Similar rotary converters were used in military radio equipment during WW2.
In those applications, the input was usually from a 12v battery in the case of a land vehicle, or 26/28v in the case of an aircraft.
The output was usually DC at whatever voltage the particular equipment required.
I'm not sure that the type of battery would be important.
What is important is for the battery to deliver whatever current is needed to power whatever load you have on the output, plus the conversion (in)efficiency.
Remember to factor in the voltage ratio when you calculate that. Even with a conversion efficiency of 100%, 1 Amp of output current at 115 Volts will need ~ 19.2 Amps of input current at 6 Volts.
As I said in post #2, it obviously would have been designed for lead acid, as that's all there was back then - but in modern times there are alternatives available. It's also rated for 6V, which is a common lead acid battery size, and what military vehicles used in the ancient times the converter was made.
What is important is for the battery to deliver whatever current is needed to power whatever load you have on the output, plus the conversion (in)efficiency.
Remember to factor in the voltage ratio when you calculate that. Even with a conversion efficiency of 100%, 1 Amp of output current at 115 Volts will need ~ 19.2 Amps of input current at 6 Volts.
Motor generator sets are far more common than Nigel would lead you to believe. A web search will show this.
One large-scale application is on board submarines. Batteries are an emergency power source. In normal operation, they do something called ring-around. The sub's turbine generator supplies AC to one MG set. The DC output is connected to the batteries.
The other MG set is operated from the batteries, and supplies AC to the power buss. The AC-->DC unit keeps the batteries topped off. The DC-->AC unit provides instantaneous power to the AC buss in the event of loss of power from the turbine generator.
I suspect we have wildly different understandings of 'far more common' - presumably you are one of those who consider Covid vaccine blood clots an unacceptable risk? (and it's probably at a similar level).
One large-scale application is on board submarines.
MG sets of various types are still relatively common in industry, though steadily being replaced by solid state control systems.
These types are used for power control or as servo drives and act as AC to DC converters with eg. 415V AC in.
(ie. With the generator field controlled by a much smaller electronic system than would be required for the final motor the generator operates).
They are often multi-section with the AC motor driving two or more generators on the same shaft.
Example; the factory that is in has three other machines at least with MG set drive systems, all different.