One of our smoke detectors was regularly beeping after we got home from dinner, so I suspected the battery might be low. Amanda was amused that I stuck it on my tongue to test it; it was a standard nine volt. “Surely there are higher tech ways to check” said she. “Not easier, though” said I.
As it turned out, the battery was fine. I dunno what the detector was complaining about, but it eventually stopped beeping.