Patent Publication Number: US-9906067-B1

Title: Apparatus, system and method to wirelessly charge/discharge a battery

Description:
TECHNICAL FIELD 
     The present invention is directed, in general, to wireless power transmission and, more specifically, to an apparatus, system and method to wirelessly charge and/or discharge a battery. 
     BACKGROUND 
     A consideration in the design of consumer products is the use of a rechargeable battery to provide adequate, reliable, and unconstrained power to a consumer device. Up until about 20 years ago, most electrically powered consumer devices were simply coupled to the utility power grid. Rechargeable batteries saw limited use because earlier battery technology allowed only a very limited number of charging cycles with limited charging efficiency. 
     With the adoption of lithium-based batteries that allow for a large number of charge/discharge cycles, rechargeable batteries began to see increasing use for consumer electronics applications, facilitating the proliferation of electronic devices without tethering to the utility grid. Despite the great advantage of allowing consumers to use electronic devices such as cellular phones, tablets, and laptop computers, the battery operated electronic devices still needed to be connected to the utility grid to recharge the batteries. 
     In recent years, wireless power systems have been developed that allow recharging of the batteries without making a physical connection between the battery and the charger. The wireless power systems use resonant operation to transfer power from a charger to a battery. The battery itself is electrically/metallically tied to the load it will eventually power and charging is accomplished through a metallically isolated wireless interface. There are many reasons that the battery has been electrically/metallically tied to the load it operates including that both power transfer and communication in standard wireless interfaces is set up to allow transfer of power in only one direction. Additionally, standard wireless power interfaces are inefficient, so too much battery life would be lost by driving an electronic device through a wireless interface. 
     Standard wireless interfaces also require post regulators such as linear regulators because the control loop through a wireless interface is too slow for the wireless battery interface to adequately regulate the output of the battery. This regulator presents a further impediment to processing power in both directions. Wireless interfaces also tend to be very limited in power, both because of poor coupling efficiency and because of the heat generated by the poor coupling for any appreciable levels of power. The poor coupling efficiency of wireless power systems also produces a loss in the voltage that can be produced by a system component, which causes a mismatch in the voltage that could be wirelessly produced by the battery compared with the voltage necessary to wirelessly charge the battery. 
     There are many advantages associated with a battery that can be wirelessly charged or discharged, that is, one which interfaces wirelessly over a metallically isolated path for both charging and discharging. What is needed in the art, therefore, is a power system that can wirelessly charge a battery that overcomes the deficiencies in the prior art. 
     SUMMARY OF THE INVENTION 
     These and other problems are generally solved or circumvented, and technical advantages are generally achieved, by advantageous embodiments of the present invention, including an apparatus, system and method to wirelessly charge and/or discharge a battery. In one embodiment, the apparatus includes a removable first magnetic core piecepart having a surrounding first metallic coil and configured to be coupled to and aligned with a second magnetic core piecepart having a surrounding second metallic coil to form a transformer. The apparatus also includes a battery metallically coupled to the first metallic coil and configured to be charged and discharged through an electrically isolating path of the transformer. 
     The foregoing has outlined rather broadly the features and technical advantages of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter, which form the subject of the claims of the invention. It should be appreciated by those skilled in the art that the conception and specific embodiment disclosed may be readily utilized as a basis for modifying or designing other structures or processes for carrying out the same purposes of the present invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a more complete understanding of the present invention, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which: 
         FIG. 1  illustrates a block diagram of an embodiment of a power system with a wireless battery interface and a wireless battery; 
         FIG. 2  illustrates a schematic diagram of an embodiment of a power system with a wireless battery and a wireless battery interface; 
         FIGS. 3 and 4  illustrate schematic diagrams of embodiments of a model of the transformer of  FIG. 2 ; 
         FIGS. 5 and 6  illustrate graphical representations of waveforms demonstrating an embodiment of a dc transformer mode of operation of the power system of  FIG. 2 ; 
         FIG. 7  illustrates a graphical representation of waveforms demonstrating an embodiment of a boost mode of operation of the power system of  FIG. 2 ; 
         FIG. 8  illustrates a schematic diagram of an embodiment of a power system with a wireless battery and a wireless battery interface; 
         FIG. 9  illustrates a graphical representation of waveforms demonstrating an embodiment of an operation of the power system of  FIG. 8 ; 
         FIG. 10  illustrates a graphical representation of waveforms demonstrating an embodiment of a boost mode of operation of the power system of  FIG. 8 ; 
         FIG. 11  illustrates a diagram of an embodiment of a magnetic device; 
         FIGS. 12A and 12B  illustrate horizontal and vertical cross-sectional views, respectively, of an embodiment of a permanent magnet aligner; 
         FIG. 13  illustrates a plan view of an embodiment of a permanent magnet aligner; 
         FIG. 14  illustrates a diagram of an embodiment of a portion of a magnetic device; 
         FIG. 15  illustrates a diagram of another embodiment of a portion of a magnetic device; and 
         FIG. 16  illustrates a diagram of another embodiment of a portion of a magnetic device. 
     
    
    
