The battery of my 15" old MacBook Pro Mid 2009 was pretty much done after 4.5 years and getting a new battery for on older MBP model isn't that straight forward as you may think. Lots of cheap knockoffs and fakes.
I got all the information, tools (just a Pentalope P6) and a better battery with higher mhA than the original on ifixit.com and everything was delivered only 2 days later! I was prepared for at least a week since I am from Europe.
Repair went without any problems. I read the according guide on ifixit.com several times before doing it.
Everything including removal of the old HDD and installing a new SSD was done in 10 Minutes.
My old trusted MBP mid 2009 got a huge performance boost and the new battery works perfectly (6-8 hours easily).
I saved $3.000 I was about to drop for the latest MBP retina but I read about the LG / Samsung panel problems Apple is using for the new models and decided to upgrade my old MBP instead.
Read the ifixit.com guides carefully and repeatedly. It will help a lot. Don't buy cheap knockoff parts from ebay / amazon, look on ifixit.com first. It will save you money, time and a lot of headache.
P6 Pentalobe Screwdriver 2009 15" MacBook Pro Battery
MacBook Pro 15" Unibody (Mid 2009/Mid 2010) Replacement Battery