I haven't used my kindle for a while and when I opened it, it gave me the "Batterie Faible" ("Weak Battery") error message, asking me to connect it to a power source, which made sense after a month of inactivity so I've put it on charger. However, an hour went by but the error didn't go away.
The first thing I blamed was the wobbly cable, but because this is an older Paperwhite from 2015, I then assumed it's about time for a battery fail. I did keep it on a charger for another hour thinking maybe it takes longer when it's on zero, but still nothing.
The solution was to simply restart it. Following the official guide, all I had to do was press and hold the power button for about a minute. And when the device booted, it was actually at 80% already. Apparently, this was a software glitch of some kind and the battery seems to be fine.