My S3 was working beautifully with battery life the first 3 weeks. Normally after about 1.5 - 2 hours of work calls over bluetooth, moderate usage with emails, texts, and Facebloke, I'd be at about 65-70% or sometimes a little bit under by 5pm when my work day ended, which is typically 9-10 hours after taking the phone off the charger. Mind you I work from home so all my data activity is WiFi.
Suddenly the last 2 weeks or so, with the same usage I'm in the 40% - 50% range by 5pm. The one thing that I noticed changed is Cell Standby. I know this is a known issue, but my first 3 weeks with the phone, Cell Standby was consistently BELOW 10%. When I started noticing the battery drain the last couple of weeks, Cell Standby has been in the 30% - 40% range. It seems just turning on the phone for 30 seconds to check an email drops 1% - EVERY TIME. And on standby with no direct usage from me, it drops typically 5-6% per hour. Previously there were times before this drain started happening when I wouldn't touch the phone for an hour it would maybe drop 1-2%.
I did a factory reset about a week ago, and with ZERO apps installed other than the defaults, the drain was still happening and Cell Standby was in the 30-40+% range.
Anyway, now that I'm off my soapbox

, the one thing I changed the last 48 hours that has seemed to help is I turned off Keep WiFi Alive, and also turned off Mobile Data (obviously I only do this when I'm in range of my router). I had one hour worth of calls this morning which only drained 9% compared to the almost 20% when the problem started. So this solution is helping, but still not to the level of battery performance I had the first 3 weeks, and it sucks having to micro-manage this and turn Mobile data back on when I'm away from my router. I shouldn't have to do that just to save battery life considering it wasn't a problem the first 3 weeks. Also, there's definitely still network activity during standby because even in the above configuration I still get notifications from network apps like Facebloke and email. I had assumed doing the above would kill said activities in standby.
So anyway, I think in my case Cell Standby is the culprit, but I'm puzzled as to why the first 3 weeks of having the phone, Cell Standby usage was low, and then out of nowhere it's taking the majority of standby usage?