My iPhone 5 shutdown when the battery life hit 20%... help!!

Hello everyone....

My iPhone 5 about 3 weeks ago start to shutdown when it hit below 10%.... then it started to shutdown when it hit below 17%, and recently it started to shutdown once it hit below 20% of the battery life!!!!!

What can I do? What should I do to fix this?

Did anyone else had the same problem?

Why my iPhone can't act like a normal device and shutdown when it run off the battery life....... why 20%?