UPS issue

[ribcage]

A couple of years ago I got an UPS rated 1000VA 600W, with two 12V 7Ah SLA batteries in series. It ran 24/7 up until recently with a load of about 70 W. When it was new it could hold my setup for an hour or so. Not so long ago a power outage occurred and the UPS hardly lasted a few seconds. The UPS has only ever seen a handful of outages in it's life, and before this one, there probably weren't any for many months. So initially I thought the batteries must be dead, although I've read they typically last 3-5 years, so it's a little strange. I thought perhaps the batteries being at a voltage of 27.6 Volts for a year and never doing any work might not be healthy for them? So I took the UPS out and did some testing on it, it would hold a small desk lamp for 10 minutes, but went critical after adding a second one, it seems like it only works with a small load. I think I measured voltage at battery terminals while the UPS was generating AC, one of them was 13V while the other one was 10V. Probably one battery is faulty? Both batteries seem to be manufactured in August 2019. Could a loose terminal connector cause something like this? For several days I cycled the UPS a few times, the last time it sustained a load of about 70 W for 20 minutes until it went critical. And voltages looked even on both batteries. Not as good as new, but seemed to have gotten better somehow. I placed it back where it belongs, and configured it to perform a short battery test periodically, because I heard some higher end UPS do that. One more thing. The UPS can communicate with my computer either via USB or serial (RS-232), the USB connection is problematic, while serial works very well. I used to use USB, and tried a few times to command it to do a battery test, which didn't really work. Then some time later I found the UPS beeping, which it only does while on battery, and mains power was present, so I thought something got wrong with the firmware because of those commands I issued through the glitchy USB interface. Maybe it was indeed on battery even with mains present, or is it even possible for the circuitry to get arranged in a way that the UPS is generating AC using the batteries, while the batteries are being charged? The UPS type is supposed to be "Line interactive". Anyway just wanted to hear what you think about this...

[eccerr0r]

Batteries are the weakest link in a UPS system...

Ideally your UPS tests the batteries every few weeks or so, so you don't get surprised, but likely one of the batteries died from the UPS trying to charge them. A lot of UPS overcharges batteries, killing them.

The real issue is that even lead acid batteries need BMS like Li-ion, except they don't go nuclear if abused.

Since you have a line interactive UPS, it's charging and using at the same time so theoretically you're getting inverter output on your computer side instead of only when the power is out. Not sure if you have a sine or modified sine inverter, hopefully the latter else you'll constantly be feeding your computer modified sine wave, which may or may not be dirty for your use.

My 1KVA offline UPS ate a set of batteries after just one year - it uses three 6V 12Ah packs which is annoying, would be nice if it was a multiple of 12. I decided to stop feeding it batteries...

[redwire]

I'd say you have a bad battery, one cell out of the 6+6 is weak or shorted.
If a cell is weak or shorts, you'd have a 12V battery plus a 10V battery in series.
Under load, voltage quickly drops 2V lower than normal and the UPS thinks the "24V" battery is dead.
Also this can cause a meltdown because the pack is now being overcharged. It's a 22V battery, not 24V battery pack so it all quickly dries out. The defective battery can take out the others. I've seen many server UPS 12V 7Ah arrays some bulged out or get melted plastic housings.

I find UPS gel-cells don't last if the float charging voltage is a little high, and the temperature is high. Some UPS make a lot of heat when doing nothing and the batteries get warm.

[ribcage]

I guess you're right, just gonna have to get better batteries.

eccerr0r, I kind of assumed I'm only getting inverter output when the power is out. There's a loud click when the UPS switches to battery and back, probably a mechanical relay. And when it does, the 50 Hz hum becomes much louder. It's a modified sine as far as I know. I thought the type of UPS that always produces AC is called Online UPS.

[lti]

There are a lot of cheap batteries that only last about two years.

[kc8adu]

Random noname chinese/vietnamese agms are good for no more than 2 years if you are lucky.
I pick good quality metal units and run with external truck batteries.
Hours of backup rather than minutes.