     Corresponding numerals and symbols in the different FIGUREs generally refer to corresponding parts unless otherwise indicated, and may not be redescribed in the interest of brevity after the first instance. The FIGUREs are drawn to illustrate the relevant aspects of exemplary embodiments. 
     DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS 
     The making and using of the present exemplary embodiments are discussed in detail below. It should be appreciated, however, that the present invention provides many applicable inventive concepts that can be embodied in a wide variety of specific contexts. The specific embodiments discussed are merely illustrative of specific ways to make and use the invention, and do not limit the scope of the invention. 
     The present invention will be described with respect to exemplary embodiments in a specific context, namely, an apparatus, system and method to wirelessly charge and/or discharge a battery. The power system will be described as a switched-mode power supply or power converter. Any application that may benefit from a wireless battery charged and discharged by a wireless battery interface is well within the broad scope of the present invention. Additionally, while the principles of the present invention will be described with respect to electronic devices (also referred to as a “load”) such as cell phones, tablets, and power tools, other applications are well within the broad scope of the present invention. 
     In the environment of a conventional charging system with a cordless power tool or other battery operated electronic device, a battery is attached to low-voltage dc electrical connector that is formed with two metallic contacts. A load (or the battery operated electronic device) is attached to the low-voltage dc electrical connector that is formed with the two metallic (galvanic) contacts. A charger is connected to the utility grid and also to the low-voltage dc electrical connector that contains the two metallic contacts. A battery is metallically connected to the charger to charge the battery. To use the battery with the load, the battery is removed from the charger and is then metallically coupled to the load. Removing the battery from one device (e.g., the charger) and connecting it to another device (e.g., the load) requires breaking and then reconnecting metallic electrical contacts. 
     The terms “metallic and “galvanic” generally refer to, without limitation, an electrical connection between separate parts that is a wired or a contact that may include electrically conductive components such as semiconductor devices as well as current-conducting components such as resistors and inductors. Such wired connections conduct a current that may exceed a safety limit in response to an applied voltage difference across the ends of the electrical connection. 
     A battery-charging arrangement as set forth above allows a single rechargeable battery to be used by different battery operated electronic devices. A drawback of such a system is that the battery is configured with exposed electrical metallic contacts. The metallic contacts present several limitations such as the battery-charging arrangement is limited to environments that will not corrode the contacts. If exposed to a conductor, the metallic contacts can create a short circuit across the battery that can generate a dangerous amount of heat as well as destroy the battery. Also, a voltage of the battery-charging arrangement is limited to low voltages to address safety issues. All battery operated electronic devices that interface with the battery must be designed to operate at the same voltage as the battery. 
     In cases where the battery is removed from the charger and connected to a load for a large number of cycles, the electrical contacts can wear and eventually degrade the connection. The limitations of such conventional battery-charging arrangements could be overcome if the electrical metallic contacts were replaced with a system to transmit power wirelessly. 
     A disadvantage of the conventional wireless battery systems is poor power transfer efficiency. Another disadvantage is a slow feedback loop that necessitates the use of a post regulator such as a linear regulator. Still another disadvantage is the need to galvanically couple the battery to the load, thus making it difficult to remove the battery without breaking a metallic electrical contact. A further disadvantage is that the power flow to the battery is unidirectional, that is, the battery can be wirelessly charged, but the discharge occurs through a directly wired electrical connection. 
     In conventional battery charging arrangements using magnetic devices (e.g., a transformer), transmit and receive coils (or windings) of the transformer are coupled through a common flux path including air or other substance of equivalent magnetic permeability. This creates a substantial amount of loss due to poor magnetic flux coupling, and the resulting power transfer efficiency of the conventional wireless battery power system is typically only on the order of 50 percent. The substantially poor magnetic flux coupling of the conventional wireless battery power system makes it difficult to discharge the battery through a wireless path since the battery charging takes place at a much slower rate than discharging the battery into a load. The poor magnetic flux coupling further prevents voltage matching of the charging and discharging cycles. The conventional battery charging arrangements also prevents a bidirectional power flow to and from the battery. 
     The power system as introduced herein provides the safety advantages and voltage scaling of a wireless power system, while preserving the efficiency and bidirectional power flow obtained by the metallic contact battery power systems. The power system set forth herein eliminates the metallic contacts, thereby avoiding the concern with the corrosion of standard battery contacts. The wireless interface can be customized for any voltage interface. This allows batteries with any amount of energy storage to be standardized to produce a same voltage. High-voltage battery strings can be safely interfaced to other electronic devices. A single wireless battery can interface with many electronic devices that would traditionally require dc voltages different from that of the battery string. The nonmetallic power interface as described herein can efficiently transfer power for an electronic device in both directions to regulate an output characteristic (e.g., an output voltage), and obviate the need for a post regulator regardless of the direction of power transfer. 
     Turning now to  FIG. 1 , illustrated is a block diagram of an embodiment of a power system (also referred to as a “system”) with a wireless battery interface  120  and a wireless battery  130 . A power source/load  110  such as a utility grid power source or a power tool is electrically coupled (i.e., wired) to the wireless battery interface  120 . The wireless battery  130  is docked into the wireless battery interface  120  by a coupler. The coupler links a magnetic field  140  induced by a metallic coil (or winding)  150  surrounding a wireless battery interface magnetic core piecepart in the wireless battery interface  120  with a wireless battery magnetic core piecepart in the wireless battery  130 . When charging the wireless battery  130 , a voltage is induced in a metallic coil (or winding)  160  surrounding the wireless battery magnetic core piecepart in the wireless battery  120  by a voltage impressed across the terminals of the metallic coil  150  that surrounds the wireless battery interface magnetic core piecepart in the wireless battery interface  120 . When discharging the wireless battery  130 , a voltage is induced in the metallic coil (or winding)  150  surrounding the wireless battery interface magnetic core piecepart in the wireless battery interface  120  by a voltage impressed across the terminals of the metallic coil  160  that surrounds the wireless battery magnetic core piecepart in the wireless battery  130 . The power source/load  110  can be, for instance, a utility grid power source that is employed to charge the wireless battery  130 , and also can be arranged to absorb energy from the wireless battery  130  for utility grid power source load-leveling purposes. It should be understood that the connection between the metallic coil  160  and battery will include components therebetween. 
     Turning now to  FIG. 2 , illustrated is a schematic diagram of an embodiment of a power system with a wireless battery  200  and a wireless battery interface  250 . The wireless battery  200  is formed with a metallic coil  201  surrounding a wireless battery magnetic core piecepart  202  that can be used to both transmit and receive power. The wireless battery magnetic core piecepart  202  is typically composed of, without limitation, a soft ferrite, powered iron, or some other ferromagnetic substance with high magnetic permeability. 
     The metallic coil  201  is coupled to a resonant capacitor C 403  and a full-bridge power train is formed with power switches (e.g., metal-oxide semiconductor field-effect transistors (“MOSFETs”)) Q 405 , Q 406 , Q 407 , Q 408  and diodes D 405 , D 406 , D 407 , D 408  in anti-parallel with each respective power switch. In other words, each diode D 405 , D 406 , D 407 , D 408  is oriented in the same direction as the body (intrinsic) diode of the corresponding power switch Q 405 , Q 406 , Q 407 , Q 408 . The addition of the diodes D 405 , D 406 , D 407 , D 408  reduces the voltage drop across the corresponding power switch Q 405 , Q 406 , Q 407 , Q 408  and allows for higher switching speeds for the power train. Of course, the diodes D 405 , D 406 , D 407 , D 408  may be omitted if the body diodes of the power switch Q 405 , Q 406 , Q 407 , Q 408  can perform the intended task with the desired performance. The term “switch” generally refers to any active semiconductor device such as, without limitation, a MOSFET or bipolar transistor or a passively switched semiconductor device such as a diode. 
     The full-bridge power train formed with power switches Q 405 , Q 406 , Q 407 , Q 408  is coupled to a capacitor C 404  and a rechargeable battery (or battery) V 401 . The capacitor C 404  filters high-frequency current to provide a steady voltage to or from the battery V 401 . The capacitor C 404  may not be needed in all applications because the battery V 401  can also act as a filter. 
