I'm not actually sure about the technicalities of whether it affects the battery life or not, but I would have thought that once the battery was fully charged, it would stop accepting the charge from the mains. I tend to keep mine fully charged by using it mains powered most of the time, then I always have a few hours of battery left, should I need it, but I don't leave it plugged into the mains power all the time when I'm not using it. I guess it's more economical, and 'environmentally friendly'
to not leave it powered when you're not using it, unless your charging it, of course.
Hopefully, someone more technical will be along soon with some better advice.