ThinkPad's battery life

I am thinking of buying ThinkPad X61. Does anybody know how many hours an 8-cell battery lasts on ThinkPad X61 and how heavy is it? If I use the laptop's battery, it is only during long transcontinental flight watching movies (not from a DVD but from files on my harddrive).

Also, another question. From my past experience with laptops I noticed that during the laptop's life, its battery gets worse and worse. If at the beginning the laptop could run on batteries for three hours, now, after three years, it could barely run for twenty minutes. I don't run my laptop on batteries often, and I wonder if the battery's life gets worse with time even if the laptop is always connected to AC. Am I correct? I am thinking of getting a cheap 4-cell battery on eBay to use with the X61 to save the original 8-cell battery and make the laptop lighter. What do you think?

 

If I am correct the X61's are supposed to get about 8hrs+ on the 8 cell battery, but don't quote me on that one. Also the battery on just about any device can and will be depleted with time and usage. Typically manufacturers offer different warranties on batteries for said reason. That being said newer batteries tend to have longer lifespans than those from a few years back. On the point of buying the 4 cell battery; Its not a bad idea at all. Especially if you are doing it for weight savings reasons. Also you can setup up the power management in the thinkvantage utility to maximize battery life.