Have a look at what I posted here:
https://www.mobileread.com/forums/sho...&postcount=914
Do you see similar behavior? You have to make sure that the device is indeed turned off completely.
There seems to be a problem when leaving the device in suspend mode, from which it tries to turn itself off after some time (when having a setting like 10 min to suspend, 15 to poweroff). The automatic poweroff fails and instead the device is left running in normal mode, which of course causes massive battery drain.