A battery charger for magnetically transferring power from a power source to a battery , the battery charger comprising: a driver circuit couplable to the power source , the driver circuit configured to output a modulated signal with energy from the power source , wherein the modulated signal has a duty cycle; a sender-side tuned circuit magnetically and wirelessly couplable across a space to a receiver-side tuned circuit of a portable handheld electronic device, wherein the sender-side tuned circuit is configured to have a resonant frequency equal to a resonant frequency of the receiver-side tuned circuit ; a sender-side sensor configured to sense a voltage at a node directly connected to a high side of an inductor coil of the sender-side tuned circuit ; and a controller coupled to the driver circuit and to the sender-side sensor , wherein the controller is configured to: cause the driver circuit to output a modulated signal at a first duty cycle having a first pulse width and at a frequency equal to the resonance frequency of at least one of the sender-side tuned circuit and the receiver-side tuned circuit , and modify the first pulse width of the first duty cycle of the modulated signal based on the sensed voltage of the sender-side tuned circuit , characterized in that the sender-side sensor comprises a diode having a first terminal coupled to a high side of an inductor, wherein a second terminal of the diode is coupled to the controller through a low pass filter and a voltage divider , wherein the sender-side tuned circuit includes an inductor , and wherein the low pass filter and the voltage divider share a resistor . a driver circuit couplable to the power source , the driver circuit configured to output a modulated signal with energy from the power source , wherein the modulated signal has a duty cycle; a sender-side tuned circuit magnetically and wirelessly couplable across a space to a receiver-side tuned circuit of a portable handheld electronic device, wherein the sender-side tuned circuit is configured to have a resonant frequency equal to a resonant frequency of the receiver-side tuned circuit ; a sender-side sensor configured to sense a voltage at a node directly connected to a high side of an inductor coil of the sender-side tuned circuit ; and a controller coupled to the driver circuit and to the sender-side sensor , wherein the controller is configured to: cause the driver circuit to output a modulated signal at a first duty cycle having a first pulse width and at a frequency equal to the resonance frequency of at least one of the sender-side tuned circuit and the receiver-side tuned circuit , and modify the first pulse width of the first duty cycle of the modulated signal based on the sensed voltage of the sender-side tuned circuit , characterized in that the sender-side sensor comprises a diode having a first terminal coupled to a high side of an inductor, wherein a second terminal of the diode is coupled to the controller through a low pass filter and a voltage divider , wherein the sender-side tuned circuit includes an inductor , and wherein the low pass filter and the voltage divider share a resistor . cause the driver circuit to output a modulated signal at a first duty cycle having a first pulse width and at a frequency equal to the resonance frequency of at least one of the sender-side tuned circuit and the receiver-side tuned circuit , and modify the first pulse width of the first duty cycle of the modulated signal based on the sensed voltage of the sender-side tuned circuit , characterized in that the sender-side sensor comprises a diode having a first terminal coupled to a high side of an inductor, wherein a second terminal of the diode is coupled to the controller through a low pass filter and a voltage divider , wherein the sender-side tuned circuit includes an inductor , and wherein the low pass filter and the voltage divider share a resistor . <EOS>