Axial electrolytical capacitor
Yeah, its a cap..it has foil all over the place.
This should be featured on EEVSmoke
By the way, @Karlo_Moharic I thought you were Simon for a while. Hehe
It certainly smoked- the whole house reeked for a day until I got the fumes dissipated. Has convinced me to install one of the big 8" turbo fans I have out a window, such that I can vent the house. Oh well, another project.
In all honesty, thanks everyone for the help. I've mailed APC; they requested photos, I've sent them, no followups since then. Really just wanted to know what happened, if it was a known issue, and whether or not to junk any/all other units I've got or do PM/replacement on them. It's hard to find good UPSs at decent prices (for a reason, I know).
The main unit was going to be rewired to work off a flooded marine battery with 200AH capacity. Power the VOIP, phone, modem, and router. I'm going to be rethinking that for a bit and include some sort of firesupression or chemical fire retardant in an easily disintegrate bag (if fire, dump fire suppressant).