Hi tiputus, it should definitely last more than 10 hours. Aside from the charger possibly not charging it up as you say, there is one condition that could make this happen. Are you using very low-resistance atty/cartos? For the most part, they are not needed on Darwin, and will run your battery down much faster.
The arm is not a mechanical switch at all, it is magnetic. I found that if I open the arm very fast, it seems like sometimes it does not detect it the first time. It always works, however.