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In order to provide graceful degradation of function in case of total blackouts, I would like to have my smaller UPS plugged into the larger one so my main computer lasts the longest.
Question: Is it safe to plug a UPS into another UPS like this?
SDsolar
Posted 2017-03-11T04:15:41.487
Reputation: 1 206
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I'm assuming that you're talking about typical, cheap UPSes. These devices only convert DC to AC when they have no input AC or their input AC is out of tolerance.
First, the problem you won't have: You will never have double conversion. If one UPS is on, the other will not be. The UPS connected to the wall, if converting, will keep the second UPS from converting since it will have AC input. The UPS connected to the other UPS, if converting, must not have an AC input and hence the other UPS is not converting.
Since you will never have double conversion, there will be no degradation. Each UPS will produce the output it produces, regardless of what the other UPS is doing.
With the power on, both UPSes will charge from the AC that passes straight through them. The load will run on AC as well.
When the AC fails, the first UPS will start converting. The second UPS will see AC and its battery is already charged. So it will just pass the first UPSes output AC through itself to the load.
When the first UPS dies, the second UPS will stop getting AC. It will run normally, converting DC to AC just as it would if the first UPS wasn't there.
Really there are only two problems you'll have.
First, the UPS plugged into the wall might need to supply power to charge the second UPS's battery some of the time. This will almost never happen, but you do need to make sure the first UPS can handle it. If it shuts down under that load, you'll be in trouble.
Second, the second UPS might not be happy with the output waveform of the first UPS. Some UPSes allow you to adjust their tolerance for crappy input waveforms. If the first UPS is at least a stepped square wave, you'll probably be okay. But you may find that the second UPS refuses to see the output from the first UPS as acceptable. (This only rarely actually happens, but it's hard to guarantee that it won't.)
David Schwartz
Posted 2017-03-11T04:15:41.487
Reputation: 58 310
It should be said that doing this will not work if both UPS are outputting stepped sine waves. Any UPS supplying another UPS with power will (I think) need to be outputting pure sine waves. So under this configuration, all UPS in the chain will need to be pure sine except for the last going to devices. See the warnings here: https://www.power-solutions.com/pressroom/newsletter/2013/09/top-4-reasons-you-should-never-daisy-chain-ups-systems
– xendi – 2018-12-14T08:31:16.277@xendi That page is mostly garbage for the reasons that I explained in my answer above. The only legitimate point is that it will interfere with emergency power off, but in home applications, there is no such thing. – David Schwartz – 2018-12-14T18:39:07.910
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I have been researching this today because I was considering this. The short of it, don't do it. UPS into a UPS might work, but it can also cause damage to both UPSs or even a fire.
There are several reasons. The first has to do with how the UPS outputs power. When a UPS goes on battery, it doesn't output the same kind of power as what passes through. The second UPS will usually either also go on battery at the same time, or interpret the bad power as a spike and try to ground it. That means the 2nd UPS will send power down it's ground to the first UPS. That could very bad.
There are a number of other reasons, but it isn't something you should do. I am a chemical engineer by education, but I have been doing electrical engineering work in my role at an industrial plant and did the research today and stumbled across this thread so thought I would contribute.
Aaron
Posted 2017-03-11T04:15:41.487
Reputation: 11
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Simpler overall answer regarding UPS or surge protectors or plain old power strips : Don't plug them into each other. There's a risk from fire, data loss, and equipment damage from someone in the future overloading either device, the outlet, or the breaker. That's before we get into the power efficiency loss, or damaging downstream units from lack of active power factor correction.
Christopher Hostage
Posted 2017-03-11T04:15:41.487
Reputation: 4 751
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Can you? Sure. Should you? No. Quality of the voltage will drop, and the back and forth Ac to dc to Ac to dc to Ac to dc will be inefficient. Get a laptop or a bigger UPS.
cde
Posted 2017-03-11T04:15:41.487
Reputation: 1 665
1I assume he's talking about typical standby devices and not an online UPS, so there would be no back and forth except when the power fails. – David Schwartz – 2017-03-11T04:54:23.133
And why do you think the "quality of the voltage" will drop? – David Schwartz – 2017-03-11T04:57:16.237
@David ups do not produce a very clean Ac waveform – cde – 2017-03-11T05:26:16.643
Sure, but what does that have to do with using two of them? – David Schwartz – 2017-03-11T18:00:01.623
1I don't see any quality problems. Tested with Kill-a-Watt units in various places to be sure. – SDsolar – 2017-08-16T22:08:17.900
I connected supply from one local UPS to APC, If power to first ups is ON everything working fine. Once power to first UPS off, second UPS is not happy , is making some sound inside every 2 sec. ( may relay contact sound ) . So I stopped my experiment!! – Arun – 2017-12-10T09:49:01.277
1@Arun It sounds like you encountered a situation where the second UPS is not happy with the first UPS's waveform. If that happens, the chained UPSes will not work. You can try swapping them or seeing if the second UPS has an adjustable tolerance. – David Schwartz – 2018-12-14T18:40:36.300
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I don't get the point. If you plug the smaller one into the bigger one it will just drain the bigger one faster and wont come on till the big one peters out.
If you plug the big one into the small one.. same thing but the small one wont last long.
In all that there is an efficiency loss... A typical UPS is only between about 92 - 95% efficient when on line. While the first UPS is still up you are wasting 4-8% of it's capacity just keeping the second UPS alive.
Ultimately, you end up getting less time out of them and risk blowing up one or both UPS units.
