Data usage and battery drain has skyrocketed with iOS 8.1.2 (maybe earlier)

Question

3 weeks ago my wife got a text message on her iPhone 5 from AT&T that she had used 90% of her data for the month.  30 seconds later, she got another one saying that she had used 100%.  Granted, her data plan is small, 200 MB.  But, for the last two years she has used as little as 17 MB in a month and her largest prior to this latest month was 49 mb.  This month she got to 380 mb before we were able to shut off her cell data.  Otherwise, a second chunk of 200 MB and another $15 would have been added to the month.
We had noticed that her battery had been draining rather rapidly in standby mode with no background apps running when she didn't have access to WiFi.  We have now correlated the two results, High Cell data usage, Quick battery drain and very warm phone.  In case anyone is wondering about her iPhone 5 being one in the recall, it is.  I took it in and had it tested.  Apple says the battery is fine.  It does hold a charge well.  When all network communications are shut down on her phone and it is in standby mode, there is no noticeable loss of charge in 24 hours (our longest test).
Since I have a grandfathered Unlimited Data Plan with AT&T, I have been doing some testing on my iPhone 5s.  Sure enough, I've seen similar results.  With nearly everything turned off except Cell Data, I've seen 483 MB used in 24 hours.  Of that total, 479 MB was used for Documents & Sync.  I even have everything turned off in my iCloud account, Location services are turned off, no push notifications etc...
The best I can figure, this has been a problem since we installed 8.1.2 and yesterday I installed 8.1.3 resulting in no positive effect on this issue.
At this rate of data usage and no access to WiFi, my wife would have to have a 15 Gb data plan to keep from exceeding her limit.  I don't think AT&T even offers one that big anymore.  I might have to switch to Sprint or T-Mobile if this doesn't get resolved.
Any ideas would be greatly appreciated.  Thanks in advance for your shared insights.

Answer 1

Hey this is Brian, bjmcmurry, under a different user ID.  If I logged in as bmcmurry, original post, I was just getting a server error and couldn't get to this discussion nor post a comment anywhere.
Anyway, let me be as brief as I can.  After an extensive analysis of replicating the problem on another phone to eliminate it being a hardware specific issue and that the test phone has an unlimited data plan, I was able to stop the excessive data usage and consequential battery drain.  I believe I have a reasonable explanation of the problem, an iOS logic bug.
I had everything under my control clamped down on the test phone to minimize data usage.  Also, I was forcing all data usage to use the cell towers.  I determined that I was burning at least 11 to 13 Mb per hour doing nothing important.  I literally had everything turned off except 'Settings' was allowed to use Cell data providing a avenue to log into the Apple ID. 
The two phones had 2 things in common, they were using the same Apple ID and the same version of the OS.  One was a iPhone 5 and the other was a 5s.
Here is a synopsis of my experience via my written evaluation of the Apple support system (Apple Rep name's have been removed):
After a couple of hours on the phone with an Apple Care Senior support Analyst, it was suggested that I go to the Apple Store to 1) get my battery replace in the iPhone 5 since it qualified for the replacement program and 2) see if the Genius could provide me with any insight in the data usage issue and battery drain.
I'm certainly glad I had multiple reasons to visit the closest Apple store, a 45 min drive.  I got the battery replaced, thanks for that. 
Regarding the important issue to me that brought me to the Genius Bar, excessive Cell data usage, I thought the Genius did a wonderful job listening and documenting the problem we were experiencing.  Unfortunately, educating us on the use of the product or replacing our phone was not going to solve the problem.  He eventually figured it out, this was not a user error or broken hardware.  We surmised that the reason I was sent to him was to verify and document that there was a problem.  I clearly had a problem.  After writing copious notes on the issue, The Genius smartly referred me back the the AppleCare Senior Advisor.  So, back to AppleCare Senior Analyst I went. She, knowing this was not a simple problem to solve i.e. beyond her level of expertise, got an Apple Software engineer involved.
My primary frustration is that this should have been escalated to software engineering right away.  My trip to see the Genius was, I believe, a waste of time for both me and Apple regarding this issue.  Diagnosis of this type of problem is well beyond the Genius’ knowledge and job.  The Genius did a very good job doing what he could within the scope of his job.  He just couldn’t solve this problem and had no one within Apple to which he had access for help.
However, that initial consult with the engineer was a bit feeble.  Not only could I not talk to him/her directly, his/her focus was still working with the ‘Black Box' approach.  Having Analyst ask me to do stuff like restore the Phone was more like trying to find a 'work around' than 'identifying the real problem.'  Maybe restoring the phone was going to fix some corrupted data on the phone sidesteping the real problem, the handling of this condition.  The engineer did listen to the fact that I had restored the test phone, the 5s, and only added the use of my Apple ID.  Duh, been there!!!
Having been an previous owner of a software development company that I started in 1980 and retired in 2001, I can tell you that using the 'Black Box' method of debugging is extremely inefficient.  Instead, having access to an ‘Activity Manager’ type app to identify in real time what process was blowing up the data flow would have saved me a lost afternoon at the Apple store and hours on the phone with tech support.
Ironically, I may have stumbled onto a "work around" solution short of turning off Cell Data that stopped the excessive Cell Data usage and consequential battery drain.  Just prior to leaving the Apple Store, I used one of their Macs to log in my Apple ID on the id.apple.com website to ‘Manage my Apple ID’, I was forced to upgrade the security level of my password.  Afterward, my cell data usage seemed to drop dramatically.
I told the AppleCare Senior Analyst that I may have stumbled onto a workaround and I suggested that I give it a couple of days of monitoring.  We'd talk then and I would have results to report.  She agreed. 
Sure enough, my Document & Sync usage was back to normal, about 1/2 Mb per day.  Am I happy that the excessive Cell Data usage has stopped?  You bet!  But, there was no intuitive reason that an end user would know to log in on the ID website to unknowingly solve this problem.  As I use to say to my employees, that is not the ‘Macintosh Way.’  And, rebooting the the 'Microsoft Way.'
When I reported my findings to the Analyst, we both concluded that data usage was back to normal.  She did report to me that the engineer did provide a laundry list of suggested activities to help solve the problem.  Remarkably, they were everything that I had already done and were pretty much what the Analyst and the Genius tried to do.  All reasonable tasks.  But clearly, the engineer hadn't even ready the notes on the case before responding.  And, none of the things suggested would have uncovered the real problem, likely a logic bug in iOS.
My advice now is to have the software engineering team rule out that there was an unresolvable state that a background process was fiercely trying to handle.  The iOS does not seem to know how to intervene and prompt the user with a requirement to upgrade their password.  If this does end up being the case, I would call it a logic bug, a perpetual exception that is not being handled properly.
Anyway, I hope this helps you.