I have to ask, what is the aim of the project?
Is it to improve the charging of cellphone batteries?
If so, I think that the phone manufacturers would have that well sorted by now. One of their selling points would be good battery life between charges.
Is it to save the planet from global warming from all that horrible C02 produced by cellphone left on charge unnecessarily?
If so, you need to be in at the design stage for the whole battery/phone/charger system.
As an afterthought, you could try investigating the current drawn by the phone from the charger during the charge cycle, and use that as the basis for switching the charger off.
JimB