A charging device comprises a rotating unit, an axle unit assembled to the rotating unit and engaging the rotating unit, and two charging units assembled to the axle unit. Each charging unit includes a magnet retainer assembled to the axle unit and engaging the axle unit, a plurality of magnets received in the magnet retainer, and a case rotatably assembled to the axle unit and including a plurality of windings. Rotation of the rotating unit drives the axle unit and the magnet retainers to rotate, the rotating magnet retainers and magnets further rotate relative to the cases due to the inertia of the cases, and the magnets move relative to the windings and generate induction currents in the windings, for charging a battery.