The battery in my two year old iPhone has been steadily going downhill. I’m too cheap to take it somewhere  (Apple) for replacement, so I bought a $20 new battery online and worked up my courage to crack the phone open. The process isn’t brain surgery, but is not for the faint of heart. This photo shows  about the midway point. The battery is beneath the main board. I’m happy to say the phone is working great now. I can’t believe I didn’t lose any of the 10 or so microscopic screws!