Patent Publication Number: US-11664683-B2

Title: Wireless charging apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of and claims priority to U.S. patent application Ser. No. 16/358,891, filed Mar. 20, 2019, and entitled “Wireless Charging Apparatus,” the content of which is incorporated herein by reference in its entirety. 
    
    
     FIELD 
     Aspects described herein generally relate to wireless power transmission in mobile devices. More specifically, aspects relate to improving wireless charging capabilities of various mobile device enclosures. 
     BACKGROUND 
     The use of magnetic induction in wireless charging of mobile devices is conventional. In many instances, an enclosure (e.g., a case, wallet, or the like) may be placed over the mobile device for protection. However, use of an enclosure may limit wireless charging capabilities with mobile devices. 
     Accordingly, there exists a need for ways to improve wireless charging capabilities for mobile devices that are stored in such enclosures. 
     BRIEF SUMMARY 
     The following presents a simplified summary of various aspects described herein. This summary is not an extensive overview, and is not intended to identify key or critical elements or to delineate the scope of the claims. The following summary merely presents some concepts in a simplified form as an introductory prelude to the more detailed description provided below. 
     To overcome limitations in the prior art described above, and to overcome other limitations that will be apparent upon reading and understanding the present specification, aspects described herein are directed to apparatuses and systems for improving wireless charging capabilities of mobile devices stored in various enclosures. 
     A first aspect described herein provides a first electrical coil configured to establish a first wireless coupling with a transmitter coil of a power supply. The first aspect described herein also provides a second electrical coil configured to establish a second wireless coupling with the first electrical coil and to establish a third wireless coupling with a receiver coil of a mobile device. In one or more instances, a distance between the receiver coil and the transmitter coil may exceed a range over which the transmitter coil is able to transfer power to the receiver coil via a single wireless coupling between the transmitter coil and the receiver coil. Additionally or alternatively, the distance between the receiver coil and the transmitter coil may diminish power transfer between the transmitter coil and the receiver coil (e.g., cause slower charging). In one or more instances, the first wireless coupling, the second wireless coupling, and the third wireless coupling, when established, may enable the transmitter coil to perform a wireless power transfer to the receiver coil. Additionally or alternatively, the first wireless coupling, the second wireless coupling, and the third wireless coupling, when established, may increase the power transfer between the transmitter coil and the receiver (e.g., cause faster charging). 
     In one or more instances, the first electrical coil might not be configured to self-resonate. In these instances, a capacitor may be added in parallel with the first electrical coil, which may cause this sub-circuit (e.g., paralleled coil and capacitor) to resonate at a predetermined frequency. In these instances, the second electrical coil may include a second inductor and a second capacitor connected in parallel. 
     In one or more instances, the first electrical coil and the second electrical coil may be self-resonating coils. In one or more instances, the first electrical coil and the second electrical coil may be integrated into one of: a mobile device case, a mobile device wallet, and a removable portion of a mobile device. 
     In one or more instances, wireless power transfer via the first wireless coupling, the second wireless coupling, and the third wireless coupling may improve the power transfer and/or efficiency of the power transfer relative to that of a direct coupling between the transmitter coil and the receiver coil. As a result, by implementing the first electrical coil and the second electrical coil, the charging time of a battery may be reduced. 
     A second aspect described herein provides a first electrical coil configured to establish a first wireless coupling with a transmitter coil of a power supply. The second aspect described herein further provides a second electrical coil configured to establish a second wireless coupling with a receiver coil of a mobile device. In one or more instances, the first electrical coil and the second electrical coil may be connected via a hard wire connection. In one or more instances, a distance between the receiver coil and the transmitter coil may exceed a range over which the transmitter coil may be able to transfer power to the receiver coil via a single wireless coupling between the transmitter coil and the receiver coil. In one or more instances, the first wireless coupling and the second wireless coupling, when established, may enable the transmitter coil to perform a wireless power transfer to the receiver coil. 
     In one or more instances, the wireless power transfer to the receiver coil may be performed by causing mutual inductance between the transmitter coil and the first electrical coil, and the second electrical coil and the receiver coil. 
     In one or more instances, the transmitter coil, the first electrical coil, the second electrical coil, and the receiver coil may have identical outer diameters. 
     In one or more instances, the outer diameters may be greater than a distance between each of the respective coils. In one or more instances, the first electrical coil may include a first inductor connected in parallel to a first capacitor. In these instances, the second electrical coil may include a second inductor connected in parallel to a second capacitor. In one or more instances, the first electrical coil may be connected in parallel to the second electrical coil. In one or more instances, the first electrical coil and the second electrical coil may be integrated into one of: a mobile device case, a mobile device wallet, and a removable portion of a mobile device. 
