even w/the new lithium batteries, if you reqularly use a laptop while running on battery, you will probably start to see degradation in the battery charge after about 18+ months.
for our 'power user' laptop users, we find we have to replace the batteries after 12 months.
i was told by a reseller that ibm has a lifetime warranty on their batteries. i was very skeptical about this, but true enough, we have not had to purchase replacement batteries for our thinkpads since i found this out. our reseller has been handling getting us replacements. go figure.
maybe another reason why i like the ibm thinkpads...