After charging my Galaxy SII (ICS) overnight, I unplugged and started using the web browser. Battery indicated 100%. After 20 minutes the screen went black and the phone rebooted, then it popped up a battery low warning. On plugging back in, it seemed to charge normally for a while and then jumped from 80 to 100% almost instantly.
I would post a screenshot of the battery screen but don't have enough reputation - it shows charge starting at 100%, dropping slowly for 20 mins, then instantly down to <10%. From there I plug in and it goes slowly up to 80%, then instantly up to 100% again.
Anyone fancy having a guess at what is going on here?