In article , Simon Barr
writes

I've *never* known any UPS to boil batteries and dump acid and I've replaced
batteries in maybe 50 over the years. Some of them were very deformed from
high temperatures but all held onto their contents.

Normally, I'd agree with you, having happily installed and used a lot of
APC kit, but we had an APC Smart UPS 2200 boil its external battery pack
last week. Fortunately, we smelt something getting hot and caught it
before it could leak everywhere. The batteries are badly distorted.

It's only just over two years old, and APC refused a warranty claim.