Better to be smart about what you really need powered up through the outage. Only plug things into the backup power plugs you really need kept alive.
Also, you should remember the UPS is really there to give you sufficient time to get to your system and shut it down in a controlled fashion. It is not really intended to let you keep running for a long time. That's what backup generators are for.
Trevor_G
Posted 2017-03-11T04:15:41.487
Reputation: 117
2In theory, you get the combined total backup time when they are in series. If you use either one alone, you will have less backup time. This is what the OP is thinking, anyway. – None – 2017-03-11T04:41:40.013
1Yes I know, but you wouldn't. There is an overhead because the UPS itself consumes power from the mains too in addition to supplying the load. – None – 2017-03-11T04:44:14.043
1@mkeith theory assumes perfect conversion. They don't have superconductor ups yet. – cde – 2017-03-11T04:52:26.063
Why would there be an efficiency loss? There is no case where both UPSs would be converting DC to AC at the same time. The one plugged into the wall would feed into the second, which would stay bypassed, until the one plugged into the wall died, at which point the second one would feed the load directly. – David Schwartz – 2017-03-11T04:55:57.300
@cde It doesn't assume perfect conversion at all. Each one's backup time takes into account the conversion of that UPS. There will never be double conversion because each UPS will only be converting when the other isn't. – David Schwartz – 2017-03-11T04:56:46.730
@DavidSchwartz, because a UPS is only between about 92 - 95% efficient when on line. While the first UPS is still up you are wasting 4-8% of it's capcaity just keeping the second UPS alive. – Trevor_G – 2017-03-11T05:01:22.660
No, because the second UPS won't be online at that point, it'll have its battery already fully charged and the AC will just pass through it to the load. – David Schwartz – 2017-03-11T05:01:45.480
You still have to power the UPS itself though @DavidSchwatrz. The UPS DOES NOT take it's power from the battery when on line. It takes it from the mains, powers up the charger circuitry etc. The fact that it is switching through the mains is irrelevant. – Trevor_G – 2017-03-11T05:08:18.023
@Trevor That's so microscopic it's hardly worth mentioning. Remember, this it he case where the second UPSes battery is fully charged and the first UPS is converting. So it's powering the load and some microscopic bit of standby power the UPS uses at 92-95% efficiency. Back of the envelope, you're wasting maybe 3W while the first UPS is converting. Hardly worth worrying about. (Especially since we're talking about nearly doubling run time compares to a single UPS.) – David Schwartz – 2017-03-11T05:12:17.670
@DavidSchwartz, if you think 5% of the power your computer uses isn't much ok. Either way that does not answer the question though does it. The OP expects to get MORE time out of it. He will in fact get less.. whether that be a couple of seconds or half an hour.. it is still less. It will NEVER be more. – Trevor_G – 2017-03-11T05:22:04.603
@Trevor No, he will get more time. Just about 4% less additional time than you might expect. – David Schwartz – 2017-03-11T17:59:35.080
@DavidSchwartz, it depends on your definition of "More Time". I agree, one load plugged into two UPS in series will give you more time than just being plugged into a single unit .. yes!. However, multiple loads plugged into two in series will give you less total time over properly distributing the loads across two units in parallel. total time being CPU Time + Monitor Time + Sound System Time + Whatever else he has plugged in time. – Trevor_G – 2017-03-11T18:05:07.290
eg, two 500W loads plugged into two 500W UPS in parallel may give you 100 minutes of up time. Same loads both plugged into second unit or series connected UPS will only get you about 95 minutes. @DavidSchwartz – Trevor_G – 2017-03-11T18:07:12.493
Properly balancing your loads across the UPS in parallel is more efficient..gets you max time. – Trevor_G – 2017-03-11T18:08:06.350
@Trevor I doubt it. It's unlikely you could balance them to more than about 10% accuracy. And if you're down when the first UPS dies, you're actually better off with them in series. – David Schwartz – 2017-03-11T23:07:05.403
Better to have the UPSes plugged into separate wall outlets, feeding separate devices. High-capacity UPS for computer, low-capacity UPS for less important stuff like monitors. Don't plug printers into UPSes because printers ruin UPSes. – Christopher Hostage – 2017-08-16T19:01:53.273
@mkeith is right - combined time is important. And also graceful degradation. I want the most important computer to be the last to get shut down, and so it will get notified to do a proper shutdown when it is finally time for it to give up the ghost. As for printers, that sure is true of lasers. But my little Brother MFC-J870dw takes flea power so is on the first, larger UPS. It will go off before the computer... – SDsolar – 2017-08-16T22:10:24.307
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Most UPS systems output a stepped sine wave when on battery power. Almost all UPS equipment interprets a stepped sine wave as bad power. Therefore, as soon as the first UPS goes on battery and outputs a stepped sine wave, the second UPS in the series will read that input as bad power and convert to battery also. With both UPS systems running on battery at the same time, there is no increase in battery capacity.
James Skaggs
Posted 2017-03-11T04:15:41.487
Reputation: 1
In the end I did chain an Opti 1500VA unit for most stuff, including a small inkjet. When it gives up, then everything will be off (one laptop will have a bit more time due to its own internal UPS) including the printer. The smaller APC will buy just a little more time - finally, the router and most-important computer and one monitor will be last - and it runs on a SSD so when it gets the "now" signal it does a full clean shutdown. I posted this 5 months ago, and have been using this setup for 3 months, with one involuntary full-on test, and it seems to be fine. – SDsolar – 2017-08-16T22:02:17.460