First let me make sure it's clear that I am NOT trying to extend runtime by connecting two UPSs in series. That's been asked a million times on various forums, and is not what I'm trying to accomplish.
I've had 3 UPS units fail on me in the last 12-18 months, and I'm starting to wonder if it's the power flickers that are doing them in. My power rarely goes out for more than a minute or five, but before it does, it always violently flickers for a few seconds. Those flickers are hell on my unprotected equipment, and I'm wondering if that's what has caused my UPSs to die prematurely (the newest one barely lasted 5 months).
The old ones still function and still seem to do automatic voltage regulation, but none of them last more than 1-2 seconds once they switch to battery. I've tested the batteries, and they're fine; they were also all replaced about 9 months ago.
So, what I'm hoping is that the old ones can sit upstream of the new UPSs to take the brunt of any rapid brownout /surges and keep my new UPSs healthy. They're all pure sine wave and similarly rated.
Thoughts? Warnings/cautions?
This is kinda old information, but my understanding was that there were 3 issues with dasiy-chained UPSes.
The first is that you're potentially going to cause a ground loop, which is not healthy for the life of anything plugged into those UPSes.
The second is that there's a potential for a voltage droop going through from the first to second UPS, which means the UPSes will flap constantly and screw their batteries up, though I'd be shocked if that was necessarily still true for modern high-quality units.
And of course, the UPS itself won't be outputting a proper sinewave when it's on battery, which means your 2nd UPS in the chain will freak out (though again, maybe modern ones don't have that limitation).
Wasn't aware of that, but will look into it. Good to know
The other two, yeah, they're pretty much non-issues with modern pure sine wave UPSs. Mine both have AVR and will, within a certain range, just boost an undervoltage at the expense of a little extra current draw (or also the opposite for overvoltages). Since I'm limiting my loads to 1/2 of the rated value, even that extra current should be well within spec.
I am running a UPS after a solar battery system as it doesn't switch fast enough when the power cuts (despite having a "UPS" mode) and I can tell that the lack of a proper sinewave is still a problem. It seems to work, but the real UPS makes some very strange humming noises when running that way.