Apparently most APC UPS's, not just "smart" ones, support battery hot swapping.
The manual for the Back-UPS ES 525 advises:
Do not replace the battery when the UPS is On Battery (amber indicator is lit).
-- but that dumb warning is obviously applicable to "smart" UPS's as well.
The manual proceeds telling:
NOTE: Small sparks may occur during battery connection. This is normal.
-- can't remember if sparks fly during an offline UPS battery connection.