     A third aspect of described herein provides a power terminal configured to provide a wireless power transfer to a mobile device when the mobile device is located within a baseline distance of the power terminal. In one or more instances, a mobile device enclosure may be configured to hold the mobile device. In these instances, the mobile device enclosure may include a first electrical coil, magnetically coupled to a transmitter coil of the power terminal, configured to receive the wireless power transfer from the transmitter coil. In these instances, the mobile device enclosure may also include a second electrical coil configured to receive the wireless power transfer from the first electrical coil. In one or more instances, the mobile device may include a receiver coil, may be magnetically coupled to the second electrical coil, and may be configured to receive the wireless power transfer from the second electrical coil. 
     In one or more instances, the first electrical coil and the second electrical coil may enable the power terminal to provide the wireless power transfer to the mobile device when the mobile device is located within an updated distance of the power terminal, which may be greater than the baseline distance. In one or more instances, the first electrical coil may be magnetically coupled to the second electrical coil. In one or more instances, the first electrical coil may magnetically induce a current in the second electrical coil. 
     In one or more instances, the first electrical coil may be connected to the second electrical coil via a hard wire connection. In one or more instances, the mobile device enclosure may include a battery, and the battery in the mobile device enclosure may begin charging once the mobile device has completed charging. 
     In one or more instances, the mobile device enclosure may be configured to charge the mobile device, using the battery, based on determining that the mobile device is out of power, by inducing a current in the receiver coil based on the second electrical coil. In one or more instances, the mobile device enclosure may be configured to store power, using the battery and without transmitting power to the mobile device, if the mobile device is not within the mobile device enclosure. For example, a user may place the mobile device enclosure on the wireless charging device without the mobile device, and the mobile device enclosure may begin charging without the mobile device. 
     In one or more instances, the mobile device enclosure may provide the energy (e.g., from its battery) for the wireless power transfer to the mobile device by magnetically inducing a current in the first electrical coil based on the transmitter coil and magnetically inducing the current in the receiver coil based on the second electrical coil. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A more complete understanding of aspects described herein and the advantages thereof may be acquired by referring to the following description in consideration of the accompanying drawings, in which like reference numbers indicate like features, and wherein: 
         FIGS.  1 A and  1 B  are diagrams illustrating a mobile device and a wireless charging device that may be used to implement aspects of the disclosure. 
         FIGS.  2 A- 2 C  are diagrams illustrating schematics for wireless power transmission according to one or more aspects of the disclosure. 
         FIGS.  3 A and  3 B  are diagrams illustrating a mobile device wallet according to one or more aspects of the disclosure. 
         FIGS.  4 A and  4 B  are diagrams illustrating a mobile device case according to one or more aspects of the disclosure. 
         FIGS.  5 A- 5 D  are diagrams illustrating the use of electrical coils within a mobile device enclosure according to one or more aspects of the disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     In the following description of the various embodiments, reference is made to the accompanying drawings, which illustrate various embodiments in which aspects described herein may be practiced. It is to be understood that other embodiments may be utilized and structural and functional modifications may be made without departing from the scope of the described aspects and embodiments. Aspects described herein are capable of other embodiments and of being practiced or being carried out in various ways. Also, it is to be understood that the phraseology and terminology used herein are for the purpose of description and should not be regarded as limiting. Rather, the phrases and terms used herein are to be given their broadest interpretation and meaning. The use of “including” and “comprising” and variations thereof is meant to encompass the items listed thereafter and equivalents thereof as well as additional items and equivalents thereof. The use of the terms “mounted,” “connected,” “coupled,” “positioned,” “engaged” and similar terms, is meant to include both direct and indirect mounting, connecting, coupling, positioning and engaging. 
       FIG.  1 A  illustrates an example hardware environment  100  that may include a mobile device  105  and a wireless charging device  110 . In one or more instances, the mobile device  105  may be a smart phone, tablet, smart watch, or other electronic device capable of receiving a wireless charge. In these instances, the mobile device  105  may be configured to receive a wireless charge from the wireless charging device  110 . As is described in greater detail below, the mobile device  105  may be configured to receive charge from the wireless charging device  110  using magnetic induction. In one or more instances, the wireless charging device  110  may be a wireless charging pad, or the like, which may be configured to wirelessly charge a mobile device that is placed on the wireless charging device  110 . In order to perform such wireless charging, the wireless charging device  110  may be connected to a power source, such as a wall outlet, or may be configured to operate using battery power. Operations of the mobile device  105  and the wireless charging device  110  are described further below with regard to  FIG.  1 B . 
