Nine-Volt Battery Life: A Failure of Quality Control

Aims: To determine the distribution of battery life for batteries that often are used to power smoke alarms.
Place and Duration of Study: Pacific Institute for Research and Evaluation, Calverton Maryland, USA, between June 2007 and September 2009.
Methodology: We recorded the life of 167 9-volt zinc-carbon batteries and early failures among 60 alkaline batteries that we installed in ionization smoke alarms that, like all US smoke alarms, came equipped with battery test buttons and battery charge monitors. We tested two brands of batteries of each type. We also used a Radio Shack digital multimeter to test 100 unused zinc-carbon-batteries 7 months prior to their expiration date.
Results: Among zinc-carbon batteries, 25% of one brand and 12% of a second failed in less than 100 days. These batteries had a wide, disturbingly flat lifetime distribution, with 40% of the brand with the most early failures but only 1% of the second brand lasting more than 500 days. In a 9-volt battery eight-pack, one or two batteries are likely to be problem batteries that would not last for three months in a low-draw device like a smoke alarm. In this relatively undemanding application, the majority lasted less than the nominal one-year life reported by battery manufacturers. Among alkaline batteries, 2 of 60 failed within three months and 2 more failed between 21 and 24 months. Among unused batteries, 7% were dead 7 months before their expiration date.
Conclusion: Nine-volt batteries, especially zinc-carbon batteries, need better quality control. Consumer protection is lacking around this problem. Unless using longer-life batteries, changing smoke alarm batteries at every clock change makes some sense. Most will not last a year. Nevertheless, that advice may not be optimal for people pressed for cash. Depending on brand, a third of the batteries will last for 18-24 months. And alkaline batteries that are not defective should last more than two years.