Why Hoverboards Keep Exploding

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Those self-balancing scooters that everyone’s calling “hoverboards” don’t actually hover. But that’s not the strangest thing about them—that would be the fact that this year’s most popular holiday gift keeps catching on fire.An exploding two-wheeler burned down a house in Louisiana a few weeks ago; another scooter combusted in the same state in the past week. A gyroboard caused significant damage to a home in New York a few days ago. At a mall in Washington this week, a scooterboard caught fire and shoppers were forced to evacuate. The perceived danger is significant enough that major airlines have banned the little vehicles altogether.

But what is actually causing all these fires? In the New York and Louisiana incidents, the board was plugged in and recharging. In the mall incident, the board wasn’t plugged in at all; there have also been reports of scooters bursting into flames while people were riding them. Plugged in or not, the big problem has to do with the quality of the rechargeable lithium-ion batteries inside these things. They’re almost always tucked in one of the foot rests, and they work the same way as the lithium-ion batteries in our smartphones, tablets, and laptops. They’re just a lot more prone to defects.

Jay Whitacre, Professor of Materials Science & Engineering at Carnegie Mellon University, says that the problem doesn’t have to do with these self-balancing scooters themselves, but with the quality of the batteries being used. They’re cheap, and it makes sense: This is a hot (pun not intended) holiday product, the reputable models are pretty expensive, and more-affordable brands are using cheaper components to lure in shoppers that don’t want to spend a grand or more on a hands-free Segway. Predictably, a slew of cut-rate brands are flooding the market with shoddy scooters made from cheapo components.

“There are a lot of factories in China that now make Li-ion batteries, and the reality is that the quality and consistency of these batteries is typically not as good as what is found in top tier producers such as LG or Samsung,” Whitacre says. “These are known as ‘low cost li-ion batteries’ by most in the industry—they are not knockoffs or copies, but are instead just mass-manufactured cells.”

With these cheap batteries, a lot of things can cause fires. For one, the nature of a scooterboard—it can bang into stuff, smash into things at high speeds, and be abused by bros—makes the batteries susceptible to damage. It’s not just the nature of a cheap battery, it’s the nature of any lithium-ion battery. And when one of these batteries punctures, this is what happens:

In a cheaper battery, Whitacre 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.

This is not new at all: Lithium-ion batteries have long led to explosions—in smartphones, laptops, airplanes, cars…the list goes on. Lithium-ion batteries are great because they are small but hold a lot of energy, so electronics manufacturers are obviously going to use them. But packing all that power can come with its risks in some products—that risk specifically being fire. That’s why our high-powered, long-range electric cars, like those made by Tesla, have highly advanced cooling fans and heat-sink systems. The fan is the key component inside the vehicle that keeps the battery cells operating at a safe temperature.

The batteries in hoverboards may not be the only problem, though. It’s less common, but a defective charger could also cause problems with any electronic device.

“If there is not proper protection to the cells, and if the charger is defective, the cells can be severely overcharged,” Whitacre says. “In cases of severe overcharge, even perfectly made cells will eventually fail, though a fire is not always the outcome in this case. The cell may just pop its gas vent and dry out.”

So what can a consumer do if they really have their heart set on one of these bad boys? Conventional wisdom would say they should just stick with top tier brands, but this is where things get confusing, because this product category is totally new, and no exemplars of quality have emerged. A higher price should be an indicator of better quality, but companies such as IO Hawk and Hovertrax, which make more-expensive devices, aren’t exactly perennial tech powerhouses. This “hoverboard” trend is almost certainly a fad, and it’s hard to know whether any of these companies will be around at this time next year.

Regardless of how much you’re paying, it’s almost impossible to tell what kind of fire hazard lurks (or doesn’t lurk) inside any scooter. The scariest part is that you may not find out until it’s far too late.

“There is no way to tell when buying, since the catastrophic failure likely will not manifest until the battery is fully charged and discharged several times,” Whitacre explains. “This charging/discharging mechanically exercises the guts of the cell and typically provides the ultimate trigger for the failure.”

If all of this hasn’t diffused (that pun was intended) your excitement for a board, then at the very least, you should know how to put out a fire—keep a fire extinguisher or bucket of water handy, friends.1

1UPDATE 1:15 PM ET 12/14/15: We incorrectly said water couldn’t put out a lithium-ion-created fire—that’s only true of lithium metal. Water will work on your hoverboard-related fires.

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