A week ago I got the following error on my APC Smart-UPS 1000 which I muted.

Warning State:
Connect battery
Load: 55%
Batt: 100%

Today, I could smell a sort of sulfur/sulphur/rotten egg smell when I came into the office and the UPS is alarming again. There isn't a burning smell.

I have vented the office & server room and shutdown the UPS.

Got any other advice?

UPDATE: This is what I found in the UPS. enter image description here

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    Note to self: Always configure the remote management if your UPS supports it. It's not nice being the one to go in and press the power button. Apr 1, 2016 at 14:13
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    @BigHomie: This is a special case. I wouldn't vote to close as duplicate because this is a potentially dangerous condition that should be treated on its own. It's also not really a burning, fire, or overheating scenario more than it is a battery chemical failure. Lumping the two questions together could confuse readers.
    – bwDraco
    Apr 1, 2016 at 19:31
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    UPS batteries have a limited life - anything from 2 years in a budget UPS up to 5-7 years in a good quality UPS like your APC. I bet the batteries are End Of Life and they were last replaced 7 years ago. Buy four new batteries of the same spec and swap them out. You should inspect for any corrosion, but the UPS is probably okay. And schedule another battery replacement in 2022. Write on the case the date of last battery change.
    – Criggie
    Apr 1, 2016 at 23:29
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    Maybe the error should be replaced with "Battery on fire"
    – wizzwizz4
    Apr 2, 2016 at 11:58

2 Answers 2


To answer the question: This is almost always a lead-acid battery failure causing the battery to vent hydrogen sulfide (H2S). The battery needs to be replaced as soon as possible.

As an additional note, H2S can be extremely dangerous at higher concentrations. If you experience eye irritation or difficulty breathing or your ability to smell the odor deteriorates noticeably, the concentration of the gas is dangerously high and you should see a doctor. At that point, you may need to hire a hazmat cleanup service to remove the battery and clean up the area.

Wikipedia says this on H2S toxicity:

  • 0.00047 ppm or 0.47 ppb is the odor threshold, the point at which 50% of a human panel can detect the presence of an odor without being able to identify it.
  • 10 ppm is the OSHA permissible exposure limit (PEL) (8 hour time-weighted average).
  • 10–20 ppm is the borderline concentration for eye irritation.
  • 20 ppm is the acceptable ceiling concentration established by OSHA.
  • 50 ppm is the acceptable maximum peak above the ceiling concentration for an 8-hour shift, with a maximum duration of 10 minutes.
  • 50–100 ppm leads to eye damage.
  • At 100–150 ppm the olfactory nerve is paralyzed after a few inhalations, and the sense of smell disappears, often together with awareness of danger.
  • 320–530 ppm leads to pulmonary edema with the possibility of death.
  • 530–1000 ppm causes strong stimulation of the central nervous system and rapid breathing, leading to loss of breathing.
  • 800 ppm is the lethal concentration for 50% of humans for 5 minutes exposure (LC50).
  • Concentrations over 1000 ppm cause immediate collapse with loss of breathing, even after inhalation of a single breath.
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    Thanks @bqDraco. Follow on question: I read that "Over-charging a lead acid battery can produce hydrogen sulfide". All 4 batteries were deformed by heat/gas when we opened the case. Could one faulty battery cause all 4 to bloat? Apr 1, 2016 at 21:06
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    Either a cascading failure occurred when one battery failed, overloading the remaining batteries and causing them to fail in turn, or the UPS itself malfunctioned and drove excessive voltage into all the batteries. Another possibility is one battery overheating causing the other batteries to be exposed to high temperatures and hence failing in a chain reaction.
    – bwDraco
    Apr 1, 2016 at 21:08
  • This should be redundant, but.. store the battery outside until you can dispose of it! Also replace the entire UPS, chances are it's going to do this again to the other batteries or any replacements.
    – Joris
    Oct 27, 2020 at 15:35

It sounds like one of your lead-acid batteries has gone bad and is now leaking hydrogen sulfide (H2S) into the air.

You will obviously want to replace the battery, but I would think some cleaning may also be in order as both hydrogen sulfide and the sulfuric acid normally present in the battery are corrosive. (Do consider that hydrogen sulfide is also toxic.)

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    Perhaps a cleaning of any affected surfaces with a solution of baking soda and water to neutralize the acid? FWIW, for most people, the "rotten egg" smell of hydrogen sulfide will be repulsively strong at concentrations well below anything harmful. If it's just noticeable in the air, it probably won't hurt you.
    – Anthony X
    Apr 2, 2016 at 19:58
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    Actually, one of the dangers of hydrogen sulfide gas is that the unpleasant smell only applies at lower concentrations. At higher concentrations, the smell may actually seem sweet, and your sense of smell can be deadened very quickly, making you unaware of how much exposure you're having. link
    – barbecue
    Apr 3, 2016 at 3:10
  • @barbecue The symptom list from bwDraco's answer (and the osha pdf link mainly agrees) shows eye irritation and damage (may occur?) at lower concentrations than the sense of smell disappearing, but could take hours or days to show up for "lower concentrations. Do you know if your eyes would be burning and irritated immediately, even after it's "numbed" your sense of smell?
    – Xen2050
    Apr 4, 2016 at 20:09
  • @Xen2050 In many cases, eye and throat irritation occur very quickly, but that may not be sufficient warning to avoid injury. H2S is so toxic that even a couple of breaths can produce very serious injury, or even death.
    – barbecue
    Apr 5, 2016 at 0:28
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    Jesus, guys. You make me feel like I need to exit the office for a breath of fresh air.
    – Neil
    Apr 5, 2016 at 13:29

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