I have several modded APC Smart UPS's

Add rear contacts fused and parallel to the internal batteries.
Then you can add any batteries you like as long as the voltage matches the
originals. If you are extending the runtime, very good if you fit a fan to
the grill inside, to cool the ccts inside.
I have a smart APC 1500. It will run at 90% load for 8 hours. No problems at
all.
P.S I use 4 x exteral Chloride 100AH in a seriel parallel combo to give the
needed 24v nominal.