The 2 UPSs I have (both APC), simply have a 12v sealed lead acid battery in them. I see no reason why you couldn't simply hook up a larger 12v battery to them to get a longer life on battery power. I don't know how well or if it could charge a much larger battery, of course you could simply avoid that by charging the battery when power comes on with another battery charger. You would still need to make sure the power supply gives you enough wattage to properly power your devices, but you wouldn't have to buy a giant one just for the extra battery life.

I also checked, these will turn on after being turned off with no power (obviously can't speak of any other brands).