I have one of those absolutely terrible cheap Cyberpower ones, and I actually use it with the battery completely removed for some weird reasons. Works fine, though it gets upset and wants to sound its buzzer even with that disabled in its configuration utility. A few dabs with the desoldering tool fixed that.
But really, don't buy those things they are awful as a UPS. Never did work reliably.
Shitty UPSes are the ones which does NOT ALLOW you to replace rhe battery in-situ while critical load is being applied without disruption of critical load.
But really, don't buy those things they are awful as a UPS. Never did work reliably.