On Sun, 25 Apr 2021 17:10:12 +0100, Fredxx wrote:
On 25/04/2021 05:47, Johnny B Good wrote:
On Fri, 23 Apr 2021 19:00:40 +0100, Fredxx wrote:
=====snip=====
I learnt a valuable lesson that evening, namely to reduce the float
charging voltage from the 2.133v per cell typically set by default with
UPSes down to the less abusive 2.1v value if you want your very
expensive "consumable" to last more than just a lousy two or three
years (assuming few to no brief mains outages in that time and that the
automated weekly battery test feature is also disabled).
I've regarded 13.0V on a 12V lead acid to be a maintenance charge.
Keeping a battery at a float charge level is said to enhance plate
corrosion over leaving it to stand and charge periodically.
It's a balancing act between the risk of sulphation at too low a voltage
and premature corrosion at a high float charging voltage. 13.8v per six
cell SLA (Gel or AGM) when sustained year in, year out, in a UPS
application is IMO, way too high.
A more sensible choice of float charge voltage in this usage case would
appear to be that 13.5v value often noted as the optimum operating
voltage for mobile transceivers.
When I recommissioned my APC SmartUPS2000 with a cheap set of four 7AH
alarm batteries almost three years ago to prove its compatibility with a
cheap Lidl 1/1.2KW Parkside 'suitcase' inverter genset, I adjusted the
float voltage down from the 55.2 originally set by APC to 54v (actually,
the float charging circuit has a very soft voltage regulation and had
crept up to 55.5v over the years since I'd last tweaked the relevant
trimpot to set it bang on the 55.2v mark per the 13.8v standard, ignorant
of this high voltage setting being the cause of early battery failure at
that time).
Incidentally, APC specify an 18AH 48v battery pack for this UPS model
which would cost somewhere in the region of 200 quid for an equivalent to
the APC supplied battery pack which, afair, was priced in the region of
320 quid! Those 7AH alarm batteries will be hard pushed to give more than
a minute's run time at the full 1500W loading so I'm giving serious
consideration to investing in an 80 to 100AH 16 cell LFP battery pack to
eliminate the overpriced short lived inefficient 'consumable' from the
equation.
The larger capacity UPSes tend to be designed to safely utilise an
additional battery bank or three without overheating (they incorporate a
cooling fan or two). That SmartUPS2000 of mine has one such
thermostatically controlled fan but the ventilation owes too much of its
design to aesthetics and not enough to function. I'll be cutting out the
'vent slots' (fan and intake) and fitting a wire finger guard on the
intake - the fan doesn't really need one since it will give a warning
sting to anyone foolish enough to try and shove their fingers into where
they don't belong.
Anyhow, my point is that just last week, I decided to risk pressing the
'Test' button to see if there'd be any sign of deterioration of the
battery pack's condition after three years of service. I'm happy to say
only the top led in the column of five extinguished as per the initial
commissioning tests so it looks like my decision to dial the float
charging voltage 'back a notch' has actually paid off.
A lot of owner / users of APC UPS kit have for many years, noted and
complained about the rather short service life of the batteries
(typically just two to three years before they're totally shagged) but APC
are not alone in defaulting to this common 13.8v per six cell SLA setting
- it's an "Industry Standard" that maximises the initial autonomy for a
laughably marginal battery pack AH sizing.
I haven't seen any actual figures on this but I suspect reducing the
float voltage down to 13.5v simply reduces the effective from brand new
initial capacity by some 5 to 10 percent in exchange for a doubling,
possibly even a tripling, of battery service life. Considering just how
expensive these 'consumables' are to replenish, trading a few percent of
autonomy for a considerably extended battery life is a no brainer choice
in my view.
Most recent models of UPS can be adjusted to a lower float charge
voltage setting by the user so it's an option that's worth investigating
if you want to reduce the TCO of your UPS.
My experience over the past 5 or 6 years with that 2nd hand 12AH SLA
suggests that the 13v maintainance figure you quoted will indeed protect
an SLA against sulphation but, of course, the downside is that it won't
be charged to its full capacity.
No great problem when its being stored and not expected to provide
backup power at an instant's notice. It'll just need a boosting charge to
13.8 volts prior to being used or placed into service. If it's one of a
bunch set aside to replenish a knackered UPS battery pack, the UPS will
do this for you whether it's been set for 13.8 or 13.5 volts per 6 cell
float voltage setting.
Just one final observation (to get back on topic, sort of) SLI LA
batteries (car batteries) are totally unsuited as a cheap alternative to
SLAs for UPS service.
I found this out the hard way (I can be a slow learner at times) by
going through TWO sets of NOS 36AH SLI batteries, each only surviving 6
to 8 months tops (and ONLY THEN did I recall a similar experience with a
4A 13.8v CB power supply floating a 48AH SLI battery some thirty years
earlier which I'd simply shrugged off as a faulty battery at that time).
Luckily, each of these sets of four car batteries had only cost me sixty
quid - it could so easily have been a lot more expensive a lesson!
--
Johnny B Good