Many iPhone 4 users have reported that they seen their iPhone 4 get hot after updating to iOS 4.3 and iOS 4.3.1. This iPhone 4 heating has drained battery power up to 10% -14%. If you have this problem don’t worry, you can now fix it.
The phone overheating or running down the battery when on standby is the result of an application trying to send data and failing or sending large quantities of data; it’s the only thing that can cause that much battery drain. Games can cause heavy drain also, but that’s clearly not the issue if the phone is asleep. The troubleshooting procedure is to identify the app (or rarely apps), disable it, reboot, and then try to enable it.
If the problem isn’t email there are a few basic troubleshooting steps (stop once one works):
1. Drain the battery all the way until the phone shuts off and won’t restart, then charge for 4 hours with a wall charger.
2. Go to Settings/General/Reset and Reset Network Settings, then reboot (you will lose WiFi passwords)
3. Same, but Reset All Settings
4. Try disabling the following apps (in Settings/General/Restrictions): Multiplayer Games, Ping, App Store, iTunes Store. Then reboot. If this fixes it turn on the ones you use (one at a time) and see if the problem returns. It probably won’t.
5. Check 3rd party apps, especially Facebook, Yahoo, any app that uses Notifications (e.g., news apps) and any streaming apps. Some of them do not terminate unless you explicitly disconnect.
6. Some people have reported Facetime and Find my iPhone causing heavy battery drain. Worth checking