     Referring to  FIG.  1 B , the wireless charging device  110  may be connected to a power source  110   a , such as a wall outlet, generator, battery, or the like. In one or more instances, the power source  110   a  may provide alternating current (AC) to the wireless charging device  110 . Additionally or alternatively, the power source  110   a  may provide direct current (DC) to the wireless charging device  110  (e.g., if the power source is a battery cell, or the like). In instances where alternating current is received by the wireless charging device  110  from the power source  110   a , the wireless charging device  110  may route the current to an AC-DC converter to convert the alternating current into a direct current that may be used to charge various devices. In these instances, the wireless charging device  110  may perform an AC-DC-AC conversion process. Alternatively, the wireless charging device  110  may perform an AC-AC conversion using an AC-AC converter. In these instances, the AC-AC converter may convert alternating current of a particular frequency and amplitude to a different frequency and amplitude without converting to direct current as an intermediate step. In either instance, the wireless charging device  110  may excite a transmitter of the wireless charging device  110  with alternating current of a particular frequency and amplitude. In some instances, the wireless charging device  110  may regulate the conversion process (e.g., ensure voltage and/or current are within a predetermined range, or the like). In one or more instances, the wireless charging device  110  may use a rectifier circuit  110   b  to convert the alternating current into a direct current. After converting the alternating current to direct current, the wireless charging device  110  may use an inverter, such as inverter  110   c , to convert the direct current back to alternating current. In one or more instances, the wireless charging device  110  may perform this double conversion (e.g., AC to DC and back again) to properly regulate incoming and outgoing voltages, step up/step down the voltages, and/or to ensure the operability of sensitive circuits that may be housed in the wireless charging device  110 . The wireless charging device  110  may then route the alternating current to the transmitter coil  110   d . This may generate a changing magnetic field around the transmitter coil, which may be used to generate current in another coil (e.g., receiver coil  105   c ). 
     In one or more instances, when the mobile device  105  (and thus the receiver coil  105   c ) are located in close proximity to the wireless charging device  110  (e.g., laying on the wireless charging device  110 ), current may be generated in the receiver coil  105   c  as a result of the changing magnetic field surrounding the transmitter coil  110   d . As a result, the receiver coil  105   c  may be magnetically coupled to the transmitter coil  110   d . Once the current is generated at the receiver coil  105   c , the mobile device  105  may feed the alternating current through a rectifier  105   b  to generate direct current. The mobile device  105  may then store the energy at the battery  105   a  (e.g., charge the battery). 
     Accordingly,  FIGS.  1 A and  1 B  illustrate a conventional method of performing wireless charging between a mobile device and a wireless charging device. Such a method may adhere to the Qi standard (e.g., version 1.2.4), developed by the Wireless Power Consortium, which states the following principles. 87 to 205 kHz is defined as the typical range for coil operating frequency. With regard to coil area, an outer diameter of about 2 inches and 1.6 inches is standard for transmitters and receivers, respectively. To obtain an efficient power transfer, the transmitter and receiver coils should be identical, the transmitter and receiver coils should be aligned, the distance between the coils should be small relative to an outer diameter of the coils, and magnetic shielding may be used to reduce losses from magnetic field exciting electrical currents in unintended coils/objects The elements of efficient power transfer relating to distance between coils and coil alignment may cause problems for wireless charging of mobile devices because the coil size may be limited by a size of the mobile device. The full standard is available at the Wireless Power Consortium website for members of the organization. 
       FIG.  2 A  illustrates a coupling between the transmitter coil  110   d  and the receiver coil  105   c , as described above with regard to  FIG.  1 B  (e.g., a basic transformer model). For example, the transmitter coil  110   d  may be modeled as a resistor  204  and an inductor  203 . In one or more instances, the resistor  204  and the inductor  203  may be connected in series. For example, the transmitter coil  110   d  may have an inductance of L T . The receiver coil  105   c  may include a resistor  201  and an inductor  202 . In one or more instances, the resistor  201  and the inductor  202  may be connected in series. In these instances, mutual inductance may be produced between the transmitter coil  110   d  and the receiver coil  105   c . In one or more instances, this mutual inductance (M) may be correlated with L T  and the inductance of the receiver coil  105   c  (L R ) via the following relationship:
 
