replace UPS battery with same voltage but bigger

You can, but the unit's built in charger will take longer to charge it if you run it down. Tom

I did that. used a lawn n garden 12 volt to replace the small lead acid, of course the bigger battery sit beside the ups with wire running inside. Ran about 2 years like that. JTT.

Along with what the others have said, do NOT use a car battery. Use an RV, Marine, Golf Cart, or Wheelchair battery - a deep-cycle. Automotive batteries are optimized for heavy cranking amps, for a short time. They don't really like being on a medium discharge for very long, and cycling them is kind of hard on them. Deep-cycle batteries are designed to provide moderate currents for a long time, and won't get damaged by deep discharging. (well, you don't want _any_ battery to go completely flat).

The difference, I think, is in the plate structure.

Hope This Helps! Rich