Why does my iPhone battery drain so quickly?

I have had an iPhone 4s for almost 2 years. The phone has slowed down a lot since that time and the battery is drained extremely quikly, even when I'm not doing anything. If I just listen to music for 40 mins, doing nothing else, with a locked screen, my battery will go down 20%. It's insane. I never overcharge my phone. I charge it to 100% before I go to sleep and when I wake up its around 96%

Added (1). @ WB, No, even with everything closed it, the battery still leaks.

I found that by turning off the GPS, the battery life is greatly improved.

Do you have active apps; blue tooth, Wi-Fi, GPS, e-mail notification, auto sync, etc? These apps use power all the time unless you turn them off. Check your settings for running apps, and turn off the ones you don't need.

Turn off all apps that are running that are not In use including those running in the background that will help.

Maybe if you stop using it the battery would be better. *troll face*

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