I thought I would write about this as I had to go through loads of forums where where people asked a question ' why does my battery drain terribly when I enter recovery' and the answer was 'search the forum, its already been answered, stop wasting our time.'
If you happened to phrase your search exactly right then you might find it, and I eventually did. So here it is:
Your battery may appear to be working fine, but, if you've been using it for over a year, and remembering lithium ion batteries only last for 500 charges, it may not operate correctly in the high current environment of software update or recovery.
Samsung and Rom makers say you should always have a fully charged battery before updating the software and most custom Rom makers say you should have at least 66%. But I was seeing the battery charge drop to 8% after updating and someone I know had the update get corrupted (presumably the battery completely failed for a while) and his phone is still not working.
So the answer is easy - get a new battery. And if your phone is boot looping after a failed update you can try the instructions here to blow the operating system back - (again no warranties whatsoever - use at your own risk)
http://reviews.cnet.co.uk/mobile-apps/how-to-go-back-to-a-stock-android-rom-on-samsung-galaxy-s2-50007880/
