Bill Houghton, Sebastopol, CA
I was following up on a false alarm from one of our smoke alarms today, and the second customer service rep I talked to (the first one having given me bad information) told me that there's a scienterrific reason that smoke alarms containing batteries are more prone to some types of false alarms in the middle of the night than during the day. Houses are generally cooler overnight, and the battery output goes down as it cools. If it's borderline, that can be enough for the smoke alarm to decide it needs replacing, leading to the "replace the battery" signal. Then, during the day, as the house and thus the battery warm up, the battery gets above the threshold, and the smoke alarm decides it's fine.
And all this time I thought it was Satan, or at least a lesser demon.
He told me that he'd asked this very question during training, which is where he learned the answer above. My kind of customer service person.
He also mentioned (public service announcement here) that Kidde has concluded that lithium batteries in replaceable-battery smoke alarms are a bad idea, because a failing lithium battery sometimes emits a chemical that can damage the electronics. Too bad: three of our smoke alarms require getting out the six-foot stepladder, and one requires the eight-footer, so I'd put lithium batteries in all of them and was feeling good about not having to replace batteries every year.