Swap your optical drive for a second battery to increase your unplugged usage time. The ThinkPad Advanced UltraBay Battery fits neatly into the UltraBay media bay of ThinkPad Z Series notebooks and can be used in conjunction with the system battery. For all day computing use the Advanced UltraBay Battery with the optional ThinkPad Z60t Series 7 Cell Li-Ion Battery or the optional ThinkPad Z60m Series 9 Cell Li-Ion Battery. These combinations provide up to 8 hours run time on select systems with the 7 cell battery and up to 8.5 hours run time on select systems with the 9 cell battery. Battery life (and recharge times) depends on many factors including screen brightness, applications, features, power management, battery condition and other customer preferences.