To provide backup during power cuts (to my shame I can't live without
broadband) I'm thinking of buying one these:
It's a UPS circuit designed to charge a lead acid battery (not supplied)
and to maintain 12V in case of a power cut. It seems to be ideal for my
purposes as I have an 85AH leisure battery to hand.
However the spec says, quaintly, "It is allowed to connect batteries
that is below 12V 20AH".
It's been decades since I did any electronics but I don't understand the
limit. Surely, other than that it will take a long time to charge a big
battery (of no concern to me), what harm can come from using the 85AH
battery? I will, of course, be sure not to exceed the current limit.
It does not imply nor indeed state to my mind that bigger batteries are
bad merely that smaller are acceptable in what I taker to be some
translation from another language? Some chargers might overcharge
smaller capacity batteries?
On Wed, 19 Oct 2016 15:21:13 +0100, Zephirum wrote:
If you can read past the Chinglish translation, it appears to be more
than just a simple battery charger for float charging a 12v L/A battery
to which you also directly connect the 'protected' load (as in a simple
homebrewed 12vdc UPS set up).
Interestingly, I noted the use of 13.5 volts instead of the more
typical 13.8 volts float charging level used by virtually every UPS
manufacturer for each half dozen SLA cell's worth of battery - a float
charge voltage level that I've come to suspect as being the main reason
for the limited 3 to 5 year service life of a typical UPS battery pack
whether they sit idle or are actually used in anger during a mains outage
(and also the reason why substituting SLAs with cheap car batteries
results in a set of knackered car batteries in a mere 6 to 12 months!).
The protected load is connected to terminals on the UPS charger rather
than directly to the battery terminals so it's quite clear that it
contains a battery undervolt load shedding switch (relay or solid state)
to save the battery from being over-discharged and suffering sulphation
damage as a result. The 10v cut-off corresponds to the 20.2 and 40.4 volt
cut-off voltages I've measured with APC SmartUPS700 (24v BP) and an APC
SmartUPS2000 (48v BP) units.
The major difference between this unit and a classic UPS supplying
ersatz mains power during an outage to IT kit which uses this ersatz
source of mains voltage to provide low voltage DC to power the actual
electronic loads is the elimination of "The Middleman" 12vdc to 230vac
inverter and its attendant conversion losses.
Virtually every bit of wall wart powered IT kit can be powered from a
standard 12vdc almost regardless of the claimed dc input voltage
Indeed, there was one piece of kit that I had (still have?), after I'd
checked it out, which had been designed for 7.5 vac power input from a
cheap transformer only wallwart, which worked perfectly fine with dc
input voltages ranging from 7.5 to 12v dc, drawing less current as the
voltage was increased - a sure fire sign of the use of a switching
regulator after the fullwave rectifier and 25v rated smoothing cap that
had been incorporated to save the need of having these space consuming
components in the wallwart itself thus reducing the wallwart to nothing
more complex than a simple overheat protected mains to 7.5v transformer
no longer subjected to the heat from the additional rectifier components.
Since most SoHo ethernet switches and routers are powered from wallwarts
providing 'stated' dc input voltages ranging from 6 to 12 volts, this "12
volt UPS" can be the ideal solution to keeping your LAN and internet
connections up and running during protracted mains outages without adding
to the loading on a more conventional mains voltage supply UPS used to
protect desktop PCs against sudden loss of power.
As for the choice of battery, that depends on how much run time you feel
is desirable which, in turn, depends on the total protected load (which
might range from as little as 24 watts (a couple of ethernet switches and
a VM SuperHub2 for example) to maybe as much as 50 watts in a more
ambitious SoHo environment (ymmv- you'd have to measure the total
consumption of all of this "12 volt powered" kit to get the actual figure
by which to calculate an autonomy rating).
I'd say that that 'under-used' 85AH 'leisure battery' is just crying out
to be pressed into such service. As someone else mentioned, you're well
advised to use a fused connection to the battery to minimise the risk of
a house fire.
I used a 100A exchange rack fuse bolted to the positive terminal of the
200AH 12v battery I used to use as my shack supply which supplied a bank
of DC circuit breakers feeding my 12v powered amateur radio kit. This was
simply to eliminate the risk of a battery explosion should the heavy duty
cable feeding my bank of circuit breakers be shorted out.
