On Thursday, the U.S. Consumer Product Safety Commission issued a letter to hoverboard manufacturers, importers, and retailers warning that devices that don’t meet new safety standards could be detained, seized, or recalled.

The letter followed reports to the CPSC from people in 24 states of 52 hoverboard fires resulting in more than $2 million in property damage over an 11-week period.

K.M. Abraham, research professor at Northeastern’s Center for Renewable Energy Technologies, penned a technical commentary for the Electrochemical Society on why hoverboards are exploding and what role the lithium-ion batteries powering them play in their combustion.

We asked him to take us under the hood, so to speak, to break down the science and explain why devices such as smartphones and laptops, which use the same battery technology, aren’t experiencing the same explosive problems.

Haste makes waste

Lithium-ion, or Li-Ion, batteries are rechargeable and have four to six times the energy of your standard nickel-cadmium batteries. That makes them an excellent power source for everything from smartphones and laptops to electric cars such as the Tesla Model S and the Chevrolet Volt. Yet you don’t hear stories about iPhones or Macs, Teslas or Volts, self-immolating in record numbers.

That’s because the Li-Ion batteries in those technologies are made by “experienced and highly reliable manufacturers,” says Abraham, who is also the principal of E-KEM Sciences, a battery-consulting company in Needham, Massachusetts. They know how to construct them in a way that balances the amount of power produced with the amount of power  consumed by the device during its operation.

“When that balance is compromised, the battery can heat up ,” he says, “leading to a thermal runaway reaction and the uncontrolled release of large stores of energy.” Translation: an explosion. The race to feed the hoverboard fad brought in scores of less-than-expert battery suppliers using perhaps defective materials or improper engineering of parts.

The parts determine the whole

What happens when battery engineering runs amok?

A Li-Ion battery has three primary parts: Two “electrodes”—an “anode” made of graphite and a “cathode” made of lithium cobalt oxide or a similar metal oxide—and a very thin, but porous, polyethylene “separator” that keeps the two apart.

Make sure that the batteries are reliably made with good materials as well as proper engineering and tested for use in a hoverboard specifically.
—K.M. Abraham, research professor at Northeastern

The electric current flows between the anode and the cathode via a liquid, called the “electrolyte.” If the anode and cathode are not engineered correctly for the power draw or the separator is imperfect—say, it’s been punctured by mechanical impact or even impurities—a short circuit can result. When that happens, the electrolyte heats up, the cathode and anode become unstable, and the two react violently with the electrolyte. The temperature may reach the boiling point, says Abraham, “causing the battery to eject its hot internal contents, which catch fire or explode when they come in contact with oxygen in the atmosphere.”

Hoverboards pose additional risks, given the operation and construction of the machines themselves: They draw energy from batteries much faster than, for example, cellphones and laptops do, which strains the electrodes and ratchets up the internal heat. They also bang into things or, as Abraham gently puts it, “are subject to more mechanical as well as electrical abuse.”

So, is there a way to avoid danger, other than hanging up your hoverboard? Abraham suggests the following: “Make sure that the batteries are reliably made with good materials as well as proper engineering and tested for use in a hoverboard specifically.” To test your device, he recommends investigating the consumer-technology battery-testing division of UL, the independent safety-science group that set the new standards.

In January, Northeastern banned using or charging the popular “levitation” devices inside residence halls or other university-owned buildings, citing safety concerns.