[frostschutz]

You should convert your image into an attachment before a moderator visits your home and removes the device for your own safety (or if he's lazy he might just delete the image instead).

It might get fried if lightning hits your house and the device is connected to an outlet at that time.

Seriously though, the battery should be fine, Kobo stops charging once it's full so it's no longer involved at all. I've never tested with Kobo but there are devices where you can even take out the battery entirely while external power supply is connected and it would run until you have a power outage.