Something to keep in mind with SLA - They do not like being discharged at anything greater than C/20, and charging them much faster than that almost always has made me wish I hadn't.
For a 7.2Ah, this means ~360mA. In my experience, and as G mentioned, SLA is happiest when cycled to 50% on a typical discharge. You don't want to yank them down too far regularly, but they get "lazy" if they don't get a workout too.
I'm with Woof on this one... If you're not too worried about portability, just use a car battery. Even though they aren't meant for deep cycling, you're unlikely to take it far enough down to shorten the life of it all that much over time. Just don't use one that shares terminals with a starter, just in case.
If you decide to go SLA, sizing is the key; Its just one of those things.
Steve