When I got my iPhone 4 I was impressed with the battery life (for a smartphone). I could easily last 48 hours even with regular usage (and much longer that on lower usage).

Even running Skype in the background it only burned though around 4-5% an hour.

However, just before Christmas, around the time that I installed iOS 4.2.1 it changed. It suddenly started burning 4-5% an hour even when idle, even if it had just been rebooted and no apps had been run.

I tried fiddling with various things, but it kept doing it. I assumed it was:

- a problem with the hardware (which would get sorted out when I got round to swapping the hardware due to the failing button)
- a problem with iOS 4.2.1 (which might get sorted in an update)
- something about my setup or one of my many many apps (though couldn't quite see how as the apps weren't running after a reboot)

I got the hardware swapped last week and the new phone kept draining at 4-5% per hour. So I was then fairly sure it wasn't the hardware.

I spent a couple of days working away from home this week and it magically seemed to go back to sipping 2% and hour (even though it was having to use 3G rather than wifi most of the time).

So when I got home I had a thought about anything I had changed at home around the same time. I remembered that I had been experimenting with setting up a second wifi network on the wifi on my router. I had connected the iPhone to this second network to test it.

The two networks are identical apart from the ssid and key. But it looks like for some reason being connected to that second network was draining the battery.

I switched it back to the old wifi network yesterday and battery life seems to be back to <2% hour. All very odd and the last thing I expected to be causing the problem.

"It just works", some times...