     The wireless battery interface  250  is formed with a metallic coil  251  surrounding a wireless battery interface magnetic core piecepart  252  that can be used to both transmit and receive power. The wireless battery interface magnetic core piecepart  252  is typically constructed with a soft ferrite, powered iron, or some other ferromagnetic substance. The magnetic core pieceparts  202 ,  252  link most of the magnetic flux that passes between the metallic coils  201 ,  251 . There is a small air gap in the magnetic path created by the magnetic core pieceparts  202 ,  252 . The air gap is typically due to the enclosures of the wireless battery  200  and the wireless battery interface  250 . In practice, however, the air gaps can be kept quite small such as 3 or 4 millimeters (“mm”). It would be advantageous to maintain the air gaps to be smaller than about 1.5 times the square-root of the cross-sectional area of the magnetic core pieceparts  202 ,  252  to reduce (e.g., minimize) fringing of the magnetic flux. The magnetic core pieceparts  202 ,  252  with the corresponding metallic coils  201 ,  251  form a transformer of the power system. 
     The power system illustrated in  FIG. 2  demonstrates one of many ways that the magnetic core pieceparts  202 ,  252  can be designed to create a flux path that passes through the metallic coils  201 ,  251 . The metallic coil  251  is coupled to a resonant capacitor C 402  and a full-bridge power train is formed with power switches Q 401 , Q 402 , Q 403 , Q 404  and diodes D 401 , D 402 , D 403 , D 404  in anti-parallel with each respective power switch. In other words, each diode D 401 , D 402 , D 403 , D 404  is oriented in the same direction as the body (intrinsic) diode of the corresponding power switch Q 401 , Q 402 , Q 403 , Q 404 . The addition of the diodes D 401 , D 402 , D 403 , D 404  reduces the voltage drop across the corresponding power switch Q 401 , Q 402 , Q 403 , Q 404  and allows for higher switching speeds for the power train. Of course, the diodes D 401 , D 402 , D 403 , D 404  may be omitted if the body diodes of the power switch Q 401 , Q 402 , Q 403 , Q 404  can perform the intended task with the desired performance. 
     The full-bridge power train formed with the power switches Q 401 , Q 402 , Q 403 , Q 404  is coupled to a capacitor C 401  and terminals  257  including a positive terminal POS and a negative terminal NEG. The capacitor C 401  filters high-frequency current to provide a steady voltage to or from the terminals  257 . The terminals  257  can be connected to either a power source or a load depending on whether the battery V 401  is charging or discharging, respectively. The power source may be, for example, a power converter that regulates voltage from a utility grid such as a power-factor corrected power converter. The power source may also be a battery or a dc voltage source connected to a universal serial bus (“USB”) power port. There are many possibilities for a load including, without limitation, a string of light-emitting diodes (“LEDs”), a battery, or a power converter that pushes power into or receives power from the utility grid. The terminals  257  may connect to a power source or load within the same enclosure as the wireless battery interface  250 . For instance, a power converter may be located in the same enclosure as the wireless battery interface  250  and have electrical connections leading to an external load or power source. Of course, a portion of or all of the power source or load can be located external to the enclosure of the wireless battery interface  250 . Many implementations are possible as would occur to one skilled in the art. 
     The power system illustrated in  FIG. 2  can process power from the terminals  257  to the battery V 401  or from the battery V 401  to the terminals  257 . If transmitting power from the terminals  257  to the battery V 401 , the full-bridge power train formed with the power switches Q 401 , Q 402 , Q 403 , Q 404  produces a pulsed voltage waveform to the resonant capacitor C 402  and the metallic coil  251 . The full-bridge power train is switched so that the power switches Q 401 , Q 404  are simultaneously turned on and off with a duty cycle slightly less than about 50 percent (such as 45 to 49 percent). Also, the power switches Q 402 , Q 403  are simultaneously turned on and off with a duty cycle slightly less than 50 percent and 180 degrees out-of-phase with respect to the power switches Q 401 , Q 404 . The duty cycle of each power switch is slightly less than 50 percent to decrease a possibility of simultaneous conduction with an opposing power switch and to allow enough time for a magnetizing current in the metallic coil  251  to resonate with the parasitic capacitance of the power switches Q 401 , Q 402 , Q 403 , Q 404  to commutate a voltage thereacross. This process results in soft-switching, meaning the voltage across or the current through each power switch Q 401 , Q 402 , Q 403 , Q 404  is naturally resonated to substantially zero just prior to turning that respective power switch on or off. 
     For cases in which the wireless battery interface  250  is used as a battery charger, it is also possible to configure a controller X 401  to turn off the power switch Q 403  and turn on the power switch Q 404  continuously. As a result, the full-bridge power train formed with the power switches Q 401 , Q 402 , Q 403 , Q 404  will act as a half-bridge power train with the resonant capacitor C 402  absorbing a dc offset caused by half-bridge operation. Reverting from a full-bridge to a half-bridge power train may be useful, for example, when a charging circuit connected to the terminals  257  switches connection from a 115 Vac utility grid power source to a 230 Vac utility grid power source since a half-bridge configuration will transmit only half as much voltage as a full-bridge configuration. The controller X 401  can therefore be configured to selectively cause at least a portion of the power train to switch between full-bridge and half-bridge operation in response to a sensed voltage level (e.g., the voltage at the terminals  257 ). 
     A pulsed voltage that appears across the metallic coil  251  induces a voltage across the metallic coil  201  that is scaled by a transformer turns ratio of the metallic coils  201 ,  251 . The voltage across the metallic coil  201  appears across the resonant capacitor C 403  in series with the full-bridge power train formed with the power switches Q 405 , Q 406 , Q 407 , Q 408 . The diodes D 405 , D 406 , D 407 , D 408  rectify the pulsed voltage that appears across the metallic coil  201  and the resonant capacitor C 403  and the resulting power is sent to the battery V 401 . The power switches Q 405 , Q 406 , Q 407 , Q 408  may be turned on during some or all of the time that the corresponding diodes D 405 , D 406 , D 407 , D 408  are conducting to reduce conduction losses therein. 
     If the power system illustrated in  FIG. 2  transmits power from the battery V 401  to the terminals  257 , the process as described above is reversed. That is, a controller X 402  drives the full-bridge power train formed with the power switches Q 405 , Q 406 , Q 407 , Q 408  to produce a pulsed voltage across the resonant capacitor C 403  and the metallic coil  201 . The induced voltage in the metallic coil  251  is then rectified by the diodes D 401 , D 402 , D 403 , D 404  to send power to the terminals  257 . Accordingly, the resonant capacitors C 402 , C 403  in conjunction with the metallic coils  201 ,  251  and the full-bridge power trains form a resonant topology. 
     Turning now to  FIGS. 3 and 4 , illustrated are schematic diagrams of embodiments of a model of the transformer of  FIG. 2 . The magnetic core pieceparts  202 ,  252  with corresponding metallic coils  201 ,  251  of  FIG. 2  are modelled as a transformer  320 . The transformer  320  is formed with inductors L 501 , L 502 , L 503  and an ideal transformer TX 501 . The ideal transformer TX 501  represents an ideal transformer with no leakage inductance and infinite magnetizing inductance. The actual leakage inductance caused by non-ideal coupling between the metallic coils  201 ,  251  is modelled by the inductors L 501 , L 503 . The magnetizing inductance of the transformer  320  is modelled by the inductor L 502 . The system galvanic isolation and turns ratio is modelled by the transformer  320 . The turns ratio N of the ideal transformer TX 501  equals S 1 /P 1 . The leakage inductance of the transformer  320  is naturally split between the two sides of the ideal transformer TX 501  such that the ratio between the inductance of the inductors L 501 , L 503  is N 2 . If the inductance of the inductor L 501  is L s , then the inductance of the inductor L 503  is N 2 L s . The capacitance values for the resonant capacitors C 402 , C 403  should preferably be chosen to obtain the same ratio of impedance as the inductances for the inductors L 501 , L 503 . So, if the resonant capacitor C 402  has a capacitance C s , then the resonant capacitor C 403  should be chosen with a value C s /N 2 . The input to the transformer  320  has a voltage v p  across the input terminals and a current i p  enters the top terminal. The output of the transformer  320  has voltage v s  and current i s . 
     For the purposes of analysis only, the galvanic isolation barrier can be removed from the circuit in  FIG. 3  resulting in the diagram of  FIG. 4 . In  FIG. 4 , a circuit block  420  models a transformer and replaces the transformer  320  from  FIG. 3 . In  FIG. 4 , the inductors L 501 , L 502 , and the resonant capacitor C 402  are the same as the correspondingly numbered components illustrated in  FIG. 3 . The inductor L 503 S and the resonant capacitor C 403 S have been appropriately scaled translated to the other side of the circuit block  420 . Furthermore, the output voltage and current are scaled by the transformer turns ratio to Nv s  and i s /N, respectively, as illustrated in  FIG. 4 . 
     With continuing reference to  FIGS. 3 and 4 , an advantageous mode of operation of the power system illustrated in  FIG. 2  is a dc transformer mode of operation. In the dc transformer mode of operation, the full-bridge power train on the transmitting side is driven at the resonant frequency of the resonant capacitor C 402  and the inductor L 501 . The resonant frequency of the resonant capacitor C 402  and the inductor L 501  is: 
     
