I doubled the batteries on my APC SmartUPS 750 in expectation of
doubling the run time. If I didn't use APC's software, the runtime
more than doubled, as expected. However, when I used APC's Powerchute
software, there was no increase in run time. Despite doing the
hardware-firmware run time recalibration with doubled batteries, the
software had a mind of its own about how long the UPS could run with
the load I had on it. Different makes/models of UPS probably react
differently. If the load is under constant supervision (and perhaps is
not a computer), the UPS software is a non-issue.

Recharging 4 batteries took longer than 2, (but if I remember correctly,
not twice as long.)

krousson wrote:
> hello group; a post with 2 linked questions about UPS:
>
> i have a mobile application requiring UPS online supply, for a "small"
> load (approx 250W) that needs to run for 1 hour. more than half of that
> power is drawn by 12VDC equipment.
>
> question A: can i extend a UPS's runtime by simply adding similar
> batteries? in general, what do the chargers know about voltage and
> capacity of the batteries? if there are more batteries, same voltage,
> will it only take more time to charge? (and then, to discharge...)
>
> question B: is it OK to tap the 12VDC of the batteries directly before
> the inverter or will something (charger or inverter) be thrown off by
> such a "leak"? by doing this i hope to drop the 120VAC - 12VDC
> transformer that adds significant weight to the setup (which needs to
> be as light a possible).
>
> (safety note: this setup will never run unattended; it powers equipment
> that is constantly monitored)
>
> thanks!
> kamut.
>