Mike,
This is an excellent article and helps me allot with a similar problem I have. I am curious though in that if, 24V (i.e. in your case, 2 times 12 batteries wired in series) provided you with approx 2 hours, what do you speculate would happen if you added additional 24 v batteries in series? Would you have in effect extended the up time by almost double? Who would the outcome be if you added another 24V but in this time in parallel?
Cheers
Mark