I've tried much the same with an APC Smart UPS with puzzling results.
When I ran it as a dumb UPS, with no monitoring software, I got twice
the run time, as you would expect. When I ran the paralleled batteries
along with APC's Powerchute software I could not extend the run time,
(despite lots of recalibration). It was as if Powerchute knew the
maximum capacity of two batteries and would not recognize/accept/deal
with 4, (despite the fact that it monitors battery voltage, seemingly as
an indicator of when to shut down a system).

I've no idea whether this is relevant to your UPS.

I think the problems warned of here in running them series/parallel are
misplaced. To quote from GE's "The Sealed Lead Battery Handbook" 1979,
page 91:

"With sealed lead batteries there are no major problems with parallel
connections."

Gerard Bok wrote:
On Sun, 22 Jan 2006 07:15:01 GMT, "Jixter"
wrote:


I have a 700VA UPS that has dead batteries... I'm about to purchase
replacement batteries but got to thinking..
The batteries this UPS takes are 6v 12aH and there are 2 of them running in
series = ~12v together (13+ realistically)

I can replace the 2 batteries running in series, but was thinking about
extending the runtime of the UPS by running 4 batteries.. (2 sets of 2
running in parallel)


NEVER EVER connect batteries in parallel!


Or should I just get 2 larger 6v batteries with more aH rating instead? Of
course they might have to be outside the UPS instead.
Or I could get 1x12v sealed lead acid battery instead.

I realize that a larger battery might not charge well or properly on this
UPS.


And you must keep in mind, that some cheaper devices are
constructed in such a (lousy) way, that the components used will
only survive for the backup time provided by the batteries.

(Double the capacity, your UPS will run longer, parts may reach a
higher temperature and fail.)