I would like to have a detailed specification of this battery pack. I miss at least the specification of the internal thermal fuse. During charging at 400mA the fuse inside the pack always trips at about 1280mAh charge (from empty battery of course).

First, are you using a charger that is designed for use with NiMH batteries? A fully discharged NiMH battery looks like a dead short to a standard power power supply. The sudden surge of current causes the battery pack to over heat and the thermal switch opens. The charger for NiMH or Lithium batteries has current limiting to prevent the sudden surge of current and allows for a controlled charge rate. The thermal switches are not part of the normal Digi-Key part line so I do not have access to any of the documentation… There may also be different thermal switches in use depending on the size of the battery pack.