After about 10 years of testing, I'm absolutely certain that, as long as the circuitry doesn't fail, you can leave a lithium battery on a charger for months with no harm. (If the charger fails and applies 110 Volts to the phone, of course there will be problems - like talking to the fire inspector.) In fact, I have an old one (about 7 years old) that's been sitting on its charger for a few days now, and every few hours it charges for a few minutes. (Needless to say, it's not in very good condition, but that's because it was badly abused.)