IPhone battery life shorter.dying early?

I currently have the IPhone 5 and for some reason it keeps dying at 7-20 percent. Sometimes I'm in the middle of a call and it just dies when its at 20%. I have IOS 7 currently and I make sure I close all my apps when I'm done with them. Does this happen to anybody else? How can I avoid this problem? Please help out.

Thank you.

Added (1). It is also not jailbroken

Not sure. I have the 5 but not ios 7 and that doesn't happen to me…

The problem is simple. You have been plugging your Phone in to recharge when it is not dead. Doing this to any rechargeable batteries will shorten it's life considerably in very short time. The only remedy is get a new battery and do not recharge it until it absolutely dead.