On Sun, 04 Oct 2009 10:46:55 -0700, Jeff Liebermann
wrote:

On Sun, 04 Oct 2009 11:02:14 +0800, who where wrote:

Rhetorical question: Why don't UPS manufacturers use a decent
charging circuit in their SLA-backed UPS's?

Many years ago, I accused APC of intentionally setting the charger in
some models to prematurely destroy batteries and create hazardous
conditions (bulging, leaky, and overheating batteries). Specifically,
the early APC 1400RH 4ru model was the major culprit.
http://www.LearnByDestroying.com/pics/home/apc1400.jpg
It has 4ea 12V 7A gel cells in a series-parallel derangement. We had
about 35 of these installed at various installations, all of which
rapidly ate batteries. Eventually, these UPS's were removed when it
was found that the batteries had bulged and leaked so much that
extraction was impossible. I ended up with most of them and tried
redesign the charging circuit. APC was totally uncooperative. I
don't want to go into the details, but eventually APC released a
totally new 1400RH model, with a slightly improved charging circuit.

During this adventure in frustration, I learned a few things about UPS
charging philosophy. The customers want the batteries to recover as
fast as possible after being run for a while. That's because power
outages tend to come in clusters, like during a storm. Fast recharge
is an important requirement. Given the choice of long battery life
and fast recharge, most customers will choose fast recharge. More to
the extreme, when faced with the possibility of killing the battery
just to get it charged quickly, most customers will accept the cost of
a new battery pack rather than risk any additional server downtime.

I suspect there's a bit of assumption in there. Most consumers of
"small/desktop/soho" UPS don't have the slightest clue how they work
or what goes on with them. The manufacturer makes assumptions on the
buyer's behalf, but obviously slanted towards the manufacturer's ends.
I have found UPS owners who maintain that they had to have failed
units refurbed by the mnaufacturer.

So, rather than a modern staged charging system, that tapers off near
the EOC, and is intentionally easy on the battery, the typical UPS
battery charger is designed to get as close to 100% of charge as
quickly as possible and never mind going into overcharge. That
results in dramatic changes in EOC threshold with aging batteries,
connector losses, manufacturing variations, etc. Basically, you can
have long battery life, or fast recharge, but not both.

You can achieve a *reasonable* compromise if a bit of effort is spent
on charger design. SLA's are best fed with a current-limited constant
voltage regime. Bulk charge recovery is achieved under the CL phase,
and setting the limit high to achieve fast replenishment isn't of
itself a battery-killer. BUT once the reasonable SOC has been
achieved and even before transition occurs, the CV fgure should be
reduced. And float voltage should similarly be reduced in recognition
that the average UPS spends say an hour a year (max) on discharge.
They don't need to be kept at 100%. They *should* be kept at a
sustainable (aka survivable) float condition, and if that loses
capacity the user needs then the user should have bought a better
sized unit.

None of this is news to you of course.

As an aside, I have equipped a several torches here with 6V 4Ah SLA's
in place of the original dry cell (4F?) pack (and changed to a 6V
krypton lamp). These are recharged as required using the Unitrode/TI
UC3906, to whose charge regime I continually refer people when they
enquire about care and feeding of SLA's. The current batch of SLA's
are over 10 years old.

My current guess(tm) is that UPS charging circuits are designed first
for fast charge and secondarily for maintaining as close to 100%
charge as possible. Both of these are detrimental to long battery
life

Yep.

so the charging is selected for a "reasonable" battery life of
about 3 years (depending on model).

IMOE there you are being kind to the UPS makers.

I've also experienced the drama of extracting dried/cracked/swollen
SLA's from their enclosures. Just like laptop manufacturers, UPS
makers are guilty of self-serving design without regard for the
downstream cost to the end user.

Some of my previous rants on the subject:
http://groups.google.com/group/sci.electronics.repair/msg/e99a0f155fc198c0
http://groups.google.com/group/sci.electronics.repair/browse_thread/thread/cf5ea1e3f3a01d4f/