My iPhone 5c dies with plenty of battery life left.

My iPhone 5c first began dying at 8% when I first got the phone about 5 months ago. It slowly has increased to what percentage it dies at since then. It now dies at 75% and it is very frustrating. I still have iOS 8, as I don't have enough storage to download iOS 9. What could be wrong with my phone? How do I fix this problem?