The load was otherwise connected directly to the battery so had no
automated over-discharge protection, relying on manual intervention to
avoid this undesired state (the packet radio set up only took a tiny
fraction over an amp so could be run for just over a week on battery
power alone in this case).
If your load on the protected 12 volt supply is, say, just three amps,
you'll have a charging current reserve of 7 amps which is enough to fully
charge (well, 90% or so) that 85AH leisure battery from flat in just 12
With a specified charging voltage of 13.5 volts, there's very little
danger (compared to the 13.8v choice) of corrosion damage to the plates.
However, I suspect the 13.8v level chosen by the UPS manufacturers was a
choice determined by the maximum they could get away with in order to
maximise the effective battery capacity for a minimum 3 to 5 year service
life to 'plump up' their autonomy ratings whilst minimising initial
I reckon that if the UPS manufacturers were to "Do The Right Thing" and
use 13.5 volts instead, they'd either have to deflate their autonomy
claims by 10 to 15 percent or else specify a 10 to 15 percent larger
capacity battery pack to compensate (and effectively triple the service
life of said battery packs to reduce the overall total costs of
 I've been keeping a 12v 12AH SLA parked on my office window ledge for
some 4 or 5 years now (a 5 quid 2nd hand purchase from a local flea
market). When I rechecked the voltage on getting it home, it was barely
above the 12v mark so flatter than the 13.something reading I'd observed
with a borrowed DMM at the time of purchase (I guess I must have missed
the low batt indicator on the dmm which would have accounted for the over-
Anyhow, since I didn't have an actual 13.8v charger I could safely
charge this from, I hung one of those 1.5Wpp solar panels out of the
office window to catch the afternoon sun to use as a battery charger. It
took about a fortnight to fully charge as indicated by the voltage
hitting the 14v mark and over before I disconnected it. The voltage
settled down to 12.78v, dropping to 12.75v a few months later where it
more or less stayed until the next round of late Spring/early Summer
sunshine by which to apply another dose of 'refreshing' charge.
In between times, I had used it now and again as a test supply as well
as checking it with a 55W quartz lamp capsule from time to time to
confirm that there was more than just voltage alone in the battery.
Indeed, a year or two later, I got to use it to jump start the wife's car
(an Astra 1.6 automatic with a totally knackered battery) so it certainly
seemed to be in very good condition all things considered. Indeed, having
receiving its annual solar panel refreshing charge about 4 or 4 months
ago, it's now showing a voltage of 12.88 volts (and still lights the 55W
test lamp with no signs of fade during the brief 10 seconds or so test -
I don't want to "burn up" charge/discharge cycles, just do a brief test
to confirm it hasn't "dried out" and gone high resistance).
The point of mentioning this is that I think keeping a 12v SLA on a
permanent 13.8v float charge year in, year out, does far more harm than
simply charging them right up on a 13.8v charger once a year and leaving
them sat on a shelf for the rest of the year.
The big problem with lead acid batteries is finding the optimum float
charge voltage, too high and it'll suffer corrosion, too low and it
suffers sulphation. The fact that this 12AH SLA has done quite nicely out
of being left to 'float' at a mere 12.75 volts for more than 11 months at
a time suggests to me that the lower 13.5 volts is still way more than
ample to keep the battery in a fully charged state (perhaps effectively
just 90 to 95 percent of that resulting from a float charge voltage of
13.8 volt) without excessive corrosion taking place.
IOW, use a lower voltage to sacrifice a few percent of effective
capacity for the benefit of extending the service life by a factor of two
to four times that seen when using the higher 13.8v charging regime
employed by the UPS manufacturers who are only concerned that the buyer
doesn't discover the "Squeeze the most autonomy out of the minimum
required battery capacity when charged at the maximum short term safe
float voltage" deceit until well after the warranty period has expired.
Battery packs are to UPS manufacturers what inkjet cartridges are to
inkjet printer manufacturers. I think the secret to getting a ten years
or longer life out of a UPS battery pack is to (where possible) adjust
the universal 13.8v per 6 cells float voltage setting to the less
aggressive 13.5 volt level. The fact that the "12 volt UPS" in question
is already set to this lower voltage suggests that this theory (untested
as yet by me) has validity.
