When the normal supply of AC power fails, the emergency driver/ballast senses the power failure and immediately switches to the emergency mode to illuminate the designated lighting load at a reduced lumen output for a minimum of 90 minutes. If multi lamp or module operation is selected, light output is evenly divided among the lamps/modules for better distribution of emergency illumination. When AC power is restored, the emergency driver/ballast returns to the charging mode until the next power failure or emergency situation. The battery fully recharges within 24 hours.