Either a timer or something along the lines of the my500e device which tracks the SOC via OBD and controls the activation and deactivation of charging.There is currently no way to have the car stop at 80% charge is there? You’d have to gander a guess via timer?
None of that data is really secret. It's just undocumented. The 500e is likely easy to figure out as the Bosch developers used different components to put the car together along with CanBus for communication. A diligent documentation effort to capture the relevant data and commands could put together a system to have complete control of charging.
I had held out hope for the Wifi my500e. But now that I see that the plan is to charge online server fee subscriptions to use the device, I'm really not that interested anymore.