I'm certainly encouraged to try out a *third* set of cheap car batteries
to place my SmartUPS2000 back into service again. This time I'll be
reducing the 55.5v float charge voltage (intended to have been 55.2
volts, btw) to a mere 54.0 volts. This isn't a very large reduction but
it can make a critical difference to batteries that were never intended
to be kept on a permanent 13.8 volt float charging regimen in the first
place (leisure batteries are better optimised for such permanent float
charging but they're more expensive than normal car batteries and almost
as costly as those expensive SLA battery packs being pushed by the UPS
That, I completely agree with. I used to keep my caravan, with its 12v
leisure battery parked in my drive with its 13.8v charger/SMPSU powered
up permanently. It went through batteries at a rate, failed due to
drying out, despite being sealed. Now I recharge every time the battery
has been used, for a couple of days, then a couple of days every few
months, or just prior to the caravan being used.
Corrosion? Not sure what you mean, as said the electrolyte evaporates
due to being on a too high constant charge level.
The trickle charge voltage needs adjusting so that a fully charged
battery draws a minute fraction of an amp. Something like 0.001 to
0.003A. Charge the battery fully. Check the battery voltage immediately
after you disconnect the charger. Set the charger to that voltage. After
24 hours check the charge rate. Adjust the charger voltage to achieve
the above current rates. That works well.
The alternative is to use an intelligent charger. These also work well.
On Thu, 20 Oct 2016 19:41:11 +0100, Harry Bloomfield wrote:
The corrosion problem is confined to the positive plates of modern paste
filled lead alloy grid construction (presumably there's much less of a
problem with the negative plates when the fully charged state is pure
The wikipedia article here:
explains this better than I can.
Incidentally, I've changed my mind about using cheap car batteries now
that I've re-read that article. It seems the problem with extremely (and
surprisingly) short service life under seemingly the most benign of
conditions is not, in this case, the result of excessive float charge
voltage so much as a lack of stirring of the electrolyte to prevent
When used for their intended purpose, the accelerations (due to
acceleration, braking and cornering) cause the electrolyte to slosh about
and keep it thoroughly mixed. I suppose I could make up a rocking
platform to mount the batteries upon, driven by a small electric motor to
provide the necessary agitation but that seems an extra complication too
far. However, if I can snaffle another bargain set of batteries out of my
local car breakers/spares dealer, I might reconsider this option. :-)
It seems my best bet is to find a supplier that doesn't charge the UPS
battery pack price premium and invest in a decent set of 25AH Gel SLAs
(and reduce the float charge voltage at 20 °C to 53.52 volts as well). A
good source of such 'battery bargains' could often be found at major
amateur radio (mobile) rallies so I might hang on until the next major
rally to try out my luck with the battery specialists.
An alternative to using 25AH or larger capacity SLAs would be to fit
12AH SLAs and invest in a 3KVA inverter based genset featuring eco-
throttle control. The last time I looked several years ago, I saw prices
as low as 600 quid for some brands of this genset type (four times what I
paid for a cheap Aldi 2.8KVA emergency genset several years ago now which
proved totally unsuited due to capacitive loading by the SmartUPS2000
causing the genset to overvolt beyond the UPS's line interactive voltage
bucking range - the genset overvolting north of the 275vrms mark in this
case and a problem common to *all* such gensets).
For anyone contemplating such a genset based emergency power backup
solution, I'd *strongly* advise against investing in such a basic genset
(the type that drives the AC generator at a fixed 3000/3600rpm to
generate the required 50/60Hz mains voltage). The only type that is
immune to such capacitive load induced voltage instability is the
inverter type where a high efficiency permanent magnet generator is used
to feed a mains inverter with a dc voltage, allowing the engine speed to
be varied according to power demand.
The Old Skool type gensets whilst ok with resistive loads such as
incandescent lamps and able to cope with inductive loads such as pump
motors and the less inductive loads from universal AC motors used in
vacuum cleaners and washing machines, I suspect the increasing use of
capacitively ballasted LED lamps (capacitor volt dropper circuit) might
be sufficient to cause problems *before* you even get as far as
connecting the more critical and capacitive load from your IT kit.
If you're going for the luxury of a backup power system that can do more
than provide you with a 15 to 30 minute margin by which to properly shut
down your PCs and allow you to ride out an extended mains outage,
you'll need the more expensive inverter type genset. The basic, 'cheap as
chips' Aldi bargain gensets simply don't cut the mustard with today's
domestic electricity loads.
