Recently I acquired an iPhone 4S. Luckily, despite hearing the horror stories of poor battery performance by some users, I had never experienced any issues myself. I could go a whole day with only a few percentage drops in battery life. Admittedly though I am not a heavy user of my phone, in so far as making and receiving calls go. I was very happy with it.

Then all of a sudden, about a week ago, I noticed that the battery was losing charge at a significantly faster rate, without any increase in my usage.

I would leave the phone fully charged, 100%, next to my bed before I went to sleep and 9 hours later in the morning it was at 58%. WTF?

This went on for days, so I began some experiments in order to isolate the cause.

Turn off all unnecessary notifications, alarms, usage reports, auto time zone detection etc.
No change.

Turn off Bluetooth.
No change.

Turn off WiFi.
No change.

This was crazy. No Bluetooth and no WiFi and yet the battery was dropping 30% to 40% overnight.

My last resort, something that in hindsight I should have tried earlier; a full power-off i.e. holding both buttons down for a couple of seconds and sliding off the power, waiting a few more minutes and starting it up again.

I charged the phone back to 100%, turned Bluetooth and WiFi back on and left the phone next to the bed again ready for the morning, fully expecting no improvement.

The percentage charge value the next morning? 99%!!

So, while I am still none the wiser as to the root cause of the abnormal battery drain issue, in my case at least, performing a complete power off/on cycle solved the problem.

My thinking, since I had turned off both Bluetooth and WiFi in my tests, was that power drain through radio use was not the cause. My software engineering background leads me to think that perhaps there was some process caught in a tight loop, sucking up the CPU and in turn draining the battery. A full power cycle cleared out the culprit process. It’s certainly a possibility.