       
         
           
             
               f 
               res 
             
             = 
             
               
                 1 
                 
                   2 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   π 
                   ⁢ 
                   
                     
                       
                         C 
                         s 
                       
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                         L 
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               . 
             
           
         
       
     
     The resonant frequency of the inductor L 503 S and the resonant capacitor C 403 S is the same as that of the resonant capacitor C 402  and the inductor L 501 . As a result, when driving power switches at the resonant frequency, the circuit block  420  of  FIG. 4  can be simplified to a parallel inductor L M  with the voltage and current at the battery scaled by the factor N. The magnetic structure thus acts like a dc transformer in parallel with an inductor. Operating in dc transformer mode of operation is possible because the magnetic core pieceparts  202 ,  252  provide consistent and tight coupling between the metallic coils  201 ,  251  illustrated in  FIG. 2 . 
     In practice, the driving frequency is usually slightly lower than the resonant frequency of the resonant capacitor C 402  and the inductor L 501  because the diode bridge (the diodes D 405 , D 406 , D 407 , D 408 ) of the wireless battery  200  prevents the resonant current from continuing to flow after the current in the wireless battery  200  passes through zero, thereby allowing a window of driving frequency over which the power system will behave like a dc transformer. That is, the voltages v p =Nv s  for a small range of frequencies are at and above the resonant frequency. The ratio of output voltage to input voltage is independent of load current and remains a fixed ratio that is the same as the ratio of turns in the metallic coils  201 ,  251 . The resonant capacitors C 402 , C 403  can thus be selected to produce substantially zero-current switching of a switching circuit of the power train in conjunction with at least one inductor (e.g., an inductor L 501 , L 502 , L 503  representing the inductances of the transformer  320 ). The inductor(s) can be formed at least in part with a metallic coil of a transformer. 
     Turning now to  FIGS. 5 and 6 , illustrated are graphical representations of waveforms demonstrating an embodiment of a dc transformer mode of operation of the power system of  FIG. 2 . In the dc transformer mode of operation, the current in the power system resembles a sequence of half sinusoids as shown by the waveforms in  FIG. 5 . The first waveform  510  in  FIG. 5  represents gate drive signals for the power switches Q 401 , Q 404 . The second waveform  520  represents gate drive signals for the power switches Q 402 , Q 403 . The third waveform  530  represents the current i p  as illustrated in  FIGS. 3 and 4 . The fourth waveform  540  represents the current i s /N as illustrated in  FIG. 4 . The sequence of half-sinusoid current waveforms allows for efficient control of the power switches of the full-bridge power train while avoiding (or at least reducing) the possibility of a shoot-through because, as can be seen by time interval  550  in  FIG. 5  (the dead-time), the resonant current in the wireless battery  200  goes to zero for a short period of time prior to switching polarity. This dead-time occurs because the diodes D 405 , D 406 , D 407 , D 408  substantially prevent reverse current from flowing. As long as the switching frequency of the power switches Q 401 , Q 402 , Q 403 , Q 404  is slightly higher than the resonant frequency, the current i s /N will reach zero before the alternate conduction of the power switches of the full-bridge power train is enabled, as can be seen by the timing of the gate drive signals for the power switches Q 401 , Q 402 , Q 403 , Q 404  with respect to the time interval  550 . 
       FIG. 6  superimposes the gate drive signals for the power switches Q 405 , Q 406 , Q 407 , Q 408  of the wireless battery  200  of  FIG. 2  in addition to the waveforms introduced in  FIG. 5 . The first new waveform  610  in  FIG. 6  represents gate drive signals for the power switches Q 405 , Q 408 , and the second new waveform  620  represents gate drive signals for the power switches Q 406 , Q 407 . At a time  630 , shortly after the current i s /N has fallen to zero, the power switches Q 405 , Q 408  gates are switched off. At a time  635 , just after the current i s /N goes negative, the power switches Q 406 , Q 407  are turned on. The controller X 402  for the wireless battery  200  controls the conduction periods of the power switches Q 405 , Q 406 , Q 407 , Q 408  by monitoring the current i s /N. The current i s /N in  FIG. 4  is analogous to the current i s  through the resonant capacitor C 403  in  FIGS. 2 and 3 . 
     If the gate drive signals for the respective full-bridge power trains of the wireless battery  200  and the wireless battery interface  250  are synchronized, another advantage occurs above that of merely reducing conduction and/or switching losses due to diode voltage drops and diode recovery times. If the receiving side full-bridge power train (e.g., the wireless battery power train) is driven at the same frequency as that of the wireless battery interface  250 , then the wireless battery  200  can function as a wireless battery interface  250  with no change of operation, thereby allowing power flow into or out of the wireless battery  200  to instantly switch direction with no change to the gate drive signals (or duty cycle thereof) of the full-bridge power trains. The roles of wireless battery  200  and wireless battery interface  250  can thus be switched very quickly without concern for response times of control loops. In fact, no voltage control is needed at all in the dc transformer mode of operation provided the wireless battery interface  250  and the wireless battery  200  operate at fixed voltage levels. The power system of  FIG. 2  thus behaves like an actual battery in its ability to both charge and discharge through the same two terminals without any significant change to its voltage level. 
     The addition of a bidirectional dc converter (such as a half-bridge with an inductor tied to the switching point of the half-bridge) to either the battery V 401  or at the terminals  257  can provide the necessary regulation when the wireless battery interface  250  or wireless battery  200  does not operate at a fixed voltage level. It is thus possible to use the power system of FIG.  2  in applications that allow the battery V 401  to be successively charged and discharged without changing a duty cycle of the power trains. Examples of such applications would include using the battery V 401  for load leveling of a utility grid or using the battery V 401  to provide peak load demands. So, for example, if a distributed generation plant provided power to a varying load, the battery V 401  could switch between charging mode when the load was below the capacity of the distributed generation plant, and the battery V 401  could discharge when the load exceeded the capacity of the distributed generation plant. 
     Thus, the dc transformer mode of operating the power system of  FIG. 2  has several advantages. For instance, the voltage between the wireless battery interface  250  and the wireless battery  200  is held in a substantially fixed and predictable ratio regardless of the load, thereby obviating the need for a voltage regulation loop. Also, the diode rectifier losses are reduced. Moreover, the wireless battery  200  can instantly change mode of operation to a wireless battery interface  250  to allow quick change of power flow direction without any communication signals sent between the two wireless components and thus causing the power system to act like a battery with galvanically isolated terminals. 
     Another mode of operation that allows the possibility of reduced switching losses is a pulsed boost mode of operation or boost mode of operation. The boost mode of operation is employable with a single direction of power flow for any given control loop. Reversing the power flow is more easily accomplished in this mode of operation by an external input to a controller, for example, by use of a push-button switch. While the boost mode of operation is more difficult to control than the dc transformer mode of operation, the boost mode of operation presents the possibility of reduced switching losses as well as the possibility of fast voltage regulation, both boosting and reducing the voltage at the wireless battery  200  compared to that of the wireless battery interface  250 . 
     Referring again to  FIGS. 3 and 4 , in the boost mode of operation, the power system is driven at a frequency far below that of the resonant frequency f res  of the inductor L 501  and the resonant capacitor C 402 , but above the lower resonant frequency of the power system which occurs at: 
     
