Electronics question

To provide backup during power cuts (to my shame I can't live without broadband) I'm thinking of buying one these:

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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.

Another Dave

Reply to
Another Dave
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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?

Reply to
Zephirum

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.

Reply to
Lee

Doh! thought that was an inverter :) Maybe it's just about the charge current or the rating of the relay then?

Reply to
Lee

An example from further down the page: "It supports float-charging with high charging efficiency, battery plumpness could be up to 90%."

Yes, we have a dubious translation. Quite funny, though.

Reply to
Davey

Argh! me hearties, always clamp hard on yer rug terminal!

Reply to
The Natural Philosopher

Curiously described device. It is the battery that provides the UPS function. 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 fuse.

Reply to
Bob Minchin

You have not considered one thing in this scenario. I know that many of the broad band cabinets which Virgin uses are powered by mains with no back ups, so at least in SW London, when the power is off where the cabinet is, both phone and broadband go off as does the TV cable service. It could well be that BT the other supplier of the Fibre connections to these boxes do the same for broadband. You would need to find out in some way before wasting your money on back up supplies. Brian

Reply to
Brian Gaff

When our power has gone off in the past, I just use the laptop which will last for about 5 hours on battery and tether my mobile phone to the laptop which gets me back online. Luckily I get a good 4G signal of up to 92Mbps down and 33 up.

Reply to
Bod

There are certinly batteries in the BT Fibre box which feeds my house. I've seen them.

Reply to
charles

I find that worrying. Plump batteries in a UPS are extremely difficult to prize out when their sides have bowed out by more than about 5%.

It hardly inspires confidence for an emergency backup system!

Reply to
Martin Brown

Just buy a laptop.

Reply to
harry

I blame the EU it's all that cheap fatty food making everyone obese. :)

Reply to
whisky-dave

I'm the OP.

I'm working on the assumption the landline broadband will be up even if my house is down. I thought it was a requirement that the landline had a battery backup for emergency use by a landline-powered phone, although admittedly the broadband may not have. Time will tell.

If I can't see a fuse in the battery circuit I'll put one in and consider my options if it blows.

Another Dave

Reply to
Another Dave

Ok, but...:

It hasn't worked that way with all of the routers that I've had. When the power supply went off the router went off with it.

Reply to
Bod

There is zero chance the broadband will have battery backup. Do you pay extra for it to have? They don't need to and they aren't a charity.

Reply to
dennis

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[1] 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 requirement.

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 hours!

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 battery costs.

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 ownership).

[1] 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- reading).

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 makers).

Reply to
Johnny B Good

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.

Reply to
Harry Bloomfield

Build you own. ;-)

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(UPS by Photonic Induction)

Tim

Reply to
Tim+

To solve the same problem, I have contemplated buying one of these from Amazon:

Mini-Box picoUPS - 100?micro-UPS système 12 V DC/Batterie backup system

(Search for: Mini-Box picoUPS)

It does much the same thing but does not have the metal cover.

Reply to
Michael Chare

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