In 2014, on the tarmac of Melbourne airport, moments before passengers were due to board, white smoke was seen pouring out of the hold of a Boeing 737.
The captain and ground crew sprung into action, activating the internal fire suppression system, declaring an emergency and cancelling the flight.
The source of the blaze turned out to be a suitcase, which investigators discovered contained eight lithium-ion batteries, one of which had apparently short-circuited.
In September 2010 a 747 UPS flight heading from Dubai to Germany experienced a catastrophic fire in the cargo hold. The plane crashed, killing the two-person crew.
A subsequent investigation concluded the fire was likely caused by lithium batteries listed on the manifest.
The directives issued this week by US and UK aviation authorities requiring all lithium-battery powered devices other than smartphones – including laptops, tablets and e-readers – to be stored in aircraft holds has been justified in part as a response to intelligence “chatter”.
This implies that spooks somewhere along the line have reason to believe that terrorists are planning on using a battery-powered device to cause an explosion on a commercial flight. From that perspective, the ban might be good policy, but it is very, very poor science that, on the evidence, increases rather than decreases the chances of a catastrophic mid-air event.
Lithium-ion batteries are fundamentally flammable, fundamentally explosive, and fundamentally unstable. In January this year tech giant HP recalled 100,000 of them because of safety fears.
They are also, of course, extremely useful, powering pretty much all the world’s electronic devices, from laptops to e-cigarettes. Managing the danger is the price we pay for the convenience of connected portability.
The batteries release or store energy because lithium ions constantly bounce between two electrodes – one made from graphite and the other from lithium cobalt oxide. If nothing goes wrong, the cobalt and oxide ensure the lithium remains stable as ions zip in and out of it. At high temperatures, however, this situation quickly deteriorates, initiating what the tech industry euphemistically terms a “thermal runaway”.
One of the key triggers for this sort of event is when the two electrodes touch each other, creating a short circuit. In the normal course of events this is prevented by a thin sheet of polypropylene, which functions as a separator.
The polypropylene, however, is not itself the most stable and sturdy of materials. It is heavily perforated with micron-size holes, through which the lithium ions pass. It sits in a soup that contains a solvent called ethylene carbonate. This provides a low friction environment in which the ions can move, but it is also flammable, literally adding fuel to the battery fire.
Battery explosions have occurred in a wide variety of circumstances, some of them quite bizarre. In 2016, for instance, in the US, over half a million hoverboards were recalled after at least 99 reports of explosions during charging. Many airlines banned them because of safety fears.
And in 2016, two unnamed people achieved the dubious honour of being the first to enter the medical literature with burns to their thighs caused by exploding e-cigarettes.
The wide variety of environments and circumstances in which lithium batteries have exploded leads inevitably to the conclusion that there is an equally wide variety of stimuli that can set them off.
One stimulus above all others, however, stands out: vibration. Shaking and shuddering significantly increases the risk of separator failure, short circuit and structural breach.
“The causes of internal destruction of Lithium-ion batteries are external vibrations,” concluded Pawel Swornowski from the Poznan University of Technology in Poland, after an extensive investigation of lithium battery “internal destruction”.His study was published in January this year.
And if there’s one thing everyone knows about aircraft cargo holds it’s that they vibrate. A lot.
This has been well known to airline operators and regulators for a long time. On its website, the Australian Civil Aviation Authority has this instruction for passengers: “The batteries that power your phone, laptop and camera are usually under the 100 watt-hour rating. If you’re carrying a spare battery that’s not in one of these devices, it must be in your carry-on baggage only. Spare batteries, regardless of their size are not to be carried in checked luggage.”
The authority goes further, recommending that travellers employ safety measures even with their carry-on devices. This includes taping over exposed terminals, and placing batteries in individual plastic bags for the duration.