       
         
           
             
               f 
               lres 
             
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               . 
             
           
         
       
     
     By driving the full-bridge power train of the transmitting system far below the resonant frequency f res  but above the lower resonant frequency f lres , the power system is able to obtain a voltage gain because it operates as a cross between a series resonant and a parallel resonant system. While the series resonant system cannot achieve any voltage gain, the parallel resonant system can achieve very high voltage gain. 
     Turning now to  FIG. 7 , illustrated is a graphical representation of waveforms demonstrating an embodiment of a boost mode of operation of the power system of  FIG. 2 . When operating in a boost mode, it is possible to choose a frequency of operation that reduces (e.g., minimizes) switching losses as shown in  FIG. 7 . The first waveform  710  in  FIG. 7  represents gate drive signals for the power switches Q 401 , Q 404 . The second waveform  720  represents gate drive signals for the power switches Q 402 , Q 403 . The third waveform  730  represents the current i p  as illustrated in  FIGS. 3 and 4 . The fourth and fifth waveforms  740 ,  750  represent the voltage Nv s  and the current i s /N as illustrated in  FIG. 4 . One can see from  FIG. 7  that the currents i p , i s /N are close to zero when the power switches Q 401 , Q 402 , Q 403 , Q 404  transition states. Combined with the voltage commutation described above, switching the power switches Q 401 , Q 402 , Q 403 , Q 404  when the resonant current is very low considerably reduces switching losses. The voltage gain of the series-parallel resonant system will vary with load and with the resistance of the metallic coils  201 ,  251  (not shown in the circuit model). To regulate the output voltage, the wireless battery interface  251  operates in a burst mode of operation. 
     Turning now to  FIG. 8 , illustrated is a schematic diagram of an embodiment of a power system with a wireless battery  800  and a wireless battery interface  850 . The wireless battery  800  is formed with a metallic coil  801  surrounding a wireless battery magnetic core piecepart  802  that can be used to both transmit and receive power. The wireless battery interface  850  is formed with a metallic coil  851  surrounding a wireless battery interface magnetic core piecepart  852  that can be used to both transmit and receive power. Analogous components for the power system illustrated in  FIG. 8  to the power system illustrated in  FIG. 2  will not herein be described again. The power system of  FIG. 8  includes additional components and control to enable a fast regulation loop and to allow shutdown and restart of the wireless battery interface  850 . The boost mode of operation prefers a fast feedback mechanism for the wireless battery interface  850  to determine when to stop transmitting power and a process to quickly wake up the wireless battery interface  850  to start transmitting power again. 
     Consider first the case in which the wireless battery interface  850  acts as a power transmitter and the wireless battery  800  acts as a power receiver. A switch S 912  is in series with the output of the wireless battery  800  and provides a method for preventing power transfer to a resonant capacitor C 403 . A switch S 911  in series with a capacitor C 911  is in parallel with the metallic coil  801 . Back-to-back Zener diodes Z 911 , Z 912  are also in parallel with the metallic coil  801 . In practice, the switch S 911  can be realized using back-to-back MOSFETs or other implementations are possible. The switch S 912  can also be located in a circuit leg between the anode of the Zener diode Z 912  and the cathode of the diode D 406 . The capacitor C 911  can be replaced with other impedances. 
     The controllers X 901 , X 902  of the wireless battery interface  850  and wireless battery  800 , respectively, operate as follows when a voltage across the capacitor C 404  or a current into the battery V 401  rises above a predetermined set-point such as 105 percent nominal value. Initially, the capacitor C 911  is discharged. At the start of a switching cycle (representing a portion of a duty cycle of the full-bridge power train formed with the power switches Q 401 , Q 402 , Q 403 , Q 404 ), the controller X 902  opens the switch S 912  and pulses the switch S 911  closed for a fraction of a resonant switching cycle (e.g., 25 percent). After the short pulse in which the switch S 911  is closed, the capacitor C 911  is disconnected from the circuit and remains charged. 
     When the switch S 911  is opened following the short pulse, and the switch S 912  remains open, any energy stored in the resonant tank of the wireless battery interface  850  is dissipated in the Zener diodes Z 911 , Z 912 . Switching the capacitor C 911  into the circuit causes the current in the metallic coil  851  to decrease faster than usual. The controller X 901  interprets the faster fall of current in the metallic coil  851  as a signal from the wireless battery  800  and stops switching the full-bridge power train formed with the power switches Q 401 , Q 402 , Q 403 , Q 404 . The voltage across the capacitor C 404  then decreases either due to leakage or battery current. When the voltage across the capacitor C 404  or the current into the battery V 401  drops below a predetermined level such as 95 percent of the nominal output voltage, the controller X 902  closes the switch S 911  and discharges the capacitor C 911  through the metallic coil  801 . This discharge causes a spike of voltage to appear across the metallic coil  851 . The controller X 902  then closes the switch S 912 . The controller X 901  interprets the voltage spike across the metallic coil  851  as a start-up signal, so it again begins switching the full-bridge power train formed with the power switches Q 401 , Q 402 , Q 403 , Q 404 . 
     Thus, the power system includes the wireless battery interface  850  (with the wireless battery interface magnetic core piecepart  852 ) and the wireless battery  800 . The wireless battery  800  includes the removable wireless battery magnetic core piecepart  802  configured to be coupled to and aligned with the wireless battery interface magnetic core piecepart  852  to form a transformer. A battery V 401  of the wireless battery  800  is metallically coupled to the metallic coil  801  surrounding the wireless battery magnetic core piecepart  802  and configured to be charged and discharged through an electrically isolating path of the transformer. The power system also includes a power train including a first switching circuit (the full-bridge power train formed with the power switches Q 405 , Q 406 , Q 407 , Q 408 ) of the wireless battery  800  configured to form a portion of a resonant topology with a second switching circuit (the full-bridge power train formed with the power switches Q 401 , Q 402 , Q 403 , Q 404 ) of the wireless battery interface  850 . The first switching circuit may be configured to be operated with a first duty cycle and the second switching circuit may be configured to be operated with a second duty cycle. The first duty cycle and the second duty cycle are controlled to enable a bidirectional power flow between the wireless battery  800  and the wireless battery interface  850  without altering the first duty cycle and the second duty cycle. 
