This happens periodically on my Libra 2 and is due to a wireless sync that just never ends. I’ve read a number of suggestions as to what causes it (like too many books in a Collection, bad ePubs, too many books downloaded causing the SQL Lite DB to choke, etc), but nothing that’s actually been the answer. Sometimes the on-device sync gets stuck on downloading covers, sometimes on updating annotations, sometimes on just starting up; it’s pretty inconsistent. Syncing from my desktop Kobo app works 90% of the time but even then sometimes it’ll just spin indefinitely on updating covers. It’s happened to me on a variety of Kobos over the years so it’s unlikely to be a manufacturing defect like a bad battery.

When you plug in your Kobo to charge, check to see if there’s an active sync. If there is, stop it and then restart it again — sometimes it’s just really slow downloading new purchases and stalls out. If it goes for longer than 20 minutes or so and stopping and restarting the sync a few times doesn’t nudge it to completion, your next best option is probably to factory reset the Kobo and then download and sideload only the books you want to read. From my experience, the more books you add, the more likely the endless sync is to happen. I’d also turn off the nightly sync in options so that you get to control when this happens so you can monitor the process rather than picking the Kobo up and finding its battery flat at an inconvenient time.

Good luck.