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When pictures of flaming laptops blaze across the Internet, and Dell and Apple recall nearly 6 million lithium ion batteries, it's natural to wonder whether we can trust our portable devices. Billions of these batteries power everything from cell phones to cameras to Segways. And the push is invariably to pack more power into less space, increasing the severity of the damage in a worst-case scenario.

Incidents Are Rare

All 4.1 million Dell lithium ion batteries and 1.8 million Apple batteries in the recall contained cells made by Sony Energy Devices of Japan. A manufacturing defect (see the diagram) led to rare instances where batteries overheated and even caught fire (Dell reported six confirmed cases, and Apple nine).

If you're looking askance at your portable device, relax. Experts say the batteries, which can generate two to three times as much continuous power as other rechargeable battery types, are not unsafe. The battery packs incorporate multiple layers of internal safety monitoring that rely on CPUs and sensors along with control technologies such as vents and chemical fuses. Normally, a problem battery will power down automatically when a short circuit occurs, before any serious overheating or fire can develop.

Of course, if these precautions worked perfectly, Dell wouldn't have undertaken what the Consumer Product and Safety Commission says was the largest consumer electronic device recall in history (Apple's was the second largest). Sony says other batteries, including some in Sony VAIO notebooks, use the defective cells too; but safety measures in those batteries eliminate the risk, it says, and no further recalls are planned. Though any battery carries some inherent dangers, Sony has changed its manufacturing process to fix the defect.

The Odds Against

Will your laptop explode? The odds against are extremely high. Even if 50 times more incidents (or 300 total) happened than were reported to Dell or the CPSC, that would translate into only about 1 in 1000 chances that any Dell laptop battery shipped during the recall period (not just those being recalled) would overheat. Dell calculates that it shipped a total of 24.9 million batteries during the recall period (April 2004 to July 2006). Apple's problem batteries shipped between October 2003 and August 2006.

Given the sheer numbers of lithium ion batteries being cranked out every day, future recalls seem inevitable, despite manufacturers' work on safer designs and technologies. But if you follow basic safety precautions (see "Portable Power Safeguards"), you can probably leave the fire extinguisher at home.

Stuart J. Johnston