That is a good idea. Years back, I had a very old phone that I used as an alarm clock and media player. It would have been nice to not worry about charging. I did disable the mobile and wifi radios, which saved a tons of battery.
On Android, I use a terminal script called Advanced Charging Controller (ACC) to automate keeping the battery under 80% and some other battery saving tweaks. It allows you to charge to any value (like 80%), then stop charging until it discharges to another set value, which triggers it to start charging again. That was you can keep it between something like 70% and 80% while you leave it charging over night. It also has settings to keep the batter under a specified temperature and trigger cool-down intervals if it gets to a particular temperature. It can use a job scheduler to switch between different profiles for different days, or events. There are also some frontend apps, which let the user control the script with a more friendly GUI ("AccA" or "ACC Settings").
I tried this a few times as a little kid to see if it would work. If I sent it from my home, it didn't work and I got it returned with red pen notes on it explaining how to send it properly. I am sure it looked like a little kids writing, so they gave me the benefit of the doubt. Next I mailed it from the post office and that actually worked, as long as it was being sent locally.
https://candybox2.github.io/candybox/