It is disclosed a device which includes an elongated member, and an energy convertor. The elongated member has a base side and a free end. A certain direction is defined by a line connecting centers of the base side and the free end. The free end oscillates relative to the base side, in a response to a mechanical excitation, in a direction substantially perpendicular to the certain direction. The energy convertor converts mechanical energy of the oscillations of the free end to electrical energy. The energy convertor includes a coil attached to the free end, and a permanent magnet tightly fixed to the base side. Mechanical oscillation of the coil relative to the permanent magnet induces a flow of alternate current from the energy convertor. The energy convertor is electrically connected to a current rectifier for rectifying the alternate current, whereas the rectified direct current feeds a battery. The device is installed within a host mobile device, and the base side is fixed to the host mobile device. The device is enclosed within a vacuum chamber such as to prevent dissipation of energy of the mechanical oscillation by air friction.