A method of determining a value of a magnetic characteristic of a wireless-power receiver system includes: obtaining a first frequency indication of a first resonant frequency of a power reception circuit of the wireless-power receiver system corresponding to a power transmit circuit and the power reception circuit being in a first state having a first combined circuit configuration; obtaining a second frequency indication of a second resonant frequency of the power reception circuit corresponding to the combination of the power transmit circuit and the power reception circuit being in a second state having a second combined circuit configuration, the first combined circuit configuration differing from the second combined circuit configuration by at least one of component content or a value of at least one component; and using the first frequency indication and the second frequency indication to determine the value of the magnetic characteristic of the wireless-power receiver system.