I'll go look again, but last I did I didn't find much [affordable]. I do have an (no longer in production?) APC Smart UPS which can be extended with an external battery, alas vastly overpriced.

Of note, if various online resources are to be believed, you can't just hack and extend the battery capacity for longer run-times as they likely aren't rated for the corresponding higher thermal load.

> alas vastly overpriced.

they're not cheap, but i've bought cheaper and that stuff costs even more in the end.

power electronics that are safe and reliable costs.

> you can't just hack and extend the battery capacity for longer run-times as they likely aren't rated for the corresponding higher thermal load.

thermal load... of batteries?

i'd expect you'd confuse the microcontroller in the UPS. more batteries will require longer to bring up to voltage than it expects, which implies something is wrong. further all of the time remaining estimations it presents will be wrong.

[Following your advise I did find some Tripp-Lite that are almost reasonable]

Thermals referred to the power electronics which do get hot. Yes certainly better to get equipment designed for it rather than risk burning down the house.