iPhone 4s overheating and suffering from drained battery So I have this iPhone 4s, which has a serious problem: when doing absolutely nothing with it (it just sits in standby mode) the battery drains from 100% to 0% in about 12 hours or less. Also, it appears to be working very actively, as it is constantly warm.

Does anyone know what the cause of this is, and how to fix it? I do have all the latest updates, so that can't be the issue (I think).
I would suggest backing up your phone with iTunes (a whole backup and not only synchronising) and transfer all your purchase. Afterwards, restore the phone with iTunes. 

Test it for a couple days and if the issue is solved, bring back your back up. If the issue reappears, restore it again and re-download your content manually. It might be your user data or settings, or an app making your phone work when it's not supposed to.

If the issue persists even after the clean restore (without bringing back your back up), I suggest to get your device checked at a Genius Bar near you.