in that a wireless charging device  101  includes a power transmitting coil exciting circuit  151  and a power transmitting coil  161 , and is capable of charging two power receiving devices  20  and  201 . 
     In  FIG.  10   , the wireless charging device  101  includes two power transmitting coil exciting circuits  15  and  151  and two power transmitting coils  16  and  161 , and two power transmitting coil exciting circuits  15  and  151  convert a direct-current voltage that is supplied from the power transmitting control unit  14  into an alternating-current voltage of a predetermined voltage and a predetermined frequency and output the alternating-current voltage to the power transmitting coils  16  and  161 , respectively. 
     The power receiving device  201  has the same configuration as that of the power receiving device  20 , each of the power receiving devices  20  and  201  is placed on the placement surface of the wireless charging device  101 , and power is transmitted to the power receiving coil  21  of each of the power receiving devices  20  and  201  from the power transmitting coils  16  and  161 , and thus, the secondary battery  24  of each of the power receiving devices  20  and  201  is charged. 
       FIG.  11    is a diagram illustrating charging control of the wireless charging device in this Example. In  FIG.  11   , the upper diagram is a diagram in which a relationship between the transmission power and the charging time of the power transmitting coil  16  and the power transmitting coil  161  of the wireless charging device is superimposed, and the intermediate diagram and the lower diagram are a diagram illustrating a relationship between a temperature and a charging time of each of a secondary battery A of the power receiving device  20  of the charging target and a secondary battery B of the power receiving device  201 . 
     As illustrated in the upper diagram of  FIG.  11   , first, the power transmitting coil  16  starts the quick charging of the secondary battery A of the power receiving device  20 , for example, with high transmission power of 15 W. In this case, as illustrated in the intermediate diagram, electrical energy that is not used in the charging is consumed in the form of heat, and the temperature of the secondary battery A increases. Then, the temperature of the secondary battery is decreased when the temperature of the secondary battery A reaches the upper limit temperature (in the drawing, 45°), and thus, even though it is not illustrated in the upper diagram, as with  FIG.  6   , the charging of the secondary battery A by the power transmitting coil  16 , for example, is controlled such that the transmission power is decreased to 1 W, and the weak charging is performed with slight transmission power of which a charging state is uninterrupted. Accordingly, as illustrated in the intermediate diagram, a temperature decrease due to natural cooling is superior to a temperature increase due to the electrical energy that is not used in the charging, and the temperature of the secondary battery A decreases. On the other hand, as illustrated in the upper diagram, the quick charging of the secondary battery B of the power receiving device  201  is started by the power transmitting coil  161  at a timing when the weak charging of the secondary battery A is started. In this case, as illustrated in the lower diagram, the temperature of the secondary battery B increases. Then, the temperature of the secondary battery B is decreased when the temperature of the secondary battery B reaches the upper limit temperature (in the drawing, 45°), and thus, even though it is not illustrated in the upper diagram, as with  FIG.  6   , the charging of the secondary battery B by the power transmitting coil  161 , for example, is controlled such that the transmission power is decreased to 1 W, and the weak charging is performed with slight transmission power of which a charging state is uninterrupted. 
     Then, when the temperature of the secondary battery A reaches the lower limit temperature at a timing when the weak charging of the secondary battery B is started, the quick charging of the secondary battery A by the power transmitting coil  16  is restarted, and when the temperature of the secondary battery A reaches the upper limit temperature, the weak charging of the secondary battery A is performed. Then, when the temperature of the secondary battery B reaches the lower limit temperature at a timing when the weak charging of the secondary battery A is started, the quick charging of the secondary battery B by the power transmitting coil  161  is restarted, and when the temperature of the secondary battery B reaches the upper limit temperature, the weak charging of the secondary battery B is performed. 
     As described above, the peak power of the wireless charging device can be reduced by alternately repeating the quick charging of the secondary battery A and the secondary battery B with the power transmitting coil  16  and the power transmitting coil  161 . Accordingly, it is possible to reduce the size, the cost of materials, and the cost by reducing pressure resistance of components of the wireless charging device. In addition, by repeating the quick charging and the weak charging in each of the secondary batteries, as with Examples 1 to 3, it is possible to provide a wireless charging device and a power receiving device used therein that are capable of reducing a decrease in the charging efficiency and of preventing an increase in the number of times of charging. 
     Note that, in a case where the wireless charging device has a margin in which two secondary batteries can be simultaneously subjected to the quick charging, simultaneous weak charging may be performed by simultaneously performing the quick charging without alternately performing the quick charging. 