     A first controller or the controller X 902  in the wireless battery  800  is configured to provide a signal to couple an impedance (e.g., the capacitor C 911 ) across the metallic coil  801  in the wireless battery  800  that advances a zero-crossing of a current in the metallic coil  851  surrounding the wireless battery interface magnetic core piecepart  852 . A second controller or the controller X 901  in the wireless battery interface  850  is configured to disable the second switching circuit (e.g., the full-bridge power train formed with the power switches Q 401 , Q 402 , Q 403 , Q 404 ) in the wireless battery interface  850  in response to detecting the advance of the zero-crossing of current in the metallic coil  851  in the wireless battery interface  850 . The controller X 902  in the wireless battery  800  is configured to enable a voltage pulse to be applied across the metallic coil  801  in the wireless battery  800  that is reflected to the metallic coil  851  in the wireless battery interface  850 , and the controller X 901  in the wireless battery interface  850  is configured to enable operation of the second switching circuit (e.g., the full-bridge power train formed with the power switches Q 401 , Q 402 , Q 403 , Q 404 ) in the wireless battery interface  850  in response to detecting the voltage pulse across the metallic coil  851  in the wireless battery interface  850 . The controller X 901  in the wireless battery interface  850  can operate the wireless battery interface  850  in a boost mode of operation to manage a power flow to the wireless battery  800 . 
     Turning now to  FIG. 9 , illustrated is a graphical representation of waveforms demonstrating an embodiment of an operation of the power system of  FIG. 8 . The first waveform  910  in  FIG. 9  represents gate drive signals for the power switches Q 401 , Q 404 . The second waveform  920  represents gate drive signals for the power switches Q 402 , Q 403 . The third waveform  930  represents the voltage V C911  across the capacitor C 911 . The fourth waveform  940  represents the current i p  for the wireless battery interface  850 . (See, e.g., the current i p  for the power system illustrated in  FIGS. 3 and 4 .) At a time  911 , the wireless battery  800  issues a turn-off pulse by opening the switch S 912  and pulsing the switch S 911 . Opening the switch S 912  and pulsing the switch S 911  effectively replaces the battery V 401  with capacitor C 911 . Placing the capacitor C 911  across the metallic coil  801  produces a resonant current therein, which is reflected to the metallic coil  851 . The net effect of the resonant current through the metallic coil  801  is to advance the zero-crossing of the current i p  as can be seen looking at the fourth waveform  940  in  FIG. 9 . At a time  912 , the switching action of the power switches Q 401 , Q 404  is terminated. Thus, the controller X 902  in the wireless battery  800  is configured to enable the advance of the zero-crossing of the current through the metallic coil  851  in the wireless battery interface  850 . The controller X 901  in the wireless battery interface  850  is configured to disable operation of the second switching circuit (e.g., the full-bridge power train formed with the power switches Q 401 , Q 402 , Q 403 , Q 404 ) in response to detecting the advance of the zero-crossing of the current through metallic coil  851  in the wireless battery interface  850 . 
     Ideally, the pulsing of the switch S 911  should be timed to allow the voltage across the capacitor C 911  to resonate to a maximum value prior to opening the switch S 911 . To command the controller X 901  to restart the power flow from the wireless battery interface  850 , the switch S 911  can be closed once again for a short pulse. This will cause a voltage pulse to be applied across the metallic coil  801  in the wireless battery  800  that is reflected to the metallic coil  851  in the wireless battery interface  850 . The controller X 901  in the wireless battery interface  850  is configured to enable operation of the full-bridge power train formed with the power switches Q 401 , Q 402 , Q 403 , Q 404  in response to detecting the voltage pulse across the metallic coil  851  in the wireless battery interface  850 . 
     Turning now to  FIG. 10 , illustrated is a graphical representation of waveforms demonstrating an embodiment of a boost mode of operation of the power system of  FIG. 8 . The first waveform  1010  in  FIG. 10  represents gate drive signals for the power switches Q 401 , Q 404 . The second waveform  1020  represents gate drive signals for the power switches Q 402 , Q 403 . The switching frequency of the gate drive signals (when on) is much higher than the frequency of the burst mode of operation. Thus, the individual gate drive signals appear blurred together while the full-bridge power train is operating. The third waveform  1030  represents the voltage V 401  across the battery (also referred to as V 401 ). The fourth waveform  1040  represents the current i p  for the wireless battery interface  850 . (See, e.g., the current i p  for the power system illustrated in  FIGS. 3 and 4 .) Analogous to the gate drive signals, the current i p  appears blurred together while the full-bridge power train is operating. 
     The battery voltage V 401  increases when the wireless battery interface full-bridge power train is operating and decreases when the wireless battery interface full-bridge power train is off. Turn-on and turn-off voltage thresholds for the battery voltage V 401  determine the amount of voltage ripple across the battery V 401  and will determine the burst frequency of a burst mode of operation. Voltage thresholds that reduce the voltage ripple across the battery V 401  produce a higher burst frequency. In practice, the frequency of the burst mode of operation should be designed to be lower than the switching frequency of the full-bridge power train, preferably at least two orders of magnitude lower. Otherwise, start-up and turn-off losses at the transitions of the burst mode of operation may be significant and may noticeably reduce the power conversion efficiency of the power system. 
     While the operation of the power system has been described above with the wireless battery interface  850  acting as a power transmitter and the wireless battery  800  acting as a power receiver, the power system may be operated in reverse. Under such circumstances, the capacitor C 910 , the switch S 901 , the Zener diodes Z 901 , Z 902 , the switch S 902 , and the controllers X 901 , X 902  perform analogous functions to the corresponding parts described above when the wireless battery  800  acts as a power transmitter and the wireless battery interface  850  acts as a power receiver. 
