Hi!

We rarely have outages here. *So, there is no real
draw on the batteries in our UPS's. *So, they should just
be on float charges.

I rarely have power outages here, and my most recent UPS battery
change involved a unit that did nothing more than "buffer" a phone
system until a generator would come on. Its battery lasted 10 years
(1998-2008). The UPS in question is an APC Back-UPS 600.

I attribute part of that long lifetime to the fact that the battery
could have degraded to where it had almost no runtime left--but that
would have been fine as the generator started and stabilized within a
few seconds.

Can anyone with firsthand knowledge shed some light on
this?

I can't say that I have firsthand knowledge. (Meaning: I've never
designed or built a UPS circuit from scratch...) However, the trouble
is likely to come from the float charging. I'm of the impression that
at least some UPS units have an awfully "hot" float charge and it
probably leads to the battery being overcharged. I've seen some Tripp-
Lite UPS units that would slowly drive the water out of their
batteries if left float charging all the time.

I also think there are some that don't put enough of a float charge on
a battery to truly keep it up. I have an older APC Smart-UPS 1000 that
demonstrates this behavior--if the power hasn't been out in a while,
the battery will drop more quickly than it does when the unit has had
only a short time (a few days) between the completion of a battery
charge after use and a subsequent power outage.

You also have to consider that some UPS designs depend upon their
battery and inverter to deal with *every* little power line anomaly.
This too will shorten the battery's useful life. This seems to be true
of the inexpensive APC "plugstrip" UPS units that many people have.

Better designs have methods by which to stabilize, boost or trim the
power coming out of them without having to use the battery and
inverter.

William