     In addition,  FIG.  11    is a diagram illustrating charging control in a case where the temperature of each of the secondary batteries B and A reaches the lower limit temperature at a timing when the weak charging of the secondary battery A or B is started, and a case where the temperature of each of the secondary batteries B and A does not reach the lower limit temperature at the timing when the weak charging of the secondary battery A or B is started will be described by using  FIG.  12   . 
       FIG.  12    is a diagram illustrating the charging control in a case where the temperature of each of the secondary batteries B and A does not reach the lower limit temperature at the timing when the weak charging of the secondary battery A or B is started. In  FIG.  12   , the upper diagram, the intermediate diagram, and the lower diagram are illustrated in the same condition as that of  FIG.  11   . 
       FIG.  12    is different from  FIG.  11   , for example, in that the power receiving device  20  is a smart phone, the power receiving device  201  is a smart watch, the secondary battery A and the secondary battery B used in each of the smart watches have different thermal capacity, and a temperature increase time according to the quick charging and a temperature decrease time in the natural cooling according to the weak charging are different between the secondary battery A and the secondary battery B. 
     As illustrated in  FIG.  12   , in a case where the temperature of the secondary battery A does not reach the lower limit temperature at a timing T 1  when the weak charging of the secondary battery B by the power transmitting coil  161  is started, but reaches the lower limit temperature at T 2  that is later than T 1 , the quick charging of the secondary battery A by the power transmitting coil  16  is restarted by waiting until the time T 2  when the temperature of the secondary battery A reaches the lower limit temperature. Note that, both of the power transmitting coils  16  and  161  perform the weak charging between T 1  and T 2 . 
     In addition, in a case where the temperature of the secondary battery A does not reach the lower limit temperature at a timing T 3  when the weak charging of the secondary battery B by the power transmitting coil  161  is started, but reaches the lower limit temperature at T 5  that is later than T 3 , and in a case where the time T 5  when the temperature of the secondary battery A reaches the lower limit temperature is later than a time T 4  when the temperature of the next secondary battery B reaches the lower limit temperature, the quick charging of the secondary battery B of which the temperature reaches the lower limit temperature is restarted first by the power transmitting coil  161  at the time T 4  when the temperature of the secondary battery B reaches the lower limit temperature. Note that, both of the power transmitting coils  16  and  161  perform the weak charging between T 3  and T 4 . 
     As described above, by repeating the quick charging and the weak charging of the secondary battery A and the secondary battery B with a time difference, it is possible to provide a wireless charging device and a power receiving device used therein that are capable of reducing the peak power of the wireless charging device, or reducing a decrease in the charging efficiency, and of preventing an increase in the number of times of charging. 
     As described above, in  FIG.  10    to  FIG.  12   , an example has been described in which the wireless charging device is capable of charging two power receiving devices by using two power transmitting coils, and in this Example, the wireless charging device includes two or more power transmitting coils, and thus, can also be applied to a case where a plurality of power receiving devices can be charged. 
     In a case where the wireless charging device includes a plurality of power transmitting coils, the power transmitting coil may be spread extensively into a coil array. In addition, in this case, the wireless charging device selects a transmitting coil with the highest efficiency among the plurality of power transmitting coils, and thus, an operation for the user to position the power receiving device onto the placement surface of the wireless charging device is not required. In addition, the plurality of power transmitting coils may be disposed in the wireless charging device with or without a space between the power transmitting coils. Further, a part of the power transmitting coils may be disposed to be superimposed. 
     As described above, according to this Example, the wireless charging device includes the plurality of power transmitting coils, and the quick charging and the weak charging of the plurality of power receiving devices are repeated in a time difference, and thus, it is possible to provide a wireless charging device and a power receiving device used therein that are capable of reducing the peak power of the wireless charging device, of reducing a decrease in the charging efficiency, and of preventing an increase in the number of times of charging. 
     Example 5 
     In this Example, charging control will be described in which the peak power of the wireless charging system can be reduced in a wireless charging system that includes a plurality of wireless charging devices including a plurality of power transmitting coils and is capable of charging a plurality of power receiving devices. 
       FIG.  13    is a schematic external view of the wireless charging system in this Example.  FIG.  13    is an application example with respect to a charging rack, and is an example of a configuration in which three wireless charging devices  102 ,  103 , and  104  are provided on a charging rack  1000 , and each of the wireless charging devices includes three power transmitting coils and is capable of charging three power receiving devices. 
     That is, the wireless charging device  102  includes three power transmitting coils  16 ,  161 , and  162 , and charges three power receiving devices  20 ,  201 , and  202 . In addition, the wireless charging device  103  includes three power transmitting coils  163 ,  164 , and  165 , and charges three power receiving devices  203 ,  204 , and  205 . Similarly, the wireless charging device  104  includes three power transmitting coils  166 ,  167 , and  168 , and charges three power receiving devices  206 ,  207 , and  208 . Each of the wireless charging devices charges each of the power receiving devices by the same charging control as that in Example 4. 
