D Yuniskis writes:

Sylvia Else wrote:
D Yuniskis wrote:
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 :
He could always put them outside.

Yes, this was the approach I was taking. But, for most smaller
UPS's you need 5 - 10X the desired output current from the
battery pack. So, the farther the batteries are located
from the actual UPS, the heavier the wire needs to be in order
to keep IR losses manageable.

And, the batteries are then subjected to the weather (temperature
extremes, etc)

Many batteries are available with venting hoses that can be used to
vent the gasses outside. Required in cars like my old audi, where the
battery is installed under the rear seat, in good VW tradition ;-)

I noticed that the SmartUPS can be SW configured with a number of
external battery packs. I configured the UPS to believe it had one and
two external battery packs, and by interpolating the run time it
showed with the various number of external batteries it appears that
they are nominally 20Ah. So a couple of 60Ah batteries would be
equivalent to three external battery packs, and would power my stuff
for over 5 hours.