My biggest hurdle reusing old phones and tablets is the battery failing over time wile living on the charger. We need a project or company that makes dummy batteries that just report 100% full all the time.
I've seen at least one hacker do this. It's not so much the voltage but you need to provide a valid temperature sensor as well (the 3rd pin on most phone batteries).
Supposedly it’s not just a software check, but the devices actually need a battery since the peak power usage can briefly exceed the chargers capabilities.
I noticed strange power behaviour in an old phone after not using it for a period and storing it charged. It seemed to run down, and have less room at the low end between 20 and 0% charge. I'd like to do something with the phone but I am not sure if I can easily get inside and access the battery. I do have an old galaxy 2 without a battery that I could do something with but I'm not sure what.
On rooted android you can use Advanced Charging Controller (ACC) [0] to control at what levels to charge. I use it to exchange normal capacity for longevity.