I think I might have spoken too soon
The battery stayed at 95% from January 6th, 18:00 until January 7th, 9:30.
I left it in sleep mode, with an epub "active."
Switched it on 24h hours later on January 8th, 9:30... and the battery was at 55%.
That's from 95 to 55% in 24 hours is -1.67% per hour, which is 59.88 hours of sleep time, or only 2.5 days. This time, I really have no clue whatsoever as to why this is happening.
Bad battery after all? But then why did it seem fine yesterday after getting rid of that kepub? Sigh