Why lithium-ion batteries must stay in your carry-on when flying

This precaution allows cabin crew to react in case of smoke or fire while a blaze in the cargo hold may go unnoticed by the aircraft’s automated fire suppression system until it becomes a serious threat.

According to the Guardian, Australia’s Civil Aviation Safety Authority (Casa) finds that each airline passenger carries at least four rechargeable lithium battery-powered devices on average like smartphones, laptops and portable chargers.

Devices like laptops and cameras could be checked in if they are fully powered down, but spare batteries and power banks have to be in carry-on luggage due to the risk of overheating, short-circuiting and potentially catching fire, Casa says.

Batteries exceeding 160 watt-hours (Wh) are prohibited unless used in approved mobility aids. Smart bags equipped with lithium-ion batteries or power banks is permitted only if the battery can be removed prior to check-in and carried in cabin baggage.

A person using a power bank during flight. Photo by Anker

Policies regarding lithium-ion batteries differ between airlines. For instance, Virgin Australia requires all spare or loose batteries, including power banks, to be carried in cabin baggage in their original retail packaging, a separate plastic bag or protective pouch, or have their terminals taped over.

Thai Airways, South Korean Airlines, EVA Air, Cathay Pacific, China Airlines, and Singapore Airlines restrict the use of power banks during flights entirely.

Professor Neeraj Sharma, a battery specialist at the University of New South Wales in Australia, says lithium-ion batteries consist of up to 20 components, some of which are liquid, making them more reactive than the solid elements like the casing or electrodes.

Applying pressure to these batteries can trigger "thermal runaway," a rapid and uncontrollable rise in temperature, though actual explosions are extremely rare, he says.

He says power banks and other lithium-powered devices such as e-scooters and vapes are generally less regulated than smartphones or laptops. They are typically equipped with lower-quality batteries and therefore pose a greater fire risk during flights, he adds.

According to The New York Times, the U.S. Federal Aviation Administration says in many instances, passengers are able to detect when their electronic devices begin to overheat. If they alert the flight attendants, they can place the device in a thermal containment bag or submerge it in water, without causing major disruption to the flight.

Lithium-ion batteries work by allowing lithium ions to move through an electrolyte solution, flowing between two electrodes as the battery charges and discharges.

A common cause of fires is overcharging. An overheated battery can enter thermal runaway if it receives too much current. In extreme cases, excess energy causes the battery to crack, exposing the highly flammable electrolyte to air and igniting a fire.

Devices like smartphones are equipped with a "trickle system" that gradually reduces charging current once the battery nears full capacity. But cheaper power banks often lack this safety feature and are more susceptible to failure.