I was more hoping by now a more specific answer was available as I've never seen a manufacturer not have an actual specification for their equipment.
Having a 24v system and not allowing for use of a 24v off the shelf battery (with charging) seems to be a poor design.
12v or 24v are usually nominal voltage based historically on a car or truck battery systems. (with an appropriate voltage range for charging and discharging batteries)
Using a mixture of batteries to get close to the voltage is just not practical.