An old phone as an always-on server
A retired phone has a decent CPU, plenty of RAM and its own battery backup, and it draws only a couple of watts. With a Linux userland it handles small services well.
Lessons so far
- Limit charging to around 80 % if the phone supports it; the battery stays cooler and lasts longer.
- Thermals matter more than CPU speed. A stand that keeps the back open to air helps.
- Keep backups off the device. Flash storage on old phones fails without much warning.