[Lynn Eades]

Some ramblings:
I would tender the idea that most early war radio equipped vehicles were 6 volt and carried a petrol gen set (300 watt chorehorse or Brit equivalent?) The U.S. would have been the first of the allies to go over to a 12 volt vehicle electical system to cater for "in house" radio support. The belt driven (pto) generator in the Jeep was the same Autolite (different end plates) generator that was fitted in the Dodge Radio Car from circa 1943 (as well as late carriers)
These comments with respect to 12 volt Jeeps, from a thread in G503:

The W7 modification is in the February 15, 1945 SNL parts book.

I know the 12v PTO units were in production and Navy contract radio jeeps delivered by Willys-Overland in May of 1943.

The following TM for radio installations implies the conversion was in use at least by January 1944.

I think the 12 volt conversion in a WWII jeep was not so common until quite late in the war? (my comment)

Obviously, the problem with having a 6 volt vehicle and a 12 volt radio is that you need a second 6 volt battery. Keeping two separately used batteries evenly charged is not so simple. To manage this for soldiers that were in majority, farm types, required some extra care and skills.
To this day there are somewhat complicated systems in motor homes.
The radio op. would have used an hydrometer to check the state of charge of his batteries. This can be a bit vague, without familiarity.
For an LRDG radio op., with vibration, heat and evaporation, he would have been constantly mothering his batteries.

All the above stated from a position of "not much knowledge"