I have another question related to the brochure: it says 1A out on PoE. It says nothing about charging the battery.
The brochure has "Specifications" table on the top of second page. Pennultimate row reads "Battery charging current 1 A max" (and ultimate row mentions PoE out max current).
Does it draw the power for charging the battery from that 1A too or if I hook a 24V 2A PSU up to it, I get 700 mA on the PoE and 1A and change on the battery?
The brochure doesn't mention any power in limits (neither barrel-plug nor PoE-in). But let's do some math: for battery charging it'll dedicate max around 15W (13.5V and 1A gives 13.5W and let's add some losses on DC-DC converter). PoE out is limited to 0.7A @24V which is slightly less than 20W (more precisely it's 16.8W). Both combined are less than 35W, which means less than 1.5A @24V. Surprisingly supplied power adapter is rated exactly the same.
So given supplied power adapter and full rated loads on PoE out, your 18Ah battery being flat should fully charge in around 24 hours. Or around 10 hours from 30% to 80% charge.
My main concern with the 18Ah battery is that if it's not well regulated it can draw too high a current during initial charging and melt something inside the mUPS after only 20 cycles or so.
My guess is that any 12V lead-acid battery with capacity of a few Ah can draw much more than 1A when not completely charged if charger blindly applies 14.2V to its terminals (quite often that's voltage used to minimize charging time but it's too high as permanent/idle charging voltage, it causes gassing for most batteries shortening their life time). So I'd expect mUPS to have some current regulation built in or else it'd emit smoke in seconds (and we all know that electronic elements run on smoke ... after they let it out, they stop working).