Even though this thread was posted for the iphone 4, there's a person on the last page of the thread that says it fixed his issue for his
iphone 4S. Couldn't hurt to try! I'll repost for the lazy fingers.
Basically, it says to delete your email accounts, reboot, reset your network settings, and re-add your email.
"I had the same issue many have reported here and in comments on blogs, where their +_battery was draining at least twice as fast_+ as on an iPhone 3GS with iOS 4.
Some threads suggest +_this happens to iPhone 4 when restored from a 3G or 3GS_+, and indeed, that's what I'd done. A few people said that resetting to factory settings and installing everything fresh solved the issue, giving them better battery life than they'd seen on any prior iPhone.
Many of my apps have files or settings stored in them, so I didn't want to have to set everything up again. A fresh install is a real pain.
Deep in an iPhone 3G thread mentioning a similar battery problem after upgrading to iOS 4, someone said they'd deleted all their mail accounts, and set those up fresh, which solved the issue for them. I tried that today, and it worked.
*_BEFORE AND AFTER_*:
Thursday and Friday, the iPhone 4 went from 100% to 15% in under 6 hours. Today, after this fix, the iPhone 4 is at 87% after 12 hours. Thursday and Friday the phone was hot to the touch whenever I picked it up. Today it's always cool. Apps usage and movement among coverage areas on each day have been exactly the same. The only change was resetting up mail.
*_STEPS TO FIX_*:
Do this on WiFi so you can sync and check mail fast.
1. Delete all mail accounts.
2. If MobileMe user, delete MobileMe account, and remove all Calendars, Contacts, Notes, etc., from the phone (so you don't have dupes when you re-sync).
3. Turn off phone, and back on again.
4. If MobileMe user, add MobileMe account first. I enabled all options, even Find My iPhone (Thursday and Friday I had it off, in case it was using power). Wait long enough for contacts and calendars to sync, check to make sure. I use MobileMe mail on Push.
5. Add back each additional mail account. I have 5. I am in a marginal coverage area, so I set these accounts on Fetch Hourly. I also have "Preview 5 Lines" (I think the default is Preview 2 Lines. This doesn't matter.)
6. Go into Mail, go to the MobileMe Inbox, and scroll your way down letting the messages come in. You can see them come in when you see the Preview Lines update, and then scroll some more.
7. Go into each additional mail account's Inbox, scroll your way down, letting the messages come in.
8. Go to the combined Inbox if you use that, make sure all the messages are in.
9. The WiFi activity spinner may remain spinning even after Mail says it is done Checking Mail. This is because other parts of mail accounts are being synced for the first time. Let the iPhone sit in Mail combined inbox until the spinner stops. Quit Mail, go back in, and watch the spinner after it's done Checking Mail. If the spinner doesn't stop after < 5 seconds or so, let Mail sit again. Once it's caught up, going into mail will Check Mail, and the spinner will stop a few seconds after as it should.
10. You're done.
Hopefully your phone stays cool and your battery life reflects the boost from the 20% larger battery. If not, the next step to try is Reset Network Settings, and finally try a factory reset and installing everything fresh instead of doing a restore. Hopefully you won't need to do that. I didn't."
