I have exactly the same problem as "slex" . I bought my ONYX BOOX M92 4 weeks ago and it came with pre-charged battery. It lasted for more then 3 weeks of constant usage of reader, occasionally with wifi and music. Practically, I was never shutoff device. Then, I charged my reader for more than 48 hours, waiting for orange LED to shutoff. But, it stayed in ON state all the time. After charging I can use my device for just few days (battery runs out). Obviously, there is a issue with charging, not battery or power consumption related.
So, "slex" did you solved your problem? How?
I am just preparing to write review about ONYX M92 BLACK PEARL, but now I dont now what to think...