A wireless power supply transmits electrical energy from a power source to a load without the use of a connecting wire. There are a number of wireless power supply techniques including electromagnetic induction, electromagnetic radiation, and electrical conduction. Here, we have investigated methods that use electromagnetic induction. Typically, for electromagnetic induction, we assume that the load resistance will fluctuate during battery charging due to variations in the battery voltage. Therefore, to stabilize the electrical power supply, a steady input impedance of the transmission circuit is preferable such that the influence of load resistance fluctuations is minimized. To this end, we suggest a "series-parallel resonant circuit" for the transmission circuit that has this characteristic. (C) 2012 American Institute of Physics. [doi:10.1063/1.3676046]