A battery charger and methods for magnetically transferring power from a power source to a battery is disclosed. A driver circuit outputs a modulated signal having a duty cycle from the power source to a sender-side circuit and across a space to a receiver-side tuned circuit. A sender-side sensor is configured to sense a parameter of the sender-side tuned circuit. The controller modifies the first duty cycle of the modulated signal based on the sensed parameter. A receiver-side circuit capable of determining charge status of the receiver side is also disclosed.