the batteries have to be low esr, the other cheap ones with 40 mOhm dont last as they heat up too much
+1 to that, UPS replacement batteries can be a real pain to find suitable ones. "Meets or exceeds OEM specs"
always take with a grain of salt.
for example, the BB battery HR9-12 (8AH, 12v, typical UPS size)
http://www.bb-battery.com/productpages/HR/HR9-12.pdf can do ~43A for 5 minutes to 1.6v/cell, another battery by the same manufacturer, not optimized for high rate usage the BP8-12 (12v 8 AH also)
http://www.bb-battery.com/productpages/BP/BP8-12.pdf can only do 35A for 5 minutes to 1.6V/cell
Looking at constant wattage (which not all battery datasheets have), makes it even easier to compare as you can go right to watt hours. The high rate battery can do ~40WH in a 5 minute to 1.6v/cell discharge, ~32WH for the general purpose BP battery, 25% more runtime. And thats 15mOhm vs 25mOhm internal resistance. 40mOhm would practically fall on its face if you tried to do a 5 minute discharge.
If you cant find a datasheet on the batteries, I'd be leery on buying it.
The kung long WP7.5-12, which is the same physical size as the above 2 batteries, lists 19 mOhm, 30.2 WH in a 5 minute to 1.6v/cell discharge, and 35.6A in a 5 minute to 1.6v/cell discharge, so its performing no better than BB's general purpose battery of the same physical size.