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An APC UPS teardown

I don’t know why…maybe I’m just a “lucky” guy…but it always seems that devices capable of audibly alerting me when something’s wrong with them always seem to “sound off” in the middle of the night, while I’m sound asleep. It happened last year, when a carbon monoxide detector downstairs, plugged into an AC wall outlet, decided that it’d gotten too old and was due for retirement and replacement. I was so groggy, as you’ll see from reading my tale of woe, that I at first thought the sound source was the smoke detector wired up at the ceiling. I didn’t realize my error until after I unhooked it and still heard the beeping going off.

More recently, the din happened again, this time in my office next door to the master bedroom. And this time it was a constant and piercing tone, coming from one of my uninterruptable power supplies (UPSs) there, which if you reference the user manual, means one of two things:

  • A dead (or disconnected or otherwise not present, for what it’s worth) battery, when the alert sound is accompanied by topside flashing lights, or
  • Too much current being pulled through one (or multiple) of the device’s battery-backed AC outputs, when the sound’s not accompanied by topside flashing lights

Again, it was the middle of the night, and I (deludedly, as it turned out) aspired to get back to sleep. So, I manually shutdown any devices connected to the UPS that wouldn’t respond well to an abrupt power outage, then powered the UPS itself off. The next day, I turned the UPS back on and…it was fine for a couple of days, when it again woke me up in the middle of the night. At that point, I decided it was retirement-and-teardown time for this unit, albeit preceded by a bit more experimentation (only during waking hours, mind you…the ongoing noise was getting my wife really annoyed). The UPS would sit there stably humming away…as long as nothing was connected to any of the battery-backed outlets. But all it took was something as current-draw-diminutive as an LCD in backlight-off sleep mode to sooner or later raise the UPS’ audible ire.

Our patient, to be precise, is an APC (nowadays a brand of Schneider Electric) Back-UPS ES 550VA unit, the BE550G:

I was surprised to find, while researching this writeup, that APC still sells it for $99.99 direct from the company website. My email archives indicate that I bought three of these around a decade ago, all from Amazon, the first for $49.99 back in April 2012, the second for $55.99 the following (2013) February, and the third for $49.49 in October 2014. What I like about these units (and conceptually similar ones from both APC and competitors) is that the batteries are user-replaceable, and they’re standard-size and -spec. A Ni-Cd cell will sooner-or-later go kaput, whether it’s due to repeated premises power loss dependency on it or just extended trickle-charging-induced storage-capability decay, but you can then just pop in a replacement battery, and it’ll be good as new. That said, eventually the internal circuitry itself may fail, as seems to be the case with my device, with the UPS then destined only for dissection-then-discard.

Those batteries, by the way, are an interesting razors and blades story all their own. APC conveniently links, direct from the BE550G product page, to a company-branded replacement battery, the APCRBC110, which sells for $69.99. Yes, you’ve done your arithmetic right…that’s only $30 less than what the company sells the entire UPS (including a battery) for new. That said, the cell used in this model is a standard 12VDC, 7Ah model also used for backup-power purposes in home security systems, to power electric scooters and wheelchairs, and the like. Whenever I’ve needed one, I’ve easily found third-party alternatives on Amazon and eBay for around 1/3 that price; I even recently acquired six 12VDC, 9Ah equivalents in the exact same form factor for $48.99 on sale. You just need to make sure the output voltage matches, that the charge-storage spec is equal to or higher than that of what you’re replacing, and that the dimensions are the same, along with the positive and negative terminal locations (and speaking of terminals, I’ve seen both F1 and F2 options; adapters are available to as-needed translate).

A couple more notes before we dive in. As you may have already noticed from the “stock” photo I showed earlier, the BE550G combines four surge-protected outlets along the top edge with four more outlets along the bottom edge, those latter ones both surge-protected and battery-backed. Perhaps obviously, the ability to convert stored DC back into AC when premises power goes down is a key characteristic of a UPS. This particular model generates a so-called simulated (also sometimes called modified) sine wave AC output; more expensive models generate “pure” sine waves but cost roughly twice as much. This info page from Tripp Lite, a brand of Eaton, gives a good comparison summary between the two AC output options.

Note, too, that the “550” in the product name refers to its DC-to-AC “VA” output spec, not its output wattage capability (which, per the product spec sheet, is only 330W). If you’re in the market for a UPS, make sure you understand the difference between the two and select wisely, to avoid (as this Eaton info page describes) under-sizing the UPS for the required connected- equipment’s power draw needs. More generally, I’ve found this Eaton info page to be a good FAQ overview of various UPS topics (I have no particular affinity or other connection to Eaton; I don’t think I even own any of their stuff. I just like their documentation!).

With the overview commentary out of the way, let’s take a look at our patient, shown here as-usual alongside a 0.75″ (19.1 mm) diameter U.S. penny for size comparison purposes (the BE550G has dimensions of 12 x 3.3 x 7 inches and weighs 13 lbs. with a battery installed inside):

Along one end is a USB port for optional UPS monitoring and control via a connected computer, “in” and “out” RJ-45 plugs to surge-protect an Ethernet connection, a circuit breaker “reset” switch, and a status LED:

Here’s a “stock” view:

The other end and both sides aren’t as exciting (those aren’t ventilation grilles, only slots embedded in the plastic, which I’m guessing exist to finger-grip-assist in unplugging things?):

Now for the backside, and our pathway inside (note the four deep holes, with screws inside):

Closeup of the label:

Next, let’s crack open that battery compartment:

A closeup of the two battery terminals:

Now let’s get those screws out and get inside, which ended up being straightforward:

Here are the four surge-only outlets, with a plastic shield between them and the insides:

Underneath the PCB (at left are the red- and black-color cables and terminals we saw earlier in the battery compartment):

is another plastic shield, this time with the four surge-plus-battery outlets below it:

The transformer, which dominates the landscape, lifts out easily (reflective of the more general modular and easily undone unit assembly technique):

Let’s look more closely at the top-edge outlets, whose shield is held in place by a single screw:

I was surprised to see, considering that these are supposedly surge-protected outlets, that they seem to be directly fed from the AC cable (versus, for example, having in-line circuitry between them and the outside world). That said, I’m admittedly a “digital” guy with no particular power electronics expertise so I’ll rely on more knowledgeable readers for education in the comments:

Now for those other (bottom-edge) outlets, both battery-backed and surge-protected. Like the transformer before it, the PCB lifts out of the chassis with absolutely no fuss:

To fully remove the PCB and transformer requires first separating the cabling running both between them and the two outlet sets, a simple exercise given the easily detached connectors:

Here are some closeups of the now-standalone transformer (the red wire originated in the battery compartment):

And the PCB (the black wire, this time, originated in the battery compartment):

Reiterating what I already admitted, I’m not a “power” guy, so I’ll leave it to you, the readers, to sound off with things that look interesting in the comments, including noting swollen capacitors or other flaws that I may have overlooked and might explain the UPS’ emergent misbehavior. I’ll hold onto the disassembled device for a while, so I can respond to incoming queries:

In closing, here are a few more views of the inside of the chassis topside, with the PCB and transformer now removed:

And with that, it’s over to you for your thoughts in the comments!

Brian Dipert is the Editor-in-Chief of the Edge AI and Vision Alliance, and a Senior Analyst at BDTI and Editor-in-Chief of InsideDSP, the company’s online newsletter.

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