Hi Peter.
an interesting project
it looks like you must have an early warning of the battery end point, long before you end up in a heap on the floor !
battery voltage down to 10.5v may be ???
found on the internet ,the data for a 12v 9 Ah battery was as follows :-
20Hr @0.45A 9Ah
5Hr @ 1.41A 7.2Ah
1H @ 5.4 A 5.4 Ah
1C @9A 4.5Ah
running two batteries in series down to 21V , I guess thats 9.45w for 20 Hrs, down to 189W for 30 mins
we just need to know the power required 1, stand still and 2, travel 3 or 4 miles per hour
John