Jim
did you check standing voltage of the batteries 12.6 VDC is fully charged note you can use AMG battery type or SLA battery
note match the measurements LxWxH and the connection blade or lug, and the Ah rate keeping same or can go bigger with Ah but keep same battery measurements so fit the case.
higher Ah gives more staying power, if want longer run time can paralle another one if a single unit or 2 battery but need to make battery box to house them an 8 Ga wire to connect them and fuses to protect against fire, use same size as orginal (1500W uses 100amp fuse) doing this will double the run time. me have done this run time 1hr @ 60% load Network, monitors, sound system and 2 PC's.
I get niebroughs knocking on my door for playing CoD during a blackout.