A few years back I read that one should not do this with consumer-grade UPSes because they have no cooling and are designed so that during the short time the standard battery lasts they will not reach an unsafe temperature.
Anyway, I replaced the battery by a used one from another UPS, and all seems to be fine.
Perce
All I've seen so far had "snapper disk" thermal cutouts. Don't know the correct name, but usually a small bakelite cylinder with a swaged on aluminium cap and 2x 1/4" spade terminals and are usually mounted by a loose flange with 2 screw holes. The active bit is a bi-metal disk that is bowed, it snaps to bowed the other way when it gets hot.
You'll always find one on the magnetron/fan duct housing in a microwave oven, they can turn up in pretty much anything that could overheat.
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