Strange battery life issue
I have never had battery problems with my iphone 4 in the 18 months I have had it so far, but since 2nd Jan I've had an odd battery life issue. It's not as simple as it's started to lose charge v quickly. Sometimes it is fine for hours and seems to be holding up well, even though I've used it a fair bit, then I put it down and leave it alone for a few hours and when I come back to it the battery has dropped like a stone.
So for example, on Weds I had it 100% charged when I left for work at 8.45. When I got home at 6.30 it was on 80% even though I'd used it a bit for texting and for using Facebook over 3G during the day. When I get home I turn off 3G as the 3G signal at home is weak and the battery has always held up better if I turn that off indoors and just use the wifi. By bedtime at midnight it was on 75%. As it was holding up so well I didn't bother to put it on charge when I went to bed. But at 7.30 the next morning I found it was totally dead, that it had gone flat in the night! Same yesterday, it was fine all day and when I went to bed at 1am (17 hours after it was last charged) it still had 61% battery left, but 6 hours later it was totally flat.
If there's any pattern, I've tended to be at home when I experience this sort of runaway battery loss. Two days last week it held up well all day at work despite moderate use (including Facebook, Safari and Mail over 3G), then lost over 40% in the five or so hours between getting home and going to bed, even though I hadn't used it at all.
Last Friday after I'd had the problem a couple of days I rebooted it and then it lasted all weekend on one charge, so I figured I'd had a runaway process and I had just needed to kill it. This week it was behaving normally again, so I thought I had solved it, until Weds.
I've heard that iOS 6 can cause battery issues on iPhone 4 but I've not bothered to update it, it's still on iOS 5, so that's not it. Any ideas?