wire it up, start 'em running and time it
[edit] the unknown is how long the psp2 would run --
my sbm-1 would easily run for 3+ hours on 4x 1,500mah nimh batteries -- so being really conservative, figure those would suck down 2ah of your sla's 7.2 ah of juice. That leaves 5ah for the psp2, but as a battery drains its voltage drops. Fobstl mentions the low battery light coming on when running the psp2 @ 6v, the question is thus at what point the sla's voltage would drop below the shut-off threshold for the psp2 -- unless you can find somebody else running exactly the same setup, the only way I can think of to determine this is to wire these up, start recording and see when they stop passing a signal.