I posted this on reddit earlier today. I apologize for cross-posting, but I realized that there are more experts on this forum and wanted it to get feedback from the people here. Here it is with some edits:
I just realized that it should be possible to limit charging to 80% by using the car's end timer together with the charging station's timer, if you have a smart one. I have a Juicebox, which lets me set limits on the charging time.
Here's the idea: If I set the car's end timer to (say) 8:00 AM, it will try to time the charging so that it reaches 100% at 8AM. Then I configure the EVSE to stop at 2 hours before that, at 6:00 AM. I think that should stop it at about 80% each day, without having to fiddle with timers.
The times are just an estimate -- I haven't tried this yet, and so it might need some experimentation to confirm. (I have a 30kWh model, and the Juicebox can max out the 6.6.kW charger in the car.)
I also suspect the temperature also matters. I just looked at some of the charging data from the Juicebox, and I think that the charging taper at the end is longer in cold weather and shorter in warm weather. So the 2 hour gap might need to be longer in the winter, and shorter in the summer.
Has anyone out there tried doing this? Any thoughts on whether this will work?