Replacing the 12 volt lead acid battery in a hybrid gas/electric
A 50 AmpHr LiFePo4 battery should provide as much (and more consistent voltage vs state of charge) back-up power as a comparable lead acid battery. The second is cost. The LiFePo4 would cost about 5 to 6 times as much as a lead acid initially but should have something like 10 times the useful charge/discharge cycles.