I have an iPhone 4S 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. Then it died on me.
I also do not know how to calibrate.
I'm running iOS 8, non-jailbreaked.