Hello,
Just trying to help out

here is my advise to you.
Batteries can sometimes be over charged it's not a good idea to leave it plugged in constantly. A battery has sort of a memory cell and if you do not allow it to drain now and then it some what forgets, and usually leads to reading fully charged but only being 1/4 charged ect... Or not charging at all.
Sometimes it can be cured by letting the battery drain, recharging, letting it drain completely and recharging again but most of the time. It's needs a new one.
It is very important for the first few uses to let the battery drain completely and then recharge, if from the point you buy it, it stays plugged in constantly chances are the battery is going to get damaged.
This goes for Laptop's, Cell phones, and anything else that uses rechargable batteries.
Steve