Loose rivets,
A battery discharge of 10-15% / day is, unfortunately, fairly normal on Sony batteries- I'm fairly sure it's something to do with the integrated circuit which tells the computer how much is left in it using up the juice, which apart from being ironic is fairly self-defeating!!
They've also had massive recalls because some batteries exploded (notably in Dells and, though it pains me to say it, Apples).
They're one of the largest battery manufacturers in the world, but I'd never buy any Sony gear- used to be great but has gone very downhill recently!