     Turning now to  FIG. 11 , illustrated is a diagram of an embodiment of a magnetic device. The magnetic device includes a removable first magnetic core piecepart  1110  having a surrounding first metallic coil  1115 , a second magnetic core piecepart  1120  having a surrounding second metallic coil  1125 , and a third magnetic core piecepart  1130 . The first, second and third magnetic core pieceparts  1110 ,  1120 ,  1130  are coupled and aligned to form a transformer. The first magnetic core piecepart  1110  may be a part of a wireless battery  1100  analogous to the wireless battery  800  illustrated in  FIG. 8  and the second and third magnetic core pieceparts  1120 ,  1130  may be a part of a wireless battery interface  1150  analogous to the wireless battery interface  850  illustrated in  FIG. 8 . In accordance therewith, the magnetic device forms a coupler between the wireless battery interface  1150  and the wireless battery  1100 . A cavity  1160  in the wireless battery interface  1150  is configured to receive the wireless battery  1100  and consequentially the first magnetic core piecepart  1110 . The third magnetic core piecepart  1130  has a relative magnetic permeability between a relative magnetic permeability of air and the first magnetic core piecepart  1110 . As an example, the relative magnetic permeability of the third magnetic core piecepart  1130  is between 4 and 100. The use of a low relative permeability in a short section (e.g., the third magnetic core piecepart  1130 ) of the entire magnetic path causes the overall magnetic properties of the magnetic device to become relatively insensitive to small changes in the length of the air gaps between the wireless battery  1100  and wireless battery interface  1150 . Other variations of the magnetic device are possible. For example, the third magnetic core piecepart  1130  may be located in the wireless battery  1100  rather than wireless battery interface  1150 . Furthermore, the third magnetic core piecepart  1130  may be embodied within or appended to a section of the first and second magnetic core pieceparts  1110 ,  1120 . 
     Turning now to  FIGS. 12A and 12B , illustrated are horizontal and vertical cross-sectional views, respectively, of an embodiment of a permanent magnet aligner. A dotted line  1205  in  FIG. 12A  shows the cross-section used for  FIG. 12B , and a dotted line  1207  in  FIG. 12B  shows the cross-section used for  FIG. 12A . A wireless battery interface enclosure  1290  houses a permanent magnet aligner including magnetic rings  1240 ,  1250 , as well as an axially symmetric magnetic coupler formed with a winding (or metallic coil)  1230  and magnetic core sections (or pieceparts)  1220 ,  1225 ,  1228 . The magnetic coupler shown in  FIGS. 12A and 12B  is formed with a pot core, which is axially symmetric around a centerline  1209 . The magnetic coupling, therefore, remains unchanged if the entire structure is rotated about the centerline  1209 . Other types of magnetic cores can be used instead of a pot core, provided that the magnetic core sections are axially symmetric around an axis of alignment for the wireless battery interface and the wireless battery. The magnetic ring  1240  is oriented with a north pole against the top of the wireless battery interface enclosure  1290 . The magnetic ring  1250  is also oriented with a south pole against the top of the wireless battery interface enclosure  1290 . 
     A wireless battery enclosure (not shown) has a very similar (if not identical) structure to that shown for the wireless battery interface enclosure  1290  except that the inner and outer magnetic rings are reversed in polarity. When the wireless battery enclosure and wireless battery interface enclosure  1290  are positioned close to each other, the magnetic rings cause the magnetic couplers of the wireless battery interface enclosure  1290  and the wireless battery enclosure to align with each other. 
     Turning now to  FIG. 13 , illustrated is a plan view of an embodiment of a permanent magnet aligner. A wireless battery interface enclosure  1380  is formed with the permanent magnet aligner including first, second and third disk-shaped magnet aligners  1320 ,  1330 ,  1340 . The first and third disk-shaped magnet aligners  1320 ,  1340  are oriented with south poles facing against the surface of the wireless battery interface enclosure  1380 . The second disk-shaped magnet aligner  1330  is oriented with a north pole facing against the surface of the wireless battery interface enclosure  1380 . A wireless battery enclosure (not shown) also has a permanent magnet aligner with matching disk-shaped magnet aligners that line up with the first, second and third disk-shaped magnet aligners  1320 ,  1330 ,  1340 . The matching disk-shaped magnet aligners of the wireless battery enclosure have pole arrangements opposite the corresponding first, second and third disk-shaped magnet aligners  1320 ,  1330 ,  1340  of the wireless battery interface enclosure  1380 . 
     Other arrangements of permanent magnets can be used to align the wireless battery interface and wireless battery. The implementations illustrated in  FIGS. 12A, 12B and 13  are two examples showing the concept of using permanent magnets to align the wireless battery interface and wireless battery. For example, some portions of permanent magnetic material can be replaced with iron or other ferromagnetic substances. Furthermore, permanent magnet alignment can be combined with mechanical structures to help guide the wireless battery interface and wireless battery to approximately correct positions while relying on the permanent magnets for more precise end alignment. 
     Turning now to  FIG. 14 , illustrated is a diagram of an embodiment of a portion of a magnetic device. The magnetic device (e.g., a transformer) includes an aligner configured to mechanically align a first magnetic core piecepart  1410  to a second magnetic core piecepart  1420 . The aligner is formed with a first structure  1430  on the first magnetic core piecepart  1410  and a second structure  1440  on the second magnetic core piecepart  1420 . While the first and second structures  1430 ,  1440  are illustrated as square or rectangular, any geometry or configuration including, without limitation, a circularly symmetric protuberance that is configured to fit into a corresponding circularly symmetric depression may be employed to advantage. The result is that the first magnetic core piecepart  1410  is accurately mechanically aligned with the second magnetic core piecepart  1420 . In an embodiment, the mechanical coupling enables at least 90 percent of a magnetic field induced in the first magnetic core piecepart  1410  to be linked to the second magnetic core piecepart  1420 . 
     Turning now to  FIG. 15 , illustrated is a diagram of another embodiment of a portion of a magnetic device. The magnetic device (e.g., a transformer) includes an aligner configured to mechanically align a first magnetic core piecepart  1510  to a second magnetic core piecepart  1520 . The aligner is formed with a first structure  1530  within the first and second magnetic core pieceparts  1510 ,  1520 , and a second structure  1540  outside of the first and second magnetic core pieceparts  1510 ,  1520 . The result is that the first magnetic core piecepart  1510  is accurately mechanically aligned with the second magnetic core piecepart  1520 . In an embodiment, the mechanical coupling enables at least 90 percent of a magnetic field induced in the first magnetic core piecepart  1510  to be linked to the second magnetic core piecepart  1520 . The first and second structures  1530 ,  1540  may be formed of a nonmagnetic material such as a plastic. 