 Indeed, if you are unfortunate enough to be running Windows 10 on
your desktop PC, you might well find yourself caught out by an unplanned
and protracted update that cannot be interrupted without causing
additional out of service time. :-(
Lidl have sold an Inverter type on occasions , I picked one up a
couple of months ago. At only 1200 watts peak.1000 continous it would
be of limited use for many but about the only situation we could not
cover in a long power cut is the loss of the deep freeze contents with
the other needs of warmth and cooking provided by the wood burner and
lighting by great,great grandparents oil lamps.
It was only £99 compared to its normal £129 so just preventing the
loss of one freezer load would seen that covered. Would have no
hesitation in running a freezer from yonks ago on our old genny which
is still available for appropriate loads but this modern freezer with
electronics within is probably a bit more delicate than just a hefty
compressor and a mechanical thermostat.
Thought I had missed the last one in lidl when I went looking but it
is so small someone had covered the last one under a pile of coats.
Probably will come around again.
In my case the important use of electricity is to keep the heating system
going. The Boiler needs a limited amount of power and so does the pump. It
might be good to power a radio, too, to save batteries.
On Fri, 21 Oct 2016 10:53:51 +0100, damduck-egg wrote:
I knew about these 'cheap as chips' inverter gensets but the ones I saw
(in Aldi) lacked the one desirable feature that the use of an inverter
allows, "Eco-throttle", so I disregarded it out of hand for that
egregious omission alone (quite apart from the fact that its rated output
was so ludicrously tiny - a case of "All bluster and no action" - if
you're going to put up with all the faff and noise of an emergency genset,
at least let it be for a worthwhile benefit).
They're not called "Suitcase Generators" for nothing. :-)
Mind you, this one seems a particularly "Industrial" and noisy example
of the breed with a 95dBA SPL rating, assuming it's the PGI 1200 a1
And, once again, I'd pass on that offer. Although a 1.2KVA inverter
genset would be *just* sufficient to supplement the 2KVA SmartUPS in
regard of the load presented by my IT kit, I'd rather be using a
3.5KVA/3KW continuously rated inverter genset with eco-throttle to give
me not only additional capacity to run the houselights and fridge/freezer
and keep the CH going but also give an extra margin of capacity to reduce
the risk of overload induced shutdowns.
In all probability, a 3.5KVA rated eco-throttled inverter is likely to
use barely any more fuel, compared to that Aldididdle unit I was checking
out some 6 to 12 months back, when powering a 1KW/1.2KVA load (and is
more likely to be the quieter of the two under such a partial load).
 If it's the PGI 1200 a1 as described here:
then it looks like you got yourself a higher specced version than the
ones being sold in (presumably) Aldi stores some 6 to 12 months ago. When
I searched for some hard facts (as you inevitably have to with high tech
Aldididdle product), I eventually discovered that it lacked "eco-
throttle" which put it completely out of the running as far as I was
 Actually, if they do become available at that price (99quid), I think
I'd be tempted into buying one to experiment with (especially if it
*does* feature eco-throttle as per the PGI 1200 a1). It can act as a
reserve backup to a quieter 3.5KVA eco-throttled inverter genset when I
eventually do get hold of one at an even more reasonable price than the
599 quid 'bargain' pricepoint I last saw them at a few years back.
It is, I was all set to buy a bigger inverter unit last year and
mentioned it to my brother who promptly gave me a cheap open framed
2kw Chinese job he bought to run site tools now spare because he has
purchased powerful Honda set and has a changeover switch and socket
installed so he can run his house if needed,though when asked if it
was an Inverter type I got a blank look so I have advised him to take
care what he powers till he knows for sure.
I couldn't diplomatically throw it away just yet and TBH with the
wood burner and gravity heating and LED lamps we can drop the
electrical load to a minimum so the bargain Lidl will be there for
I did have an 8kw Petter diesel set but that smoked a bit and was very
noisy,it would be overkill for what we need now ,it wasn't portable
enough for my brother so I got a good price for it rather than move it
I'm going to hazard a guess here, but seeing as it's not fan cooled I
would suggest they are doing what a lot of the cheap consumer UPSs do.
That is, limit the AH of the batteries to limit the run time and prevent
the thing from cooking itself.
Curiously described device. It is the battery that provides the UPS
Hardly any difference from a battery charger then. At best it might have
a relay to disconnect the battery if its terminal voltage falls to
possible damage level.
There are trimmable power supply bricks with nominal 12v output looking
very similar but far cheaper, primarily advertised as LED power units.
Just connect power supply, battery and load all in parallel perhaps via
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