A light-emitting diode (LED) lighting system comprising a luminaire and an add-on control gear is used to replace the luminaire operated with AC mains. The luminaire comprises LED arrays and a power supply with a luminaire DC voltage. When a line voltage from the AC mains is unavailable, the add-on control gear is automatically started to provide a control DC voltage greater than a forward voltage across the LED arrays. The add-on control gear comprises a rechargeable battery, a DC-to-DC converter configured to provide the control DC voltage when enabled, and a power detection and control circuit. The power detection and control circuit comprises a transistor circuit and a relay switch, in which the former is configured to enable the DC-to-DC converter, and the latter is configured to couple either the control DC voltage or the luminaire DC voltage to the LED arrays to operate thereon.