The light on my charger usually stays on pretty much all the time, even when it's not plugged into the MBP but the plug in the wall itself. Sometimes it looks like its off also but it only has a bit of a faint green glow to it when you put your hand over to make shadow it and see the light. It's been messed up since my battery was before, but I got the battery replaced and tried to see if the adapter needed to be replaced while I was at the apple store. They said it worked fine, even though now the adapter seems a little messed up sort of, but it's fine because it doesn't affect the battery now.