I ordered the same 2 batteries for my somewhat older Sennheiser HDI 380 and must say that correspond to what I expected. However, it was one of the two contact aligned so that it could neither load nor unload the headphones, but I have a contact simply bent with a sharp object upward. Problem solved.
As regards the duration I can not as yet say much, but he keeps a long series "orgies" through. He must then stop every night back to the charging station.