I will not be opening up my motor system till later this fall but I used a small scribe and the gray ring just inside the nut and outside the "bolt" rubber, which leads me to believe it's some kind of shaft seal...
hhhmmm
Interesting question...I would start with downloading the mission control app and linking up to the bike and seeing how many recharges the battery has gone through.
Second, if you are allowed, ride it under strain and again look at the mission control app at the temperatures of the battery and...