My friend and I have owned the Galaxy S3 for a week now, and yesterday, something strange happened to my friend's S III. Before she went to sleep at around midnight, she left her phone charging. In the middle of the night, at around 4 or 5 in the morning, she unplugged it because it was fully charged. She put it next to her on the bed so that she could hear the alarm that she had set. When she woke up, she realized the phone was really hot and had shut off. When she powered it back on, the battery was at 17%. That's 100% to 17% within a few hours. She then checked to see what was going on and saw that "Android System" was using about 80% of the battery usage. When she started charging it again, she also got a temperature warning (It was the picture you see when you start charging, and she said every 5 seconds, a little thermometer icon would pop up).
Today, we compared our phone usage after a full charge with moderate to little usage, and there is a definitely a noticeable difference in battery life (see attached files). After about 2 hours 35 minutes, I'm at 91% battery, but she's already at 53%. Now her "Android System" seems to be fine, but we don't know if it will act up again.
Does anybody have an idea of what has happened with her phone? We truly appreciate all your input.