On Fri, 13 Jun 2014 17:57:15 +0100, Johny B Good wrote:

I've noticed on all the APC ones they seem to cook their SLA
batteries
after a while ....

Aye, my small APC Smart UPS iNet 750 kills a set of SLA's in 3 to 4
years.

Interestingly there are three different float charge volts per cell
figures for Gel, AGM and flooded cell types (2.23, 2.25 and 2.32 to
within a figure of +/- 0.05v[1] respectively) for 20 deg C temperature
with a temperature compensation figure of -.0235v per deg C rise per 6
cell battery).

Recently been through this loop as well. In the case of my APC UPS
the killer seems to be the temperature. I reckon that the batteries
where normally at the upper 30's to 40 C, when you apply the derating
the voltage from the UPS is really rather high. Measured charge
voltage 27.7 V, calculated charge voltage at 40 C 26.58 V, 1.12 V to
high

I have considered the use of deep discharge flooded cell batteries
(there is a trimpot to adjust the float charging voltage)

No tweaky pot in my UPS. But digging about on the web found a mod to
add one and how to enter setup mode and adjust the settings so it
thinks the charge voltage is correct. I also removed the SNMP card
slot housing and fitted a PC expansion slot exhaust fan with thermal
speed control (using the MIC502 chip). Before the UPS used to run
decidedly warm to the touch, now it is cool.

Time will tell how much longer the batteries last (or not!).

--
Cheers
Dave.