The 9A.h batteries I use would have a rated capacity of about 8A.h at a current of 0.8A - that's about ten hours' worth. Dropping the temperature to freezing will drop that to about 7 to 8 hours - still more than you're getting.

Also remember that the current drawn by some devices may increase as the voltage drops. I'd suggest checking your current draw at a range of battery voltages.

If your charger's ok, it sounds like your battery isn't.

Mike