A few days ago I left my MacBook Pro at about 40% battery for a day or two (off, not sleeping). Then when I tried to boot it, it died after a few seconds. I tried a few times but it got worse and worse until it died instantly after pressing the button. This seemed very much that the battery was bad. So I charged it up and then tried again. It lasted a bit longer but then died. Even when it was plugged into the magsafe power source! I launched the MBP on battery, and it died, but this 'drained' the battery a little and it started charging again. I repeated this a few times until it wouldn't charge anymore. Once or twice I got far enough to log in and do some stuff. Only lasted a few minutes though. I launched into the boot selector and it did not die. I also ran the Apple Hardware Test and it did not die there either. I did the basic test and it found no problems. This made me think that the OS is to blame, but I tried to load into the OS X install disk. It died there too, while loading. It lasted longer though. So what could it be?! Battery dead? But shouldn't it run off the power source when it is plugged in, not the battery?