Q:

Measure input voltage of Raspberry Pi B+ running Ubuntu

I just learned that a red flashing LED indicates voltage below 4.63V on a Raspberry Pi Model B+.
Is there a command to determine the voltage programmatically?
I tried vcgencmd measure_volts. But it yields 1.2000V, independent of the input source and the LED status. And it doesn't seem to be related to the 4.63V mentioned above.
Update
Let me describe the situation in a bit more detail:
I'm powering the Raspberry Pi with a lead-acid battery built into a moving robot. After operating the robot for a while, the voltage seams to drop below a critical minimum, causing potential damage to the file system. Therefore, I'd like to detect low voltage automatically (and trigger the robot to return to the charging station).
I'm asking here in StackOverflow, since I assume the solution not to be robotic-specific, but generally applicable to other machines.

A:

Yes you can, as it is said in this topic Under-voltage warnings you can know the low voltage reading the GPIO 35. For reading GPIO, you can refer to this topic:
Python Script to read one pin

