A supplemental arc maintenance electrical supply for a high density discharge (HID) lamp (L) for applying a high frequency supplemental arc voltage to maintain the arc during a period of unstable or intermittent AC supply in addition to or in lieu of the voltage or current being supplied to the lamp by the HID ballast (12). AC supply instability is identified in less time than the unstable or intermittent power will cause the lamp to arc to extingish and energize a supplemental arc maintenance electrical supply to maintain the arc of the lamp during the period of instability or interruption, so long as its internal, rechargeable battery is operational. The monitoring system (18) also recognizes the re-establishment of a stable (as by being a persistently repeating cycle but not necessarily a sinusoidal AC supply) repeating electrical power supply which is also of sufficient energy level to maintain operation of the HID lamp.