> All of them I've encountered take a very standard 12V lead-acid battery - the same one that's inside consumer UPS units - and you can get replacements from a dozen brands on Amazon and other vendors.

There's about 70 different sizes for these standard 12V lead-acid batteries though. And most of my consumer UPS units hide the battery behind a security screw and don't list the battery size in the docs. If the battery was removed and not replaced, it's harder than it should be to find a new one.