I've bought a half-year-old iPhone 4S a couple of weeks ago and I just ran into a really weird issue the third time now: the battery percentage would suddenly drop 30-40-50%. One minute, it's, say, 69%, the next minute it's 25%. If I put it on a charger, it jumps back to 69% after a few minutes and charges it until 100% and then it works for a while.
It happened again today, I noticed that the camera apps were getting killed even though the battery was at 54%, and then after a minute, the battery would drop to 1% saying low battery. Fortunately I have a battery case, so I turned it on and now it's charging and it's already at 69%.
Can it really be just a calibration issue? I've done a calibration a week ago, draning the battery completely then charging it to 100%. Didn't do the hard reboot at the end, though, does that really matter?
Or is this a sign of a more serious hardware issue?
I'm running iOS 6.1.2, non-jailbreaked.