Sounds to me like a design flaw.

Why wouldn't this work? No switching transient and the power supply to the servers (or whatever) fails only if both battery and AC power fail.


simulate this circuit – Schematic created using CircuitLab

Over the weekend, some server in the server room died because of a UPS battery failure while the AC power remained fine.

I don't see the reason for why this failure is inherent to the problem.

Revision: It finally occurred to me to look it up in Wikipedia, and they say there are 3 different designs, and the seamless switching design I might have been looking for is the Online/double-conversion design. All other designs have a hard switch from the AC mains to the output of the inverter.

So it just occurred to me, similar to how the power-generator with the power company is put online to the grid by carefully aligning the phase of the generator to the phase of the AC on the grid and then throwing the switch, can't they make an energy-efficient UPS that has the UPS output connected directly to the AC mains in parallel to the inverter where the amplitude and phase/frequency of the inverter output is adjusted with a control system very similar to a phase-lock loop so that the power contribution to the output from the inverter is zero or very small during normal operation. It's an AC source that is phase-locked to the mains with amplitude adjusted from fiddling with the switching transistors on the primary of the inverter output transformer. The amplitude is increased so that the net AC current is never delivering power to the inverter's output transformer but there is only a trickle of current coming out, under normal operating conditions.

Then, when the AC power main goes out, the inverter is taking over the load and the battery is supplying a lot more current to the switching transistors on the primary of the transformer. There would have to be a disconnect of the output from the mains, because when the power comes back at any random moment, we don't know that the inverter phase is in phase with the mains. Then the controller of the inverter would slowly adjust the phase of the output of the inverter to be in phase with now-energized mains, the output voltage would be adjusted to be the same as the mains, then the disconnect switch would be reconnected and the system would go back to "normal operation".

Does that make any sense?

Tim, what do you think of that?

  • \$\begingroup\$ sounds like lack of maintenance. What model link pls \$\endgroup\$ Apr 9, 2019 at 22:38
  • \$\begingroup\$ some APC Back-UPS. i ain't the IT guy. but even if maintenance isn't done and the battery is old or crappy and dies (dies really bad, becomes a short circuit, not an open circuit), but the electronics of the UPS still working fine, and AC power from the wall outlet is fine, the power to the computer should continue without a glitch. \$\endgroup\$ Apr 9, 2019 at 22:43
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    \$\begingroup\$ I think this boils down to "how expensive do you want your UPS to be?". Ideally it'd work through a variety of anticipated battery faults. Practically, the more resistant you make it to battery faults, the more expensive it's going to be. If the person putting together the server room only looks at price, and doesn't take a deep dive into the features, then it doesn't behoove the manufacturer to spend the money on those features. \$\endgroup\$
    – TimWescott
    Apr 9, 2019 at 22:57
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    \$\begingroup\$ It just uses a DPDT relay to bypass the AC input on low voltage to the inverter output which is running with no load. And if there is no juice in the battery for a decent load of say 500W. It goes as undetected battery low voltage. You need a continuous UPS type with a smart battery capacity monitor and an AC power supply that has more power than the output which adds $$ to charge and drive the inverter, rather than the cheapo UPS that has just a trickle charger and a dead battery. So maintenance is key. to prevent future surprises. \$\endgroup\$ Apr 10, 2019 at 1:29
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    \$\begingroup\$ Some type of power bridge does this or a DPDT relay,. This is what we're trying to tell you. It's a cheaper solution. \$\endgroup\$ Apr 10, 2019 at 6:34

2 Answers 2


A UPS shouldn't fail in the way you have described. A UPS should have a bypass function.

Smaller and cheaper UPSs normally operate in bypass, unless they detect a grid failure. At that point, they turn on the inverter and turn off the bypass relay.

More expensive UPSs tend to have the inverter running all the time, with the bypass switched off. A built-in test function should check that the inverter is running correctly. It should shut the inverter down and switch to bypass mode if there is a problem.

The inverter in a UPS doesn't need to synchronize with the grid. The UPS is either running the inverter or in bypass, never both at the same time. Computer power supplies have enough storage in their capacitors to ride over a momentary power outage.

It seems that your UPS either doesn't have a bypass, or else it did not operate when it should have.


I agree with Tim, but still Maintenance on replacing the battery could have avoided the failure.

To ensure the battery is good, it must be tested somehow or replaced if near 4 to 6 yr life span.

One may need to remove the battery or test it in-situ while UPS has bypassed the AC to load and battery and inverter are not in use. enter image description here enter image description here REF

THat's a lower cost hot switch UPS which takes 6 to 8 hrs to recharge the battery.

  • 1 cycle of stored energy in most PSU's means they can tolerate up to 1 cycle dropout of AC line voltage in both PC's and servers alike.

This means it does not have the power to even drive the inverter, unlike all laptop chargers which can operate without the battery. But those are only 65W. In this cost-sensitive commodity adding power to drive the inverter is a lot more than just a 12V trickle charger.

Ideally, batteries are AGM also need to be pulse charged to reduce the possibility of sulfation, even if they say "sulphate-free".

Battery choices also affect lifespan greatly.

The more expensive UPS types are continuous use inverters with a power supply that can drive both the inverter AND charge and monitor the health of the battery

Read here to see the doubler conversion or multi-mode UPS which cost a lot more. These operate from the inverter in continuous mode. - So there is no fast switching of input AC to AC inverter output on AC input failure to the inverter output

  • \$\begingroup\$ when you're hot-swapping, you pull the battery out and that essentially replaces the battery with an open-circuit, right? the DC power supply is supplying all of the current at 12 VDC that the inverter needs. is this a thing where replacing the battery with an open circuit is okay, but replacing the battery with a short circuit is not okay? \$\endgroup\$ Apr 9, 2019 at 23:24
  • \$\begingroup\$ also, the IT guy said that, besides beeping, when the battery begins to fail, there is an email sent to him from the UPS if the battery starts to die. no such thing, the battery evidently died rapidly and the whole thing came down. no mortal warning or anything. features to inform us of a weak battery is another issue. i think that a UPS should work, as long as the 115 VAC from the wall outlet is good, even if the battery instantaneously changes into a short circuit. \$\endgroup\$ Apr 10, 2019 at 0:01

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