Three times I've been lazy and set up an old phone or tablet as an always-plugged-in stationary device without excising the battery, and that has produced two spicy pillows and one completely dead battery (phone wouldn't even boot when plugged in, until I replaced the battery). Granted, these were all 5-10 years ago, but I do not trust the batteries and their controllers in these devices.

Nowadays if I want to leave a device plugged in I crack it open, remove the battery cell, solder on a power supply and capacitor, and then do the nonsense with rooted Android to keep it from shutting itself down.