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Why hoverboards explode

by Thea Singer

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.

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.

Originally published in news@Northeastern on February 23, 2016

K.M. Abraham

K.M. Abraham, research pro­fessor at Northeastern’s Center for Renew­able Energy Technologies

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