A cell apparatus (10, 20, 30) for emitting and receiving AC power via magnetic field resonance coupling to an external device, comprising: a cell interface to interface with a cell; a first circuit to generate, in a discharging mode, a magnetic field to induce an AC current in the external device, and to generate, in a charging mode, an AC current from a magnetic field received from the external device; a conversion circuit (23, 33) to generate, in the charging mode, a charge signal from the AC current generated in the first circuit, the conversion circuit (23, 33) supplying the charge signal to the cell interface; a frequency generator (13, 34) to generate, in the discharging mode, an AC current from a discharge signal received from the cell interface, the frequency generator supplying the AC current to the first circuit; an input device (37, 301) to receive a selection of the charging mode and the discharging mode; and a control section to generate a control signal based on the selection, characterized by a switching circuit (38, 39) to couple, based on the control signal, the cell interface to the conversion circuit (23, 33) and the conversion circuit (23, 33) to the first circuit in the charging mode, and to couple the cell interface to the frequency generator (13, 34) and the frequency generator (13, 34) to the first circuit in the discharging mode, wherein the switching circuit (38, 39) comprises a first switch circuit that is configured to couple the conversion circuit (23, 33) to the first circuit in the charging mode and to couple the frequency generator (13, 34) to the first circuit in the discharging mode, and a second switch circuit configured to couple the cell interface to the conversion circuit (23, 33) in the charging mode and to couple the cell interface to the frequency generator (13, 34) in the discharging mode; characterized in that the first circuit (23, 33) comprises a resonance element coupled to an excitation element, the excitation element being coupled to the first switching circuit; wherein the resonance element and the excitation element are air-core coils having a same self-resonance frequency, and the frequency generator (13, 34) generates the AC current at the self-resonance frequency. a cell interface to interface with a cell; a first circuit to generate, in a discharging mode, a magnetic field to induce an AC current in the external device, and to generate, in a charging mode, an AC current from a magnetic field received from the external device; a conversion circuit (23, 33) to generate, in the charging mode, a charge signal from the AC current generated in the first circuit, the conversion circuit (23, 33) supplying the charge signal to the cell interface; a frequency generator (13, 34) to generate, in the discharging mode, an AC current from a discharge signal received from the cell interface, the frequency generator supplying the AC current to the first circuit; an input device (37, 301) to receive a selection of the charging mode and the discharging mode; and a control section to generate a control signal based on the selection, characterized by a switching circuit (38, 39) to couple, based on the control signal, the cell interface to the conversion circuit (23, 33) and the conversion circuit (23, 33) to the first circuit in the charging mode, and to couple the cell interface to the frequency generator (13, 34) and the frequency generator (13, 34) to the first circuit in the discharging mode, wherein the switching circuit (38, 39) comprises a first switch circuit that is configured to couple the conversion circuit (23, 33) to the first circuit in the charging mode and to couple the frequency generator (13, 34) to the first circuit in the discharging mode, and a second switch circuit configured to couple the cell interface to the conversion circuit (23, 33) in the charging mode and to couple the cell interface to the frequency generator (13, 34) in the discharging mode; characterized in that the first circuit (23, 33) comprises a resonance element coupled to an excitation element, the excitation element being coupled to the first switching circuit; wherein the resonance element and the excitation element are air-core coils having a same self-resonance frequency, and the frequency generator (13, 34) generates the AC current at the self-resonance frequency.