Quoting mshiels

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

Adding in series would up the voltage, a bad thing if the UPS's voltage regulator couldn't handle the extra voltage.

If you added more batteries in parellel (I think he mentions that in the article) it would increase the up time. 2 batteries last 2 hours, 4 batteries would last almost 4 hours, 6 batteries would last about 5.5 hours. You lose a small fraction each time you add batteries because of the added resistance of the extra wiring and the batteries' internal resistance.