Hi Tom
If you look at the following discharge curve you can see that the cell voltage remains reasonably level until it gets near to the end of the discharge cycle:
The battery gauge on the BAC-601 Smart Display unit is not as accurate as a Wattmeter or Cycle Analyst as they monitor the actual battery capacity that has been used instead of trying to gauge the capacity from an almost flat discharge voltage.
I mainly run mine from a 29.6V 8 cell LiPo pack (33.6V fully charged).
I originally had the controller voltage set on 24V but I have changed it to the 36V setting which allows the regen to operate at a higher battery voltage, and the battery gauge also works much better on the Smart LCD Display:
On the previous 24V setting it always remained full even when my wattmeter indicated that over half of the battery capacity had been consumed.
Check out
this post for more information on the different battery discharge curves affect on the battery gauge accuracy.
Alan