Ok, I'll type my story.
I bought a used iphone 5c and it was about dead so I charged it up. I got busy and did not mess with it further. 3 days later it was still running when I started to mess with it. I cleared the phone by doing a full reset and it restarted and wanted to upgrade to the newest IOS. I let it finish and then setup the phone for my usage. I sat it down and the next day it was off with a dead battery.
I charged it up to 100% and then disconnected and played with some video and such on WiFi. I laid it down with around 90% left. Later that day, again the battery had died and the phone was off. I started doing all the suggestions online. I ended up turning everything off and put the phone into airplane mode.
Still dropped dead.
I bought a new battery online and installed it. Better but only added perhaps an hour to the life. So I thought maybe the charge port was having an issue, bought a new one and installed it......no difference, it still dies fast.
I was about to buy another battery, hoping I had a junk one that is new and then found a program that lets me look at the battery/life. It is well known but I'm not sure if it gets flagged here for typing the name so..............
The battery capacity for new is 1550, it is charging to 1600.
I just reset it again as the instructions in this thread have said. I'm sitting at 100% now with wifi on and airplane mode off. Most all other location options are OFF along with anything else that is useful to a cell phone user.
I'll post back how long the phone lasts with it sitting on my nightstand doing nothing but being turned on with the lock screen active.
Earlier I could pick it up and hit the upper button to wake it up, with for example it showing 80%. It would refresh instantly and drop to 77%.
If it still dies, I guess my options are:
1 Jailbreak it or whatever I have to do to return to an older IOS for testing.
2 Sell it and buy an android device.
3 Sell it and keep using my 3GS with it's original battery that will not die after sitting idle for 3 days......on IOS 6.