A system, comprising a sender and a receiver; the receiver comprising: a receiver transducer configured to receive ultrasonic waves generated by a sender transducer and to generate a receiver electrical signal based on the received ultrasonic waves, the receiver transducer including at least one piezoelectric transducer; a receiver electrical storage device configured to store electrical energy; the receiver electrical storage device is further configured to store electrical energy associated with the receiver electrical signal generated by the receiver transducer; a receiver controller coupled to at least one of the receiver transducer and the receiver electrical storage device, and a receiver communications device configured to send a control signal comprising a power requirement to a sender communications device using one or more of the receiver transducer and a radio transmitter; the sender comprising: a signal generator; an amplifier coupled to the signal generator, said amplifier is configured to receive power from an electrical power source and to generate an electrical transmission signal adapted to be used to generate ultrasonic waves; a sender transducer coupled to the amplifier, the sender transducer is configured to generate ultrasonic waves based on the electrical transmission signal received from the amplifier; a sender communications device configured to receive the control signal comprising the power requirement from the receiver communications device using one or more of the sender transducer and a radio transmitter and a sender controller coupled to at least one of the signal generator, the amplifier and the sender transducer; the sender controller is adapted to change a dwell time of the sender on the receiver based on the received power requirement. a receiver transducer configured to receive ultrasonic waves generated by a sender transducer and to generate a receiver electrical signal based on the received ultrasonic waves, the receiver transducer including at least one piezoelectric transducer; a receiver electrical storage device configured to store electrical energy; the receiver electrical storage device is further configured to store electrical energy associated with the receiver electrical signal generated by the receiver transducer; a receiver controller coupled to at least one of the receiver transducer and the receiver electrical storage device, and a receiver communications device configured to send a control signal comprising a power requirement to a sender communications device using one or more of the receiver transducer and a radio transmitter; the sender comprising: a signal generator; an amplifier coupled to the signal generator, said amplifier is configured to receive power from an electrical power source and to generate an electrical transmission signal adapted to be used to generate ultrasonic waves; a sender transducer coupled to the amplifier, the sender transducer is configured to generate ultrasonic waves based on the electrical transmission signal received from the amplifier; a sender communications device configured to receive the control signal comprising the power requirement from the receiver communications device using one or more of the sender transducer and a radio transmitter and a sender controller coupled to at least one of the signal generator, the amplifier and the sender transducer; the sender controller is adapted to change a dwell time of the sender on the receiver based on the received power requirement. a signal generator; an amplifier coupled to the signal generator, said amplifier is configured to receive power from an electrical power source and to generate an electrical transmission signal adapted to be used to generate ultrasonic waves; a sender transducer coupled to the amplifier, the sender transducer is configured to generate ultrasonic waves based on the electrical transmission signal received from the amplifier; a sender communications device configured to receive the control signal comprising the power requirement from the receiver communications device using one or more of the sender transducer and a radio transmitter and a sender controller coupled to at least one of the signal generator, the amplifier and the sender transducer; the sender controller is adapted to change a dwell time of the sender on the receiver based on the received power requirement.