Battery died while turned off...

I have a White Macbook, 2.26Ghz Intel Core 2 Duo, 4GB DDR3 memory, 320GB hard-drive.

A few months back I noticed my battery life had started decreasing, not an awful lot just a enough for me to notice. And today was the first time I shut down my mac since I installed Lion last week and after 11 hours of been turned off I went to turn my mac on and it wouldn't, the battery was completely dead. I turned on my power supply and the macbook turned on, started up slower than normal, and after I logged in. There was no status bar at the top until I change spaces, and only half the status bar was showing information which was the right side with the clock etc. After opening Finder the it all returned to normal.

So yes, what is going on? If anyone knows?