You really don't want to use the batteries. They will puff up very soon.

I wonder if having software that limits charging to top it up to 80 percent only is possible.

Did not work for me with an old Nexus 4. This is a bad idea, but everyone gets to find out for themselves if they do it

Maybe old phones can't do this compared to new ones.

Disabling the battery seems reasonable enough for a device that won't use it again.

If a battery is needed.. plug one into the usb?

AccA app mentioned earlier in the thread. Requires root and seems to only be available on the F-droid app store.

Most likely only if you keep them fully charged. If you can root the device you can control the state of charge.

If you can trust a 10 year old battery to not puff up on you, by all means ;)

If turning a phone into a NAS, it’s not unreasonable to take it apart and replace the battery. Hell, it’s easy enough in most cases to wire it up to a larger battery that wouldn’t fit into the case.

It’s even simple enough to just buy an adapter so you can split the port into data (network connection) and power (to a hilariously big UPS).

Is there a site/ video / tutorial you can recommend on such hacks?