Warning: Repairing a UPS is not for beginners. There are many high voltage components that can seriously electrocute you.

3 years ago I bought a used APC UPS BR1000MS (1000VA, pure sine wave). I have it connected to all my media center devices (PC, TV, Receiver) but it rarely gets above 50% its rated power delivery. About a month ago, while I was watching a video, I got a power outage and immediately I heard a loud pop from the UPS. Everything connected to the UPS lost power, and the error code F02 appeared on the UPS LCD.

Unfortunately F02 is somewhat common and it's a death sentence for an APC UPS. APC doesn't say what problem it indicates, but it's clear from looking at posts that it indicates a hardware problem. Some threads mention burning smells, smoke, and in one case fire. Even after a hardware revision, there are posts of people still getting the error.

Just for good measure I tried all the reset troubleshooting steps and even tried using a known-good battery but to no avail. The UPS is long out of warranty so there won't be any help from APC. A local repair shop quoted me $100 just to diagnose the thing and a minimum of $50 if they deemed it repairable. I could get a whole UPS for that price so I decided I would diagnose it myself.

I started by taking the plastic shell apart and doing a basic inspection of the circuit board. The problem immediately seemed clear, one of the mosfets had exploded!


Mosfet model: IRFB7440 IOR P806J JK GO
More pictures: https://imgur.com/a/LrQykdg

While I waited for a replacement mosfet from Digikey, I noted the highest voltage the board used was around 400v. I got some Class 00 gloves which are rated to 500v AC and 750v DC. They're very bulky and difficult to work in, but I couldn't avoid touching the circuit board in order to replace the mosfet. After the parts arrived I spent 4 hours performing the replacement and putting everything back together. I plugged in the UPS, connected the battery, turned it on, and it worked! For only 5 seconds. I heard another pop and the dreaded F02 error reappeared. https://youtu.be/aKos3EM3o2Y

I expected another broken mosfet but surprisingly they were all intact. Since the pop happened just as the UPS switched to battery mode, I suspect there's a bad relay/contactor. This could explain why some users smelled smoke. If a battery relay got welded closed or shorted, I could easily see the plastic melting. There's two 30A internal fuses but the relays are only rated for 16A so I don't know what purpose the fuses serve!

I guess the next step is to figure out which relay is broken. It's such a hassle to safely handle the circuit board I don't know when I'll get around to it.

After much pondering, I've decided trying to fix the UPS just isn't worth it. In a best case scenario where I actually find the broken component and replace it, chances are it'll break again in the future since APC clearly doesn't use quality components. Although I can find beefier relays, they won't fit in the existing circuit board which means I'd have to wire extensions and that seems messy. The bottom line is, if I can't trust a UPS to keep my electronics safe, then it's not worth having at all.

I think the smarter move is to reuse the battery from the broken UPS to extend the battery capacity of my good UPS. A 24v battery balancer is only $40 and may not even be necessary.

I think I'll save up for an Eaton UPS. Supposedly they're still quality and their 5S lineup has some options pretty similar to my APC. I really didn't want to give up on APC but when the UPS is connected to $2,000+ of equipment, I can't afford for it to fail.

Since I'm scrapping the UPS I figured I could at least document how to safely take one apart since there's not much info out there.
https://www.ifixit.com/Teardown/APC+Back-UPS+Pro+1000+S+Teardown/194060

I'm curious who else uses a UPS here and what model? Surely Cemetech isn't relying on TI-84 Plus CE batteries to power the server.

I've been using an Eaton 3S for a while, though I'm on my second one which seemed noteworthy when you mentioned their products in your original post. It lasted something like 7 years then seemed to simply turn off one day and displayed a red light when I tried to turn it on (plus the non-battery outlets were nonfunctional). I was annoyed that their support just told me to buy a new one without even coming out and saying it had failed, but at the price point I'm not really surprised that they won't provide any real support. Compared to your experience with your APC one I guess it did fail gracefully though!