You'd think no one would buy hoverboards anymore, given the stories of exploding batteries that have burned peoples' entire houses down. Apparently no one told the owner of this hoverboard, who brought it into a mall just last month. Once it started smoking, things went sideways for almost two minutes, despite people's efforts to kill the thing with fire extinguishers:
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So why does the darn thing keep exploding, even after being extinguished? Well, this isn't like a burning piece of paper, where once you smother the flame it's gone because paper is inert. The problem is that this is a lithium-ion battery, which by nature store a tremendous amount of energy. When that battery is shoddily manufactured using poor materials and quality control, as Wired explained way back in 2015, things can easily go wrong:
In a cheaper battery, [Jay Whitacre, Professor of Materials Science & Engineering at Carnegie Mellon University] says the separator between each battery's anode and cathode—which are what the current flows through—may not be aligned correctly. Image it like this: The cathode is at one end of the battery, the anode at the other, and the separator is (surprise!) between them; its job is to keep them apart so nothing short circuits. An issue, in the cheaper batteries, is there could be small holes in the separator thanks to impurities in metal particles that can puncture the anode/cathode separator. In either of those cases, the damage can cause a short circuit.
"If there is an inherent defect in the cell, it will go off at some point," Whitacre explains. "Small defects in the manufacturing or materials stream lead to the plus/minus sides of the batteries being shorted with each other after a small amount of use. When this happens, especially when the batteries are charged, a lot of heat is generated inside the cells and this leads to electrolyte boiling, the rupture of the cell casing, and then a significant fire."
That fire can build upon itself and be hard to contain. Whitacre says all lithium-ion batteries contain highly flammable electrolytes that burn "fast and hard" when air hits them. When things get hot, common cathode materials turn into additional oxygen sources, too. "This stokes the fire even more," Whitacre says.
It's a sin that these things are still being sold. And I'm thankful that you're not allowed to bring one on an airplane.