Not sure if this will be helpful but I've had another experience with nano 7. I was having the same quick battery drain experience others have posted and when I did the various draining completely the battery, reseting/recalibrating that the various posts have been describing didn't seem to help (much) in that I was still lucky to get 3-4 hours out of it.
I use it only in the car and had been connecting it via the nano's lightening port through an adaptor that went into the car stereo as the usual mp3 3.5mm jack. I then had the bright idea to remove the lightening adaptor and just connect it straight to the nano's earbud port rather than the lightening port. I now get approx 30 hours of battery.
So, for my experience anyway, it seems that the big difference is between the lightening port and the earbud port.