ReverseEMF:
So cool that we live in an age where there is such a thing as an "Old Robot" ;D
Haha, indeed 
ReverseEMF:
How about a DC-to-DC converter. It would boost the battery voltage to the nominal voltage, regardless of the battery voltage.
Cool idea, thanks! Thanks also for the link. The robot is a large beast, so I'm sure it's half empty so I can put the circuit inside it somewhere.
Perhaps I can stick a volt-meter it to monitor the voltage. Maybe also a Zener diode that would disconnect once the voltage is too low, but then the question is what voltage is the limit?