by Leland Teschler, Executive Editor
Battery fires in hoverboards have made headlines and garnered the attention of regulators who are devising safety standards aimed at heading off future troubles.
There is a video on YouTube called “Fire exploding hoverboard caught on camera” that, as of this writing, has received 4,285,155 views in the three short months since it was first posted. At about the 1:58 mark, it shows a hoverboard suddenly start to smoke and within a few seconds, burst into flame.
Unfortunately, hoverboards are getting a reputation for being flaming conflagrations thanks to the lithium batteries they typically contain. Dozens of YouTube videos pop up under the search phrase “hoverboard fire,” and the U.S. Consumer Product Safety Commission is now investigating at least 40 reports of hoverboard fires across 19 states. Hoverboards have even been the subject of a skit on the widely watched TV humor show Saturday Night Live.
Late last year, flaming hoverboards got the attention of engineers at Underwriters Laboratories. “We know that when there is a lot of lithium in the product, you really have to do your due diligence,” said Ibrahim Jilani, dusiness development manager for UL’s Energy and Power Technologies Div. These concerns led to the launch of a new safety standard, UL 2272, that applies specifically to hoverboards.
The road to the new standard, officially titled Electrical Systems for Self-Balancing Scooters, started when UL engineers purchased about a half-dozen hoverboards from various sources and took them apart. “We consistently saw a few things,” said Jilani. “A lot of the hoverboards used the UL mark in a way that was considered counterfeit. The boxes they came in typically lacked the name of a manufacturer so you couldn’t trace the product back to who made it. These are signs of a gray market supply chain. We’ve seen problems like this in the past with consumer products such as Christmas tree lights, but those don’t store a lot of energy as do hoverboards. The stored energy in a hoverboard can cause a fire at any moment.”
The standard considers only the safety of the hoverboard electrical system, not factors such as whether riders can hurt themselves if they fall off. UL 2272 drew from previous work done on standards for light electric vehicle battery packs. For example, most of the construction criteria comes from UL 2271, which covers batteries for light electric vehicles.
The new standard published in January. Hoverboard makers can start evaluating their products for compliance and submit them to UL for certification. But the standard isn’t final. As part of its review process, UL convened a meeting of stake holders in March. There, suppliers and regulators and manufacturers whose products will be affected by the new standard discussed provisions of it. As part of the normal review process, these stake holders vote on whether the final standard should include specific provisions that UL initially outlined.
One of the participants in the March meeting was Robert Bigler, co-founder of California-based HoverBoard Technologies and inventor of the single-wheeled HoverBoard. Bigler said he feared the worst going into the UL meeting: That the regulator was planning provisions that would make it all but impossible to sell anything that looked like a hoverboard in the U.S. “I was wrong,” he said. “UL took our input seriously. The standard got edited as we spoke. UL was fair and even-handed about the whole thing. They took the attitude that they don’t want to make the standard unnecessarily burdensome.”
Bigler thinks his own products won’t have any trouble meeting the new UL standard. The units most likely to encounter trouble with UL 2272, he thinks, are some of those coming into the U.S. from Asia. “By way of example, people will no longer be able to run wires willy nilly in the hoverboard. There are provisions in the standard to make sure there are no wiring pinch points,” he said.
The biggest impact on hoverboard manufacturers is likely to be in battery technology, Bigler thinks. “Chinese manufacturers tend to use inexpensive and volatile battery chemistry having a failure mode that results in a fire. The flip side is that they can advertise a hoverboard with a long range,” he said. “This new standard gives manufacturers an incentive to use better batteries. Reputable companies are already using more stable battery chemistries that are inherently safe. But you might see the advertised ranges change on some products as a result of the new standard.”
Bigler also figures other hoverboard manufacturers will have to adopt some of the same measures he uses in his design: monitoring of individual cells for voltage and thermal behavior, and reverse polarity protection.
