You can get 12v 7A hour batteries at BG Micro for $15, get a cheap battery-less UPS on Ebay or somewhere for a charger.

And all that's going to fit in the spare drive bay? Somehow I doubt it. Li-Ion is preferable, because of weight and energy density. But then you have to have special charging circuits for them. Still, it's probably the only workable solution in battery chemistry.

Stu