TP Ultrab SlimLiPolymer Batt
Swap out your optical drive with a second battery to increase your unplugged power supply. The ThinkPad Ultrabay Slim Li-Polymer Battery fits neatly into the Ultrabay Slim bay of the ThinkPad T40. It can be used with the system battery or the optional ThinkPad T40 Series High Capacity Li-Ion Battery for all day computing (up to 8 hours on select systems).
Lenovo Ultrabay Slim Li-Polymer Battery for ThinkPad T41/42, R50, R51, R50p and R50e, Battery, Lenovo
Lenovo Ultrabay Slim Li-Polymer Battery for ThinkPad T41/42, R50, R51, R50p and R50e. Type: Battery, Brand compatibility: Lenovo