Hey Mr. Green,

Q: Which are the least harmful to the environment: long-life, ordinary batteries or rechargeables? I usually buy rechargeables, except for smoke alarms, which tend to go off in the middle of the night.

—Karen in Cottonwood, California

A: The comparison between rechargeable and long-life batteries is kind of dicey because some of the materials used in the two types are so different. However, you are on the right track in favoring old-fashioned, single-use batteries for your smoke detectors, because they usually last a long time before they signal their waning power with an annoying death rattle in the form of a tweet every minute or so.

The key to deciding which type of battery to use is the number of times you’ll have to recharge it. The more frequently they need recharging, the lower the rechargeable batteries’ overall environmental impact, according to a recent comparison of single-use alkaline batteries to their rechargeable nickel-metal-hydride cousins published in The International Journal of Life Cycle Assessment.

Researchers encourage the use of rechargeable batteries mainly for high-consumption devices such as cameras, flashlights, and electronic toys, which require frequent recharging. Batteries charged less than 20 times do not diminish pollution problems such as ocean acidification, human toxicity (cancer effects), and particulate matter more so than their single-use cousins, and may even contribute more to ozone depletion unless they’re recharged around 150 times.

Single-use batteries obviously do create quite a heap of waste metal and other stuff that’s packed into their guts: Around 5 billion are sold in the United States every year, but less than 10 percent get recycled. To find out where to recycle them in your area, visit Call2Recycle. You can also find places to recycle single-use batteries at earth911.com. Otherwise, you might risk being kept awake by ghost tweets from that vast chorus of dying batteries. 

This article has been modified since its original publication.