My battery in my MBP was barely holding a charge and had several hundred cycles under its belt.
Perfect. I was a bit concerned about losing some of those infinitely miniature screws, but I was fortunate to have them all in place when the job was complete.
Get the necessary tools. You will need tham.
P6 Pentalobe Screwdriver 2009 15" MacBook Pro Battery
MacBook Pro 15" Unibody (Mid 2009/Mid 2010) Replacement Battery