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...