My iPhone 5 (factory unlocked) of three years (almost) started shutting off by itself since yesterday. It is completely unusable. Most of the time now, as soon as I boot it up again and unlock it, it shuts off. Other times, it stays on for a while before I run any app (stock, third-party, settings) and it shuts off. On rare occasions if I keep it in airplane mode, it stays on. Until, it shuts down later. I suspected something related to network connectivity was triggering something that was causing the phone to turn itself off. I tested by removing the SIM card altogether, but I couldn't find any particular pattern.
I have tested with the battery at > 50% and > 80%. If it is a software issue, I am assuming an iTunes restore to 9.2 should do (it's in the process).
It's running iOS 9.2. No jail-breaks. No recent physical abuses inflicted on the phone to warrant a reaction like this.
Can anybody please share any ideas on what might be the issue here?
Update: If it is plugged directly to a power adapter, it does not shut off with or without network enabled.
Update: 9.2 restore didn't help. I also experimented by leaving the phone connected to the power adapter but with the adapter not connected to any power source. It didn't shut off all night. Since then, I have been using it without it being connected to anything. The battery has drained from 100% down to 70% so far, with network/wifi and everything enabled. It hasn't shut off. This is perplexing.