If that is what I think it is, the smell is outgassing from the lead-acid cells. It means one or more of the cells have failed, and acid is venting.
It happened to me once as well. After powering it off and disconnecting the battery pack from the unit via the cut-off connector on the back, the smell went away. I could probably have just replaced the cells, but I decided to replace the whole unit, since my requirements changed over the years.
I’m still using Smart-UPS devices, because I want the pure sine-wve output and the Back-UPS series only provide simulated-sine-wave on battery power, which isn’t good for the longevity of power supplies.
Today, I’m using an SMT-1500C which gives me over 90 minutes of run-time using my normal load (about 110W - a Mac mini, LCD display, a few external hard drives and network equipment).
They don’t last forever, but I’ve always gotten over 5 years from mine, which is about as good as you can expect.
I don’t buy direct from APC, however - they charge too much. I always get better prices from Amazon. And I can recycle the old unit at Staples - who will take most consumer e-waste at their stores.
Oh, that’s larger the unit I was using and much larger than what I would purchase if I needed another. Also expensive. I recall the Smart-UPS I used costing around $450 at the time.
Yes, it’s bigger and more expensive than many people want.
I’ve always wanted a UPS big enough to let me continue working through outages. One that only gives me enough time to quit my apps and shutdown has always seemed inadequate to me.
But for me, this is less expensive than what I previously used. My previous UPS was another SMT-1500, but the one before that was an SUA1000XL with an external battery pack. This was back when I was using a PowerMac mini-tower and a Linux PC mini-tower, and a CRT display - consuming about 600W when it’s all powered-on. That UPS (according to its runtime chart), could power that 600W load for over 90 minutes.
Fortunately, I only needed to buy one every 5-6 years, so I could budget replacements well in advance.
This is a very useful piece of advice! I’d completely forgotten about this, and I’ve got a number of things out in my garage waiting until the next time I can take stuff to the city’s recycling location. I’ll have to go look to see what restrictions they have because it may be more convenient to take things to Staples. Thank you!
A nice thing about Staples is that if you’ve signed up for their rewards program, you get a couple of dollars worth of rewards points when you bring most electronics in for recycling. I’d guess that some of the other large retailers have similar incentives.