On Fri, 13 Jun 2014 16:40:22 +0100, tony sayer
wrote:



The UPS manufacturers could have so easily addressed this issue but
it simply wasn't in their interest to do so (UPS supplied SLA
batteries are their equivilent to the inkjet printer manufacturer's
cash cow of inkjet refill cartridges).

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

Well, after I posted that, I took a look at the charging circuit for
the 2000 and it seems incredibly complex for its function. The whole
circuit diagram for the UPS is spread over 6 sheets (the charger is
sheet6 BTW). In this case, the power comes from transformer terminals
1 and 2 on sheet 3 and following the trail takes you to the inverter
powerfet stack circuit (a full bridge cct attached to the very same
terminals that are used to feed the 5A rectifier diodes on sheet 6 -
the inverter transformer does double duty as a charging transformer).

I gave up pondering the problem any further and decided to google to
the wiki on Lead Acid battery technology with regard to best charging
regimes where it all became ever so complex over the issue of choosing
a charging voltage that's high enough to avoid sulphation yet low
enough to minimise corrosion.

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).

Reading the article suggests it might not entirely be the UPS
designer's fault but more the terrible limitations of Lead Acid
battery technology (but that doesn't explain the 6 month life under
the benign management scheme of a UPS versus the 5 to 10 years life in
the harsher conditions of starter battery use).

I have considered the use of deep discharge flooded cell batteries
(there is a trimpot to adjust the float charging voltage) but that's
an even greater financial risk than buying a set of four cheap 36AH
SLI batteries (probably still cheaper per AH's worth of the APC
recommended SLA types though).

[1] The +/- 0.05v seems to make a mockery of the specific cell type
voltages and temperature compensation coefficient figures quoted which
leaves me wondering whether or not it is a per cell tolerance or, as
in the case of the temp comp figure, per 6 cell's worth.

You can see what I mean he
http://en.wikipedia.org/wiki/Lead-acid_battery
--
J B Good