My iPhone 4 powers off if it reaches 32% of battery.

So, my iPhone alsways powers off if the Baterry still has 32%. If I want to turn it on, there is that symbol which shows me to connect it back to a power source. I don't think it has somethink to do with the battery, becaus it always powers off at 32%. Does somebody have advice?

Sorry for the bad English, I'm from Germany 😉