There is a reserve that allows you to use your lights well after the battery is “depleted” from a riding perspective.
The status of the battery information depends on which display you are using. For Purion and Intuvia, you get the 5-bars and no scale. For Nyon and Kiox, the usable (less reserve) range in on a 0%-100% scale but not mapped to voltage.
You will not get a voltage reading from the battery using a voltmeter unless you apply a constant 5V to pin 3. This is the only way I can think of the get the voltages when fully charged and when you can no longer ride.