Iphone goes dead even when I have high percentage of battery left?

My iPhone sometimes goes dead and runs out of battery even when I have like 50% battery left? One time it was 85% and It just turned off itself? I have turned the brightness down and tried to have as little apps as possible. I go on Facebook all the time and twitter etc, I also use safari and text a lot. Any advice on how my iPhone could last longer?

Switch to android.