It's often recommended to keep the charge between 20-80% and the temperature between 0-45*C.

Most of us are to lazy to watch over the charging of phones to stop the charge at 80% and there is not a lot of phones that can stop charging at 80% easily. (without rooting)

To be able to control the charge I propose the use of an USB switch for turning off the power to the device from the wall plug. An app seams like the easiest way for the phone to check the status and control the switch when charging or temperature get close to the limits. BLE is a popular communication way that works on both Android and iOS and is implemented in most of the phones today.

Cordova or similar will probably be used to make one app that works in as many devices as possible.