When i plug my samsung omnia to the usb port it starts charging automaticly. This can be turned off with a third party app but it is really a pain in the butt as it makes me change the registry every time.
Anyway, i read somewhere that nowadays batteries won't get damage if you start charging them even if they aren't fully drained. Something like "baterry memory effect is a thing from the past."
Well, the way to look at it is like so: Every rechargeable battery has a limited amount of charges that it can handle over a life time. Most cell phone batteries are li-on based, which in turn leads to more recharges over the life of the battery. Regardless of when you charge it, chances are that the battery is going to burn out eventually. There are apps out there that can help increase the life of a battery, but I am unsure of any that exist for the Omnia.