Is it bad to charge a cordless phone too much?

It will depend on the type of battery in your phone, if it is a Ni-cad, than yes it can, that type of battery you will want to take down till the phone tells you the battery is low and charge it for 8 to 12 hours. If it is a Ni-MH battery than you can pretty much charge it and leave it on the base as much as you would like, you also do not have to fully discharge this type of battery either.