A scale model of a Reaper drone rumbled down the runway and lifted into the gray Canadian sky, powered by a plastic propeller and a lithium-ion battery. When the tiny plane crashed back to earth a few seconds later, white smoke began rising from the wreckage.

“Why is it on fire?” one of the hobbyists asked the other, moments before bright orange flames began shooting from the crash site.

The weary reply, captured on video, was: “Battery.”

Small, potent lithium-ion power packs have transformed the world of radio-controlled model aircraft, much as they have allowed smartphones to get thinner, power tools to work longer and electric cars to go farther. But a pair of serious incidents this month involving rechargeable batteries in Boeing 787 Dreamliners have highlighted what model-airplane hobbyists long have known — lithium-ion technology comes with inherent dangers.

Considering the sheer numbers of lithium-ion batteries — more than 4 billion rechargeable cells were made last year, according to industry figures — fires are not common. After a battery-powered Chevy Volt ignited after a test crash in 2011, federal investigators said electric cars were no more vulnerable than gas-powered vehicles, more than 20 of which catch fire each hour in the United States.

Yet some risk persists, and the results can be startling. When a cellphone battery overheats — a rare event — it can eject itself with a loud “pop,” leaving singe marks behind. Lithium-ion battery packs can have prolonged fires as each cell, typically the size of a man’s finger, gradually ignites.

“It could be a smoke bomb. It could be a flamethrower,” said Gerard Back, a senior engineer with Hughes Associates, a fire-protection company in Baltimore that investigated the Volt incident. “I’ve seen them look like every type of firework you can imagine.”

Boeing and other manufacturers are keenly aware of this peril and have built in safety features designed to keep voltages within safe limits, prevent short circuits and confine damage when problems do occur. Lithium-ion batteries in laptop computers and other consumer devices are supposed to shut down rather than allow overheating that might cause a fire. In such ways danger is minimized, though not eliminated.

The technological trade-offs play out in especially stark relief within the sprawling world of radio-controlled airplane enthusiasts, who regard lithium-ion batteries with a mixture of fear and devotion even as they subject them to unusual stresses. The result is widespread acceptance that the same portable power source that allows for fast, long, graceful flights also occasionally causes these fragile toys to burst into flames — often, but not always, after crashes.

“They make life a lot better, a lot easier, but you have to manage the risk,” said Dave Brewster, 43, a professional pilot from Toronto who flies radio-controlled model airplanes as a hobby. “Everything in flight is mitigating risk.”

It was Brewster’s Reaper drone that crashed and caught fire on its maiden flight in August 2011, destroying a model that cost him several hundred dollars and hours of work. Like many who have suffered such mishaps, he uploaded video of it to YouTube for what amounts to an emerging genre of lithium-battery-related disaster films starring radio-controlled models — not just planes but cars and boats, too. (The flaming wreckage of a twin-engine model plane, posted in October 2009 and viewed more than 479,000 times, is accompanied by a soundtrack of bagpipes playing “Amazing Grace.”)

There is a strain of gallows humor in such videos, with audible cheering and laughter at the fiery destruction. Yet the underlying danger is treated seriously. Tales abound of burned hands and severe property damage, including battery-related fires that destroyed cars, garages and homes. Hobbyist Web sites carry stern warnings and feature long chat threads discussing the risks. At radio-controlled airplane shows, charging batteries indoors typically is prohibited unless they are kept in flame-resistant bags.

How lithium-ion batteries work

“I’ve had buddies blow the ends of their fingers off with them,” said hobbyist Craig Pitcock, 41, who posted to YouTube a video of the fiery crash of his F-4 Phantom model in an Arizona field. “It’s incredible, the voltage.”

Before the arrival of lithium-ion technology, hobbyists relied on a messy liquid fuel, or occasionally on an older generation of batteries that could barely get models off the ground. “Twenty years ago, somebody would laugh at you if you said ‘electric plane,’ ” Pitcock said.

Lithium-ion battery packs may revolutionize aviation as well, allowing for planes that are lighter and use less fuel.

The cause of the incidents with the Dreamliner battery packs remains under investigation by safety officials. The jets were ordered grounded in the United States and several other nations this week.

The batteries carried in the holds of cargo planes are suspected as possible factors in two fatal crashes in recent years, according to the Air Line Pilots Association International, which is lobbying to have cells classified as dangerous goods requiring special handling, adequate fire suppression and notification to pilots who carry them. Cargo planes sometimes carry tens of thousands of cells stacked together on pallets, increasing the danger, said Mark Rogers, director of the dangerous goods program for the pilots association.

“There’s no way to engineer your way out of the problem,” he said. “No matter what you do, you’re going to have incidents.”

Lithium-ion battery packs typically are made of a collection of individual cells. Coiled within each are three layers — one positively charged, one negatively charged and the third a “separator” film that keeps the charged layers apart. There also is an organic fluid that, at high temperatures, is flammable.

Fires start when electricity moves in an uncontrolled way between the positively and negatively charged layers. The cause can be manufacturing defects or damage — anything that allows the positive and negative layers to touch, causing a short-circuit and what experts call “thermal runaway.”

Manufacturers of lithium-ion batteries say their technology is safe when international standards are followed. Problems occur mainly with lower-quality, counterfeit batteries, or when cells are shipped improperly or mishandled by consumers, said George Kerchner, executive director of the Rechargeable Battery Association.

“When you damage these cells and you abuse them, you certainly are inviting problems,” he said.

For those who fly radio-controlled model airplanes, fires typically happen when batteries are overcharged, or when wiring mistakes cause short-circuits. And, then, of course, there are the crashes: The resulting damage can causes wires to touch, separator film to tear, or the positive and negative layers to come into direct contact with each other.

Damage can be subtle enough that an apparently undamaged battery can burst into flames long after an accident. Hobbyists say they sense danger when their battery packs get “puffy,” typically the result of gases that can be the first sign of thermal runaway.

The Rechargeable Battery Association’s Kerchner said he and his sons have long flown radio-controlled helicopters powered by lithium-ion batteries without encountering problems. “We love them,” he said, “but we don’t crash them into the ground to see if they explode.”

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