I have a Dell Inspiron 6400, which I've had for just a year. I usually keep it plugged in, pulling out the AC cord occasionally when I want to move around and work on battery power.
Yesterday, I did just that, and my laptop just shutdown immediately. Hmm. After plugging in the AC cord again and powering up, the Vista battery meter said:
"99% available (plugged in, not charging)"
I shut it down, unplugged the AC from both the laptop and the wall socket, removed the battery, checked the battery status (fully charged, all lights on the battery come on when I press the test button), cleaned the battery bay and connections, re-seat the battery, connected the AC, and powered up again.
Same thing. Although the battery seems good, the laptop doesn't seem to want to use it. It just dies completely whenever I pull out the AC.