Glue I should say. I went to the Apple store for an actual display issue a few weeks ago but while I was there I mentioned how sometimes my mac says "not charging" or the green or orange light wouldn't turn on unless I reattached the charging cable. He opened up the macbook to have a look and while he was doing his thing the actual battery fell out of the casing. They had to replace the whole battery and the housing. He said the glue on the battery has gotten loose