Carry a spare ThinkPad X60 Tablet Li-Ion Battery so you can stay unplugged longer!
4 Cell Li-Ion Battery: 40Y8314 (3 hours average battery life )
8 Cell Li-Ion Battery: 40Y8318 (7 hours of system run time)
http://www.laptoptraveller.com/battery-ibm-thinkpad-x60-tablet-laptop-notebook-accessories.html

Battery life (and recharge time) depends on many factors including screen brightness, applications, features, power management, battery condition and other customer preferences.

The ThinkPad X60 Tablet 4 Cell Battery features:
• Lithium-Ion technology
• 14.4 V, 2.0 Ah
• Three-hour average battery life
• Notched design to accommodate tablet screen swivel hinge for notebook or convertible tablet operation
• Standard system battery specifications
• Over-discharge protection
• Product approvals/certifications: CE MARK, CUL, C-Tick, UL-1950


http://www5.pc.ibm.com/europe/products.nsf/$wwwPartNumLookup/_92P1009