 M=k √{square root over ( L   T   L   R )}  (1)
 
     In these instances, k may be a coupling coefficient, which may be defined between 0 and 1. In one or more instances, as coil separation increases and/or coil offset increases, mutual inductance and the coupling coefficient between the coils may decrease. In these instances, a leakage inductance may be produced, as described below with regard to  FIG.  2 B . 
       FIG.  2 B  illustrates the aforementioned leakage inductance resulting from increased coil separation. For example, as a distance between the transmitter coil  110   d  and the receiver coil  105   c  is increased, the transmitter coil  110   d  may produce a magnetic inductance (nM)  207 . In these instances, n may refer to a turns ratio between the transmitter coil  110   d  and the receiver coil  105   c  (e.g., a ratio of a number of loops forming each of the transmitter coil  110   d  and the receiver coil  105   c ). In these instances, the leakage inductance  206  at the transmitter coil  110   d  may be determined using the following relationship:
 
Leakage inductance 206= L   T   −nM   (2)
 
     In addition, in these instances, the receiver coil  105   c  may produce a leakage inductance  205 . This leakage inductance  205  at the receiver coil  105   c  may be determined using the following relationship: 
     
       
         
           
             
               
                 
                   
                     Leakage 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     inductance 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     205 
                   
                   = 
                   
                     
                       L 
                       R 
                     
                     - 
                     
                       M 
                       n 
                     
                   
                 
               
               
                 
                   ( 
                   3 
                   ) 
                 
               
             
           
         
       
     
     In these instances, the coupling coefficient may decrease. Accordingly, unless increased, the current in the transmitter coil  110   d  and the receiver coil  105   c  might not be high enough to excite magnetizing inductance to produce a desired power output (e.g., to charge the mobile device  105 ). In these instances, as coil separation increases, efficiency of the wireless power transfer between the transmitter coil  110   d  and the receiver coil  105   c  may decrease.  FIG.  2 C , described below, presents modifications to the transmitter coil  110   d  and the receiver coil  105   c  that may improve the efficiency of the wireless power transfer. 
     Referring to  FIG.  2 C , a schematic is shown that may provide a more efficient wireless power transfer than the schematic in  FIG.  2 B . For example, such efficiency may be improved through resonant based magnetic induction. Accordingly, a discrete capacitor  209  may be added to the transmitter coil  110   d  and/or the transmitter coil  110   d  may have an inherent distributed capacitance by design. Similarly, a discrete capacitor  208  and/or a specific amount of capacitance may be added to the receiver coil  105   c . In one or more instances, an ideal amount of capacitance may be a function of coil parameters, coil coupling, receiver load, operating frequency, or the like corresponding to the discrete capacitor  208  and the discrete capacitor  209 . In these instances, the ideal capacitance may be determined via the following relationship: 
     
