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this is a theoretical thought. As we know UPS (online ups) . We have outlet from where we connect to other device. And there is a plug point from which we charge.

Now my question is what happens if, plug is connected to outlet. 1 . Does UPS will never discharge so that current is in loop. 2. Some thing wrong happens. 3. Discharge rate may be slow. 4. Or weather it blows up.

What happens ? Let me know , I want to know theoretically then we can give shot to try practically. Any valid answer with logical reason is appreciated.

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Let me make sure I understand the question: you're asking what happens if you take a charged UPS, and plug it into itself. Correct? – Remiel Jun 18 '12 at 14:11
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It depends completely on the design of the UPS, and isn't a good question for this site. – Adam Davis Jun 18 '12 at 14:21
@Remiel Or one UPS plugged into another UPS that is plugged in the AC mains (wall socket). I'm confused by the question's wording myself. – mctylr Jun 18 '12 at 14:29
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Do you mean something like this? ars.userfriendly.org/cartoons/?id=20030922 – madrivereric Jun 18 '12 at 15:39
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"In this house we OBEY the laws of thermodynamics" - (H.Simpson) – MikeJ-UK Jun 19 '12 at 9:00
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3 Answers

up vote 7 down vote

The simple answer this this: On the long-term average, the power output of a UPS is always less than the power input. If you connect a UPS like this, eventually the UPS will discharge the battery.

What happens until the battery is discharged will depend on how the UPS is designed. It is impossible for us to say how every UPS will handle this because there are too many ways it could work. My guess is that some, maybe most, UPS's will keep alternating between "normal" mode and "backup" mode. Maybe switching modes several times a second.

How long it takes to discharge is also up for debate, but it will eventually discharge. It might be as quick as several hours or it could be days. My guess is that it will be longer than my patience to find out since the "loss of power alarm" will be going off during most of that time.

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I don't think you need average I believe it is strongly always less power outputted than inputted. Your basically talking in terms of efficiency. I don't have any numbers but I suspect it is less than 90% for both the AC to DC input, and DC to AC output. As well as the battery itself. – mctylr Jun 18 '12 at 14:24
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Doh! I was working on a system like this to get free power. Oh well, back the the motor running a generator which runs the motor... I'll be a millionaire any day now as soon as I get the last few details worked out. – Olin Lathrop Jun 18 '12 at 14:25
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@mctylr I put the phrase "long-term average" in there because some pedantic person will find some irrelevant case where the output power is exactly 100% of input power. For example, some UPS's might just feed the input AC directly to the AC output without any AC/DC or DC/AC conversion (when the battery is already charged). Of course, as I mentioned, this case is irrelevant for this question. – David Kessner Jun 18 '12 at 14:39
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Another pedantic person might point out that in devices with power storage, it's quite possible for instantaneous output power to exceed instantaneous input power. If the UPS is unplugged, for example, input power is zero, and output power is not. But I'm not that pedantic. – Remiel Jun 18 '12 at 18:08
@DavidKessner and Remiel Okay, I understand your concerns, and don't wish to encourage any such faulty analysis under the guise of being pedantic. We don't need to entice the Free Energy nuts. – mctylr Jun 19 '12 at 7:53
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up vote 1 down vote

I believe you are asking about the case where one UPS in plugged into itself, and with a (fully) charged battery.

I'm not sure of the result, but I believe there is a chance for the UPS to fail (i.e. not work as intended) or develop a permanent or temporary fault (i.e. component failure / bad parts).

The problem comes from the fact that the UPS in placed into an inverting feedback loop, that is not consistent with its design for intended usage. When you turn the UPS on, after it has been plugged into itself, the UPS detects no AC from its input (the plug), so it switches to deliver power from its batteries. This causes AC to be delivered to its output (power sockets), which triggers the UPS to switch back to "mains" power mode of delivering power from its input (plug). This halts the power being sent from the batteries, and thus no further power is sent to the plug.

This could restart the this pattern ad infinity until the UPS controller determines it is not operating as intended and powers itself off (in some "smarter" models) until manually powered back on, or failure. Depending on the speed of the cycling of switching to batteries and back to the plug as the power source, various components could fail. As an UPS contains essentially back-to-back SMPS (AC to DC, and DC to AC) that have high voltage stages depending on design, these high voltages (~400-800 V) are high enough to generate a significant spark that could cause a fire.

The best case outcome is that it cycles back and forth several times rapidly and powers itself off.

I have had too many UPSes fail from harsh (outside of intended or specified usage) real-worth conditions - excess loading, bad AC voltage levels from the AC mains (wall socket), not testing and replacing batteries, that I doubt that many, if any at all, UPSes could reliable withstand such abuse.

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This answer is "more correct" than the other; when you initially turn the UPS on, it will not be receiving AC power, and will switch to "backup" mode -- feeding itself AC power, causing it to switch back to "online" mode -- cutting off its own AC power. How fast it switches will depend on the circuitry, but things WILL heat up, burn out, and/or actually catch fire. – Doktor J Aug 1 '12 at 13:48
up vote 0 down vote

Since any real system is lossy, the output of a system will always be smaller as the input into the system. No physical (and thus, electrical) system exists with 100 percent efficiency. Your idea would work only if the efficiency of the UPS boxes would be at or above 100 %. So it is not possible.

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