By installing LineageOS on a galaxy s7 the old phone was turned into a self-hosted Navidrome server, with this approach you can use old smartphones like single board computers. The problem with this could be that depending on the model the battery could overheat if you leave it charging. It would be interesting to have a list of which smartphones are compatible with this approach.
Yes, many smartphones are made so that batteries don’t keep taking in power when they’re charged, but not all of them. I honestly don’t know how to figure out which ones do.