     Turning now to  FIG. 16 , illustrated is a diagram of another embodiment of a portion of a magnetic device. The magnetic device (e.g., a transformer) includes an aligner configured to mechanically align a first, second, third and fourth magnetic core pieceparts  1610 ,  1620 ,  1630 ,  1640 . The aligner is formed with a first structure  1650  within the first, second, third and fourth magnetic core pieceparts  1610 ,  1620 ,  1630 ,  1640 , and a second structure  1660  outside of the first, second, third and fourth magnetic core pieceparts  1610 ,  1620 ,  1630 ,  1640 . The result is that the first, second, third and fourth magnetic core pieceparts  1610 ,  1620 ,  1630 ,  1640  are accurately mechanically aligned. In an embodiment, the third and fourth magnetic core pieceparts  1630 ,  1640  have a uniform relative magnetic permeability between about 4 and 100.  FIG. 16  also illustrates gaps such as gaps  1670  in the magnetic circuit path of magnetic device. The gaps  1670  include a substantially nonmagnetic material such as air or a plastic. A total length of all gaps  1670  in the magnetic circuit path of the magnetic device is typically less than about 1.5 times a square root of a cross-sectional area, for instance, of the first magnetic core piecepart  1610  perpendicular to said magnetic circuit path. 
     Thus, an apparatus, system and method to wirelessly charge and/or discharge a battery have been introduced herein. In one embodiment, an apparatus includes a removable first magnetic core piecepart (e.g., a wireless battery magnetic core piecepart  802  of the power system of  FIG. 8 ) having a surrounding first metallic coil (e.g., a metallic coil  801  of the power system of  FIG. 8 ) and configured to be coupled to and aligned with a second magnetic core piecepart (e.g., a wireless battery interface magnetic core piecepart  852  of the power system of  FIG. 8 ) having a surrounding second metallic coil (e.g., a metallic coil  851  of the power system of  FIG. 8 ) to form a transformer. A battery (e.g., a battery V 401  of the power system of  FIG. 8 ) is metallically coupled to the first metallic coil and configured to be charged and discharged through an electrically isolating path of the transformer. 
     The first magnetic core piecepart and the second magnetic core piecepart may be configured to be aligned with a permanent magnet (see, e.g., the permanent magnet aligners illustrated in  FIGS. 12A, 12B and 13 ). The apparatus may include an aligner configured to mechanically align the first magnetic core piecepart to the second magnetic core piecepart. In accordance therewith, the first magnetic core piecepart and the second magnetic core piecepart may include a structure (e.g., the first and second structures  1430 ,  1440  of the magnetic device of  FIG. 14 ) configured to mechanically align the first magnetic core piecepart to the second magnetic core piecepart. The mechanical coupling may enable at least 90 percent of a magnetic field induced in the first magnetic core piecepart to be linked to the second magnetic core piecepart. The apparatus may include a cavity (see, e.g., the magnetic device of  FIG. 11 ) configured to receive the first magnetic core piecepart. 
     The apparatus may include a power train (e.g., a full-bridge power train of the power system of  FIG. 8 ) including a first switching circuit (e.g., power switches Q 405 , Q 406 , Q 407 , Q 408  of the power system of  FIG. 8 ) coupled to the first metallic coil configured to form a portion of a resonant topology with a second switching circuit (e.g., power switches Q 401 , Q 402 , Q 403 , Q 404  of the power system of  FIG. 8 ) coupled to the second metallic coil. The power train may be intermittently operated in a burst mode of operation to control a characteristic (such as the voltage V 401  illustrated in  FIG. 10 ) of the battery. A capacitor (e.g., the resonant capacitor C 402  of the power system of  FIG. 8 ) in the power train may be selected to produce substantially zero-current switching of the first switching circuit in the power train in conjunction with an inductor (e.g., formed at least in part with the first metallic coil). The power train is also configured to enable the battery to be successively charged and discharged without changing a duty cycle of the first switching circuit and the second switching circuit. 
     A controller (e.g., a controller X 401  of the power system of  FIG. 2 ) of the apparatus may be configured to selectively cause at least a portion of the power train to switch between full-bridge and half-bridge operation in response to a sensed voltage level. The apparatus may also include a third magnetic core piecepart (see, e.g., the third magnetic core piecepart  1130  in the magnetic device of  FIG. 11 ) couplable to the first and/or second magnetic core piecepart and having a relative magnetic permeability between a relative magnetic permeability of air and the first and/or second magnetic core piecepart. The relative magnetic permeability of the third magnetic core piecepart may be between 4 and 100. 
     Other effective alternatives will occur to a person skilled in the art. For example, the battery within the wireless battery can be replaced with a battery coupled to a battery management system. Those skilled in the art should understand that the previously described embodiments of the power system and related methods of operating the same are submitted for illustrative purposes only. In addition, various power converter topologies are well within the broad scope of the present invention. While the wireless battery interface and the wireless battery have been described in the environment of a bridge topology, it may also be applied to other systems such as, without limitation, a power amplifier and a motor controller. An example of another wireless power system is disclosed in U.S. patent application Ser. No. 14/754,915, entitled “Wireless Power System and Method of Operating the Same,” by Garrity, et al., filed concurrently herewith, which is incorporated herein by reference. 
     For a better understanding of power converters, see “Modern DC-to-DC Power Switch-mode Power Converter Circuits,” by Rudolph P. Severns and Gordon Bloom, Van Nostrand Reinhold Company, New York, N.Y. (1985) and “Principles of Power Electronics,” by J. G. Kassakian, M. F. Schlecht and G. C. Verghese, Addison-Wesley (1991). The aforementioned references are incorporated herein by reference in their entirety. 
     Also, although the present invention and its advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the invention as defined by the appended claims. For example, many of the processes discussed above can be implemented in different methodologies and replaced by other processes, or a combination thereof. 
     Moreover, the scope of the present application is not intended to be limited to the particular embodiments of the process, machine, manufacture, composition of matter, means, methods, and steps described in the specification. As one of ordinary skill in the art will readily appreciate from the disclosure of the present invention, processes, machines, manufacture, compositions of matter, means, methods, or steps, presently existing or later to be developed, that perform substantially the same function or achieve substantially the same result as the corresponding embodiments described herein may be utilized according to the present invention. Accordingly, the appended claims are intended to include within their scope such processes, machines, manufacture, compositions of matter, means, methods, or steps.