       FIG.  14    is a schematic configuration block diagram of the wireless charging system in  FIG.  13   . In  FIG.  14   , the same reference numerals will be applied to the same functions as those in  FIG.  10   , and the description thereof will be omitted.  FIG.  14    is different from  FIG.  10    in that the wireless charging device  102  includes a power transmitting coil exciting circuit  152  and the power transmitting coil  163 , is capable of charging the power receiving devices  20 ,  201 , and  202 , and includes the wireless charging devices  103  and  104  having the same configuration. In this Example, three arms are provided in the wireless charging device, the power transmitting coil and the power transmitting coil exciting circuit are mounted on each of the arms, and three systems of wireless charging devices are provided. 
       FIG.  15    is a diagram in which a relationship between transmission power and a charging time of each of the plurality of power transmitting coils of the wireless charging device in this Example is superimposed. In  FIG.  15   , each of the upper diagram, the intermediate diagram, and the lower diagram is a diagram illustrating a relationship between the transmission power and the charging time of each of three power transmitting coils of the wireless charging devices  102 ,  103 , and  104 . 
     In the upper diagram of  FIG.  15   , as with  FIG.  11   , the wireless charging device  102  repeats the quick charging and the weak charging of the secondary battery A of the power receiving device  20 , the secondary battery B of the power receiving device  201 , and a secondary battery C of the power receiving device  202  with the power transmitting coils  16 ,  161 , and  162  in a time division manner. 
     In addition, in the intermediate diagram of  FIG.  15   , as with  FIG.  11   , the wireless charging device  103  repeats the quick charging and the weak charging of the secondary battery A of the power receiving device  203 , the secondary battery B of the power receiving device  204 , and the secondary battery C of the power receiving device  205  with the power transmitting coils  163 ,  164 , and  165  in a time division manner. 
     Similarly, in the lower diagram of  FIG.  15   , as with  FIG.  11   , the wireless charging device  104  repeats the quick charging and the weak charging of the secondary battery A of the power receiving device  206 , the secondary battery B of the power receiving device  207 , and the secondary battery C of the power receiving device  208  with the power transmitting coils  166 ,  167 , and  168  in a time division manner. 
     As described above, in this Example, nine power receiving devices are divided into three groups that is the number of wireless charging devices, in each of the groups, so-called rotational charging is performed in which the quick charging and the weak charging are repeated in a time division manner. Accordingly, the secondary batteries A of the power receiving devices  20 ,  203 , and  206  represented by a broken line in  FIG.  15    are simultaneously charged, but the quick charging of three power transmitting coils of three wireless charging devices is repeated with three power receiving devices in a time division manner, and thus, the peak power of the wireless charging system may be for three power receiving devices but not nine power receiving devices. Further, in a case where the charging time of nine power receiving devices is shifted not to overlap, the peak power of the wireless charging system may be for one power receiving device. 
     Note that, in a case where the temperature of the other secondary battery does not reach the lower limit temperature at a timing when the weak charging of one secondary battery is started, as illustrated in  FIG.  12   , the quick charging may be performed by waiting until the temperature of the other secondary battery reaches the lower limit temperature. In addition, in a case where the temperature of the other secondary battery does not reach the lower limit temperature at the timing when the weak charging of one secondary battery is started, but one secondary battery reaches the lower limit temperature first, as illustrated in  FIG.  12   , the quick charging of one secondary battery that reaches the lower limit temperature first may be restarted. 
     In addition, in a case where the charging rack in  FIG.  13    is regarded as a stand for anchoring a plurality of electrically assisted bicycles, this Example can also be applied to a case where the power receiving device is a vehicle such as an electrically assisted bicycle or an electrical kickboard. 
     As described above, according to this Example, in the wireless charging system that includes the plurality of wireless charging devices including the plurality of power transmitting coils and is capable of charging the plurality of power receiving devices, the plurality of power receiving devices are divided into a plurality of groups, and the quick charging and the weak charging are repeated in a time division manner, and thus, the peak power of the wireless charging system can be reduced, a decrease in the charging efficiency can be reduced, and an increase in the number of times of charging can be prevented. 
     Examples have been described, but the invention is not limited to Examples described above, and includes various modification examples. For example, Examples described above have been described in detail in order to explain the present invention in a simple way, and are not necessarily limited to having all the having all the configurations described above. In addition, a part of the configuration of one Example can be replaced with the configuration of the other Example, and the configuration of the other Example can also be added to the configuration of one Example. In addition, a part of the configuration of each of Examples can be added/deleted/replaced with the other configuration.