Safety in lithium-ion batteries
Many of the lithium-ion cells used in hoverboards contain cobalt cathodes. This lets them hold twice the energy of a nickel-based battery and four-times that of lead acid. To lengthen the time between recharges, battery manufacturers have, over the years, packed more active material into each cell and made the electrodes and separator thinner. Such measures have doubled the energy density of lithium-ion cells since they were introduced in 1991.
The high energy density can potentially make cells more prone to defects. Manufacturing methods become more critical as cells grow denser. With a separator thickness on the order of 20 to 25 µm, any small intrusion of metallic dust particles can potentially destroy the cell. The worst case is where such microscopic metal particles lead to a dead short circuit within the cell.
The complex assembly techniques involved in constructing high-energy batteries make it tough to eliminate all metallic dust. A resistive short will only bring an elevated self-discharge. This generates little heat because the discharging energy is low. But enough microscopic metal particles on one spot can lead to a major electrical short and a sizable current flowing between the positive and negative plates. This causes a temperature rise leading to a thermal runaway.
Manufacturers of lithium-ion cells with cobalt cathodes say these batteries should stay below 130° C (265° F). At 150° C (302° F) the cells become thermally unstable and could end up going into a thermal runaway in which flaming gases vent. If this happens, the high heat of the failing cell can propagate to the next cell, causing a thermal domino effect.
In some cases, the chain reaction can be such that each cell disintegrates at its own timetable. A pack can self destruct within seconds or roast for several hours as cells die one-by-one.
To reduce the chance of such catastrophic difficulties, some lithium-ion cells use chemistries that are more stable. For example, some substitute manganese for cobalt cathode material. Manganese can sustain temperatures of 250° C (482° F) before becoming unstable. Manganese also has a low internal resistance and can deliver high current.
However, a cell having a pure manganese cathode only provides about half the capacity of cobalt. So battery makers sometimes mix the materials as a compromise. Typical cathode materials are cobalt, nickel, manganese and iron phosphate.
Though it has not made the same kind of headlines as thermal issues, cold-temperature charging is another safety concern. Consumer-grade lithium-ion batteries cannot charge below freezing. The sinister thing about a sub-freezing charge is that though the packs appear to be charging normally, metallic lithium plates on the anode. The plating is permanent. Repeated charging at low temperature can compromise the safety of the pack.
A big shutdown
Most major hoverboard makers in the U.S. go with more conservative lithium-ion chemistries, trading off range for a lesser chance of thermal runaway. Nevertheless, the new UL standard may affect how manufacturers build-in safeguards against abnormal battery conditions.
Take as an example the handling of an overvoltage on a battery cell. The typical battery powered consumer appliance might just shut down if it senses this problem. But that’s not a practical solution for a hoverboard. “You can’t just shut the system down because that could throw the rider,” said Bigler. “The standard will likely compel hoverboard manufacturers to devise ways of slowing the board. We might have to trick the rider into slowing down. Board manufacturers might have to go through a redesign to accomplish this.”
But battery issues might not be the main problem for hoverboard manufacturers, particularly those based outside the U.S. “There will be an impact on foreign hoverboard makers not so much because they have to design to spec, but because they’ll need a different kind of structure,” said Bigler. “When you are making hundreds of thousands of hoverboards a month, you are doing it in thousands of factories. You don’t have the connections to get this kind of certification done. The factories are beholden to those with western connections.”
Hoverboard manufacturers that want to certify their products to the new standard can start submitting samples to UL for testing. UL’s Jilani said the certification process can take four to eight weeks in the absence of surprises. “But compliance doesn’t just involve testing,” he said. “Compliance with UL standards mean meeting construction requirements, labeling and marking requirements, user instruction elements, and other factors. So it isn’t just testing.”
Meanwhile, the 2272 standard continues to undergo refinement. It will eventually be put through the ANSI accreditation process to establish it as an American National Standard. It will go through a similar process in Canada. Also the ASTM standard for pocket bikes and e-scooters will reference UL 2272 for electric system safety.
The upshot: Hoverboard difficulties of a different type. “I think there will be a lot of counterfeit UL marks on hoverboards,” speculates Bigler.