       
         
           
             
               
                 
                   C 
                   = 
                   
                     1 
                     
                       
                         w 
                         2 
                       
                       ⁢ 
                       
                         L 
                         L 
                       
                     
                   
                 
               
               
                 
                   ( 
                   4 
                   ) 
                 
               
             
           
         
       
     
     In instances where the transmitter coil  110   d  is identical to the receiver coil  105   c , w may be an angular frequency and L L  may be the coil leakage inductance. In one or more instances, a coupling between the transmitter coil  110   d  and the receiver coil  105   c  might not be fixed and/or well controlled. In these instances, the ideal capacitance may be unknown. As a result, the transmitter coil  110   d  may tune an operating frequency in real time based on feedback from the receiver coil  105   c  during resonant based magnetic induction. In these instances, the transmitter coil  110   d  and the receiver coil  105   c  may be configured in accordance with the Qi standard or some other open or proprietary wireless power transfer standard, and may be configured to support efficient power transfer over a range of relative coil positions through the use of an allocated frequency range. 
     Such techniques for wireless power transfer may be used in the charging of wireless client devices such as the mobile device  105 . However, problems may arise through implementation of these techniques in various situations as discussed below. 
       FIG.  3 A  illustrates a first wireless charging device enclosure embodiment. For example, in some instances, a user may keep the mobile device  105  in a mobile device wallet  305 . This mobile device wallet  305  may be used to provide protection for the mobile device  105  and may hold additional personal items (e.g., a credit card, cash, driver&#39;s license, or the like). Although the mobile device wallet  305  may provide numerous beneficial purposes, it may cause difficulty in wireless charging as shown below with regard to  FIG.  3 B . 
     Referring to  FIG.  3 B , which shows the mobile device  105 , inside of the mobile device wallet  305 , laying on the wireless charging device  110  for purposes of receiving a wireless power transfer. As is evident from  FIG.  3 B , the mobile device wallet  305  may increase a distance between the wireless charging device  110  and the mobile device  105 . As described above, increasing this distance may, in some instances, cause wireless charging to occur at a reduced efficiency level (e.g., when compared to a scenario where the mobile device  105  is place directly on the wireless charging device without the mobile device wallet  305 ). For example, a higher leakage inductance may be generated in these instances. In other instances, increasing this distance may move the mobile device  105  outside of a range within which the wireless charging device  110  may provide power. Additionally or alternatively, the material making up the mobile device wallet  305  may also impede the wireless power transfer. In each scenario, the mobile device wallet  305  negatively affects the wireless power transfer between the wireless charging device  110  and the mobile device  105 . 
       FIG.  4 A  illustrates a second wireless charging device enclosure embodiment. For example, in some instances, a user may keep the mobile device  105  in a mobile device case  405 . Similar to the mobile device wallet  305 , the mobile device case  405  may be used to provide protection for the mobile device  105 , but may be smaller and less cumbersome than the mobile device wallet  305 . In one or more instances, the mobile device case  405  may cause difficulty in wireless charging for similar reasons to those described above with regard to the mobile device wallet  305 , and as shown below with regard to  FIG.  4 B . 
     Referring to  FIG.  4 B , which shows the mobile device  105 , inside of the mobile device case  405 , laying on the wireless charging device  110  for purposes of receiving a wireless power transfer. As is evident in  FIG.  4 B , the mobile device case  405  may increase a distance between the wireless charging device  110  and the mobile device  105 . As described above, increasing this distance may, in some instances, cause wireless charging to occur at a reduced efficiency level (e.g., when compared to a scenario where the mobile device  105  is placed directly on the wireless charging device without the mobile device case  405 ). For example, a higher leakage inductance may be generated in these instances. In other instances, increasing this distance may move the mobile device  105  outside of a range within which the wireless charging device  110  may provide power. Additionally or alternatively, the material making up the mobile device case  405  may also impede the wireless power transfer. In each scenario, the mobile device case  405  negatively affects the wireless power transfer between the wireless charging device  110  and the mobile device  105 . 
