I installed iOS 6.1.3 a couple of days back and my iPhone began heating up like crazy and started running out of charge in under 4 hours. Realizing something was wrong I tried Apple care as well as the Genius bar, both to no avail. Both had very helpful people, but the diagnosis was incomplete and didn't solve my problem. The Genius at the Apple store thought that my phone was having a charging issue, which wasn't the case and he offered to give me another phone (for $149) which should solve my problem. Ofcourse I didn't want to take that because I'm eligible for an upgrade and could buy an iPhone 5 for about the same amount.
Symptoms:
1. Phone was heating up un-normally.
2. Battery lasted for < 4 hours without any apps open.
3. Standby time and Usage time were pretty much almost equal (~4 hours).
Anyway, I did a little bit of poking around on the phone and found that the cause of all the problems atleast for me was that the "dataaccessd" daemon was crashing over and over again. This is the daemon that manages keeping data in sync between the phone and web-servers such as iCloud or Google's exchange servers. I think Apple may have modified how this data exchange is done to improve sync with iCloud, but this may have resulted in it crashing on my phone when talking to other exchange servers.
Now, this made sense - if the dataaccessd process keeps re-spawning, trying to access the internet to get to the exchange servers and crashing, the phone will have high CPU usage and possibly RF-usage since it is trying to get data accross to start failed syncs. I tried disabling all syncs first and found that this appeared to be helping the phone run a little cooler over a few minutes. I re-enabled iCloud and it seemed like this didn't make the phone consume more power. So I let the phone charge completely once again and used it like I normally do.
Right now as I write this the phone battery is still at 77% usage after 17 hours - Usage 2 hours, 15 mins; Standby 17 hours, 11 mins. Which means - this solves the power usage issue (atleast for me).
I'm guessing most of us are probably having the same issue. To do this:
1. Go to "Settings" -> "Mail, Contacts, Calendars".
2. Under "Accounts" turn off "Calendars" for everything other than iCloud. Do this especially for Gmail accounts.
This should fix power consumption issues.
Unfortunately this doesn't solve the root cause of the problem which appears to lie in iOS 6.1.3. Apple needs to release a fix for that soon - I do rely a lot on my Gmail calendars and this is inconvenient. That being said though, there are times when even Apple is going to slip up. I just hope they manage to fix this issue soon and are more careful with testing iOS updates before releasing them.
