1/2AA size cylindrical cell with button top. 3.6 Volt Lithium Battery. Fits models; Advantech Industrial Computer 610, Apple Desktop Power Mac G3 266 Desktop, Apple Desktop Power Mac G3 Desktop, Apple Desktop Power Mac G3 Desktop series, Apple Macintosh Centris 610, Apple Macintosh Centris 630, Apple Macintosh Classic, Apple Macintosh Cla