Q. I just spent $20, again, on batteries for my smoke alarm. The Duracell batteries I bought are good for five and 10 years. Is the old idea of replacing batteries every six months still relevant with the new-technology smoke alarms?—Ken Vanhoesen, South Walpole, MA

A. You have installed those smoke alarms in order to stay safe, and the battery-change schedule is conservative because of an abundance of caution. It’s true that a lithium 9-volt battery will probably last longer than an alkaline 9V (which in turn should last longer than a carbon-zinc 9V). But how much longer depends on a smoke alarm’s power drain. If you think that your alarm’s old battery has some “life” left, use it in a product that doesn’t have to do with safety. Or consider a switch to a newer model of smoke alarm with a lithium-ion power source that lasts 10 years.

For more on smoke alarms and carbon monoxide alarms check our buying guide, plus you can read more tips on battery use and storage.

Send your questions to ConsumerReports.org/askourexperts.

This article also appeared in the May 2015 issue of Consumer Reports magazine.

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