       FIGS.  5 A- 5 C  illustrate a solution to the problems described herein with regard to wireless charging of mobile devices. Referring to  FIG.  5 A , an arrangement of the mobile device  105 , the wireless charging device  110 , and a mobile device enclosure  505  is described. In this arrangement, the mobile device  105  may be laying on the wireless charging device  110  and the mobile device enclosure  505  may form a layer between the mobile device  105  and the wireless charging device  110  such that the mobile device  105  is not directly touching the wireless charging device  110 . In one or more instances, the mobile device enclosure  505  may be a mobile device wallet (e.g., mobile device wallet  305 ), a mobile device case (e.g., mobile device case  405 ), or the like. As described above with regard to  FIG.  1 B , the mobile device  105  may include a receiver coil  105   c  and the wireless charging device  110  may include a transmitter coil  110   d . To address the problems associated with wireless charging in the presence of a mobile device enclosure (e.g., due to the increased distance between the mobile device  105  and the wireless charging device  110 , a type of material of the wireless charging enclosure, or the like), two electrical coils may be added to the mobile device enclosure  505 . In these instances, a first electrical coil  505   a  and a second electrical coil  505   b  may be integrated into the mobile device enclosure  505 . 
     In one or more instances, dimensions of the first electrical coil  505   a  and the second electrical coil  505   b  may be substantially the same, and may be an average between the dimensions of the transmitter coil  110   d  and the receiver coil  105   c . In these instances, the transmitter coil  110   d  and the receiver coil  105   c  may be manufactured and/or designed by different entities (e.g., a phone enclosure manufacturer, a wireless charging station manufacturer, or the like), and thus dimensions of the transmitter coil  110   d  and the receiver coil  105   c  may be different. In these instances, for example, diameters, number of windings, or the like of the first electrical coil  505   a  and the second electrical coil  505   b  may be an average of the diameters, number of windings, or the like of the transmitter coil  110   d  and the receiver coil  105   c . In other instances, the transmitter coil  110   d  and the receiver coil  105   c  may have substantially the same dimensions, and thus the dimensions of the first electrical coil  505   a  and the second electrical coil  505   b  may be substantially the same as the dimensions of the transmitter coil  110   d  and the receiver coil  105   c . In these instances, the transmitter coil  110   d  and the receiver coil  105   c  may establish a stronger wireless coupling with the first electrical coil  505   a  and the second electrical coil  505   b , respectively, than instances in which the transmitter coil  110   d  and the receiver coil  105   c  have different dimensions than the first electrical coil  505   a  and the second electrical coil  505   b . Further, in these instances, each of the first electrical coil  505   a  and the second electrical coil  505   b  may be self-resonating coils. 
     In one or more instances, dimensions of the first electrical coil  505   a  and the second electrical coil  505   b  may be based on a size of the mobile device enclosure  505 . For example, if a first mobile device enclosure is larger than a second mobile device enclosure, the first mobile device enclosure may have larger electrical coils than the second mobile device enclosure. In one or more instances, dimensions of the first electrical coil  505   a  and the second electrical coil  505   b  may be based on the recommendations and criteria of the Qi standard. 
     In one or more instances, a wire thickness of the first electrical coil  505   a  and the second electrical coil  505   b  may be substantially the same, and may be selected to minimize loss due to ohmic heating. In these instances, the wire thickness may be chosen based on the recommendations and criteria of the Qi standard. 
     In one or more instances, the first electrical coil  505   a  and the second electrical coil  505   b  may be integrated into the mobile device enclosure  505  using conventional techniques for making and embedding radio frequency identification (RFID) tags. For example, the first electrical coil  505   a  and the second electrical coil  505   b  may be integrated into a flexible printed wiring board (PWB). In this example, the first electrical coil  505   a  and the second electrical coil  505   b  may be printed onto the flexible PWB, and the flexible PWB may be integrated into an outer layer of the mobile device enclosure  505 . As another example, the first electrical coil  505  and the second electrical coil  505   b  may be laminated and integrated into an outer layer of the mobile device enclosure  505 . 
