Continuing the discussion from Surge Protectors:
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.