1998 Ford Escort battery light in dash

Replaced original battery a week ago with new one. Key was left on overnight by accident and new battery went stone dead. Put it on slow charge overnight and car starts and runs fine except for the charging light in dash. At very low RPM light is out. As soon as RPMs go up light comes on. Makes no difference if lights, heater, etc. Are on. Seems like light is working in reverse. I would think if alternator was faulty light would be on at idle. Could this be caused by key being left on? If so, what could it have done and where should I start?