Sudden major battery drain in sleep mode?

I have had my kobo Libra 2 for a few years. I have it set to never power off. I would sometimes leave it sitting there in sleep mode for a week and the battery would drain a few percent.

Now, starting a few days ago, I haven't changed any settings or synced or updated firmware or software, the battery is draining like 20% overnight.

Has anyone seen this sort of thing, can you think of anything that could be causing this? I have powered off and on, no change in battery drain.

Thanks,