     . Accordingly, and as described further below with regard to  FIGS.  5 B- 5 C , the wireless power transfer may occur from the transmitter coil  110   d , through the first electrical coil  505   a  and the second electrical coil  505   b , and into receiver coil  105   c  of the mobile device  105 , rather than merely from the transmitter coil  110   d  and to the receiver coil  105   c  through the mobile device enclosure  505 . In one or more instances, by adding coils designed to resonate by themselves or with discrete capacitance added within, a practical range of wireless charging may be extended between the wireless charging device  110  and the mobile device  105 . Two embodiments of this design are further described below with regard to  FIGS.  5 B and  5 C . 
     Referring to  FIG.  5 B , power may flow into the wireless charging device  110  from a power source, and alternating current in the transmitter coil  110   d  may result in a changing magnetic field at the transmitter coil  110   d . This process is further described above with regard to  FIG.  1 B . Rather than causing mutual inductance between the transmitter coil  110   d  and the receiver coil  105   c , as described above, mutual inductance may be caused between the transmitter coil  110   d  and the first electrical coil  505   a , located in the mobile device enclosure  505 . For example, the transmitter coil  110   d  and the first electrical coil  505   a  may establish a wireless coupling. In one or more instances, the transmitter coil  110   d  and the first electrical coil  505   a  may be magnetically coupled. In one or more instances, the first electrical coil  505   a  may be modeled as an inductor and a capacitor in parallel with the inductor coupled to the transmitter coil  110   d . Once alternating current is induced in the first electrical coil  505   a , the first electrical coil  505   a  may establish mutual inductance, a wireless coupling, and/or a magnetic coupling with the second electrical coil  505   b , located in the mobile device enclosure  505 . In one or more instances, the second electrical coil  505   b  may include an inductor and a capacitor connected in parallel. Once current is induced at the second electrical coil  505   b , the second electrical coil  505   b  may establish mutual inductance, wireless coupling, and/or magnetic coupling with the receiver coil  105   c , resulting in induction of current at the receiver coil  105   c , which may be used to charge a battery at the mobile device  105 . Accordingly, by channeling power to the receiver coil  105   c , the first electrical coil  505   a  and the second electrical coil  505   b  may improve the efficiency of power transfer. 
     It should be understood that although  FIG.  5 B  shows the first electrical coil  505   a  and the second electrical coil  505   b  stacked on top of each other, in one or more instances, the first electrical coil  505   a  and the second electrical coil  505   b  might not be stacked on top of each other, yet may still establish a wireless coupling. For example, in some instances, the first electrical coil  505   a  and the second electrical coil  505   b  may be located side by side. However, small proximity between the first electrical coil  505   a  and the second electrical coil  505   b , and close alignment of their respective axes may increase strength of the wireless coupling between the first electrical coil  505   a  and the second electrical coil  505   b.    
     Furthermore, effects of a material of the mobile device enclosure  505  on induction may be reduced by shortening the distance over which mutual induction occurs. This may decrease both the efficiency and practical range of wireless charging for mobile devices enclosed by a case, wallet, or the like. Additionally, detrimental effects from the proximity of undesired materials may be reduced through the use of frequency tuning, as described in the Qi standard. In one or more instances, the transmitter coil  110   d  may tune the frequency based on feedback from the receiver coil  105   c . In these instances, the transmitter coil  110   d  may optimize the frequency in real-time using one or more algorithms. For example, the transmitter coil  110   d  may perturb the frequency and determine whether it increased or decreased power transfer. In this example, if the transmitter coil  110   d  determines that the power transfer was increased, the transmitter coil may maintain the new frequency. If the transmitter coil  110   d  determines that the power transfer was decreased, the transmitter coil  110   d  may perturb the frequency in the opposite direction.  FIG.  5 C  illustrates a similar wireless charging configuration to that described above with regard to  FIG.  5 B . However, rather than causing mutual induction between the first electrical coil  505   a  and the second electrical coil  505   b  at the mobile device enclosure  505  (as shown in  FIG.  5 B ),  FIG.  5 C  illustrates an embodiment in which the first electrical coil  505   a  is connected to the second electrical coil  505   b  via a hard wire connection. In one or more instances, the wireless power transfer occurring via the configuration shown at  FIG.  5 C  may be more efficient that the wireless power transfer occurring via the configuration shown at  FIG.  5 B  because the current may flow directly to the second electrical coil  505   b  through a hard wire rather than depending on a wireless coupling between the first electrical coil  505   a  and the second electrical coil  505   b.    
