Lithonia Lighting® Fluorescent Emergency Ballast, Low Profile, Series: PSL550, Nickel-Cadmium, 3.6 VDC Nominal, 24 hr Charging, Steel Case, 3 Years Shelf Life, For Use With: Most One, Two, Three or Four-Lamp Electronic, Standard, Energy-Saving and Dimming AC Ballast, 18.5 in L x 1.18 in W x 1.18 in H