An electronic key including an internal power source is provided for transferring electrical power to a merchandise security device to operate a mechanical lock mechanism. In one embodiment, the key transfers power to the device via electrical contacts disposed on a transfer probe of the electronic key and corresponding electrical contacts disposed within a transfer port of the device when the transfer probe engages the transfer port. In another embodiment, the key transfers power to the device via inductive transfer as a result of passing an electrical current through an inductive coil disposed within the transfer probe to generate a magnetic field in the vicinity of a corresponding inductive coil disposed within the transfer port and thereby induce an electric current in the inductive coil of the device. In other embodiments, the electronic key is programmed with a security code and the key initially programs the merchandise security device with the security code and subsequently determines whether the security code of the key matches the security code of the device to permit the key to transfer power to the device.