My 2009 13" Macbook Pro's battery only held about half the charge it use to. Additionally, I began to notice that my trackpad would stop clicking when the battery charge was greater than about 80%, which was odd. After some research it sounded like my battery was swelling and putting pressure on the back of the trackpad preventing the mechanical click.
Using iFixit's guide, buying the triwing Y1 and replacement battery the repair only took a few minutes.
If you're lucky enough to have one of these unibody Macbooks, use iFixit to keep them running as long as you can. Apple's new retina laptops are simply not as repairable.