Sylvia Else wrote:
Michael A. Terrell wrote:
Thomas Tornblom wrote:

[attributions elided]

I have been contemplating using two full size 60Ah car batteries
externally instead of the small 7Ah internals.

Make sure you keep a couple fire extinguishers handy.

Such a modified UPS would clearly run for much longer, but given the run
time of a typical UPS on its standard batteries, I'd have thought any
heat sinks would be approaching thermal equilibrium, and wouldn't get
much hotter on an extended run.

Charging?

I think the point was that lead acid car batteries produce H2.
Quite easy to ignite *in* a house :