Stupidly long runtime estimates are used to make it obvious that the runtime isn't properly calibrated. A battery bench would indeed allow a better calibration to be made. I don't know what the status is for the nano 2G, but if it is saying 80 hours I would guess it is uncalibrated.
As for car adaptor mode, the manual says "Car Adapter Mode. When using the player in a car, Car Adapter Mode automatically stops playback on the player when power (i.e. from cigarette lighter power adapter) to the external DC in jack is turned off. If the Car Adapter Mode is set to On, Rockbox will pause playback when the external power off condition is detected. Rockbox will then shutdown the player after the length of time set in the Idle Poweroff setting (see above). If power to the DC in jack is turned back on before the Idle Poweroff function has shut the player off, playback will be resumed 5 seconds after the power is applied. This delay is to allow for the time while the car engine is being started. Once the player is shut off either manually, or automatically with the Idle Poweroff function, it must be powered up manually to resume playback."
However, it doesn't seem to be in the nano manuals. I assume from your question that it is enabled for the nano 2G?