     Accordingly, by implementing the configurations shown in  FIGS.  5 B and  5 C , the practical range for charging a mobile device, associated with a mobile device enclosure, via wireless charging may be extended. 
     In one or more instances, as shown in  FIG.  5 D , the mobile device enclosure  505  described above may also be configured with a battery  505   c  that may receive and store charge under certain situations. For example, the mobile device enclosure  505  may include the first electrical coil  505   a , the second electrical coil  505   b , and the battery  505   c . If the mobile device  105  reaches a complete charge, the battery  505   c  may begin to charge. Accordingly, in some instances, when the mobile device  105  is removed from the wireless charging device  110 , it may begin receiving charge from the battery  505   c  if the mobile device  105  is at less than full charge. Similarly, in one or more instances, the mobile device  105  might not be in the mobile device enclosure  505 . In these instances, the mobile device enclosure  505  may still be placed on the wireless charging device  110 , and may be configured to wirelessly receive and store a charge. For example, as described above, mutual inductance may be caused between the transmitter coil  110   d  and the first electrical coil  505   a , located in the mobile device enclosure  505 . The first electrical coil  505   a  may be either magnetically coupled or connected via a hard wire connection to the second electrical coil  505   b . In either case, power may be transferred from the first electrical coil  505   a  to the second electrical coil  505   b , and then subsequently to the battery  505   c.    
     Although the figures and description herein primarily describe a mobile device enclosure (e.g., a case, wallet, or the like) to house the first electrical coil  505   a  and the second electrical coil  505   b , it should be understood that the first electrical coil  505   a  and the second electrical coil  505   b  may be embedded into a detachable portion of a mobile device (e.g., a detachable battery cover or the like). For example, the first electrical coil  505   a  and the second electrical coil  505   b  may be located in the detachable portion of the mobile device  105 , and may be used to perform a wireless power transfer as described herein. Additionally, the first electrical coil  505   a  and the second electrical coil  505   b  may be implemented in any other way so as to cause the first electrical coil  505   a  and the second electrical coil  505   b  to be placed between the transmitter coil  110   d  and the receiver coil  105   c . Additionally, it should be understood that although the embodiments described herein utilize two electrical coils in the mobile device enclosure, it should be understood that other numbers of electrical coils (e.g., one coil, three coils, or the like) may be implemented in the mobile device enclosure to maximize charging capabilities (e.g., based on dimensions, parameters, or the like corresponding to the mobile device enclosure). For example, in one or more instances, the mobile device enclosure may include a third electrical coil in addition to the first electrical coil  505   a  and the second electrical coil  505   b . In these instances, the second electrical coil  505   b  may establish a wireless coupling with the third electrical coil and induce a current in the third electrical coil. Alternatively, the third electrical coil may be connected to the second electrical coil  505   b  via a hard wire connection, and current may flow directly from the second electrical coil  505   b  to the third electrical coil. In either instance, the third electrical coil may establish a wireless coupling with the receiver coil  105   c  in the mobile device  105 , and may induce a current in the receiver coil  105   c . Accordingly, the wireless charging device  110  may charge the mobile device  105  through the mobile device enclosure  505  (e.g., through first electrical coil  505   a , the second electrical coil  505   b , and the third electrical coil), in a similar method as described above with regard to the two coil configuration. 
     Although the subject matter has been described in language specific to structural features and/or methodological acts, it is to be understood that the subject matter defined in the appended claims is not necessarily limited to the specific features or acts described above. Rather, the specific features and acts described above are disclosed as example forms of implementing the claims.