Abstract: A monitoring system is described which comprises a sensor device for generating sensor data, the sensor device having a secondary coil, and a receiver device having a controller, and a primary coil for wirelessly communicating with the sensor device, the receiver device being operable to wirelessly charge the sensor device via inductive coupling between the primary and secondary coils. A quality factor of the primary coil is controllable, and the controller is operable to control the quality factor of the primary coil to be higher when the receiver device is wirelessly charging the sensor device than when the receiver device is receiving sensor data from the sensor device. As a result, the same coil can be used both for efficient power transfer (wireless charging) by using the coil in a (relatively) high quality factor mode, and for reliable data communications by using the coil in a (relatively) low quality factor mode.