I bought a preowned iPhone 4S a couple of days ago, which came with iOS 6.1.3 installed. I updated it to 7.0.6 immediately as I wanted to restore a recent backup. Noticed the phone's battery is draining excessively quickly, especially when in standby mode. It then shut down with the low battery & plug into charger displayed when the battery showed 37%. However, a few minutes later it powered back up again at 37% and continued to work for a few more hours! This included using it regularly, when actually in use the battery seems to drain at a more appropriate rate. I discharged it to 3% and charged back up to 100% leaving it on the charger after for a couple of hours (all in airplane mode). The battery continued to drain in standby mode too much. The usage showed it had been in use for 6 hours in only just over 6 hours since being unplugged. Thing is, I'd been asleep and the phone in airplane mode for more than 4 of those hours! Hers a screenshot of the usage, and a log of the low battery crash. Would this be a software issue, maybe an app constantly running in the background, an issue with the backup I used (it was from an iPhone 4 running the same firmware version) or is the battery on its way out? I couldn't find an estimated purchase date for the phone in Apples support section of their website and the warranty appears to be long expired so guess the phone is about 2 years old. Just debating whether to return the phone or have a go at replacing the battery - just want to rule out any software issues first.