6hearts:
Can anyone think about a hack around this? Either to increase the voltage somehow or possibly modify the hardware? Is it possible there is an obvious hardware part that measures the voltage I could rewire?
So cool that we live in an age where there is such a thing as an "Old Robot" ;D
How about a DC-to-DC converter. It would boost the battery voltage to the nominal voltage, regardless of the battery voltage.
Like this one: https://www.ebay.com/itm/LED-DC-DC-Digital-Boost-Step-up-Voltage-Converter-Board-LM2577-3V-34V-to-4V-35V/132056133502?hash=item1ebf28af7e:g:QPEAAOSwt5hYb2Nx
I'm basing this on the Eveready datasheet for an Alkaline D-Cell which is rated to 500mA, and with an internal resistance [worst case when fresh] of 400mΩ. So, if the battery arrangement in your geezer-bot is 6 cells in series, then I would expect the current demand to not be much more than a half an amp [might be current surges to 1A to drive the motors, but @ 400mΩ, that's a loss of 400mV at 1A, so probably not -- there is probably sufficient capacitance to even out the current surges].
6 cells, at 1.5V is 9V nominal. So, adjust the DC-to-DC converter for that voltage.
On downside, though. There is the danger of running the batteries down too low, and ruining them, or at least, impacting their longevity. So, either a battery manager is indicated, or because this thing has a nice big voltage readout (defaults to Input Voltage), you can keep an eye on the battery condition. Plus, this will give your robot yet more "tech-vizage"!