Can you take lithium batteries on a plane?

The rule for lithium batteries comes from the Civil Aviation Bureau's list of restricted items and is the same on domestic and international flights.

The limit of 2 counts spare batteries and power banks over 100Wh together. Outside medical devices, a lithium metal battery over 2g is not allowed in the cabin or in checked baggage, in a device or loose.

Japan's list first asks which kind of lithium battery it is, then where it sits: inside a device, loose as a spare, or built into a power bank.

Grams for lithium metal, watt-hours for lithium-ion

Japan's list measures the 2 kinds in different units. Lithium metal batteries are sorted by lithium content, with the line at 2g. Lithium-ion batteries, which the list says include lithium-ion polymer, are sorted by watt-hour rating, with the top line at 160Wh. JAL's FAQ uses plainer names for the same split: the disposable type and the rechargeable type.

JAL: a lithium battery on its own is not checked

JAL's domestic FAQ says a lithium or lithium-ion battery by itself is not accepted as checked baggage. It applies the same answer to spare batteries, to batteries removed from a device and to chargers that have a battery built in. In the cabin JAL accepts rechargeable ones of 100Wh or less with no limit on quantity.

ANA puts numbers where the national leaflet has none

The Civil Aviation Bureau's leaflet sets no limit on the number of spare batteries of 100Wh or less. ANA's FAQ does: up to 20 spares of that size per person, cabin only. ANA also counts devices, allowing up to 15 per person with a lithium metal battery of 2g or less or a lithium-ion battery of 160Wh or less.

Fitted in a device and checked: off, not asleep

A lithium battery fitted in its device can be checked. Japan's list requires the device to be switched completely off only when it holds more than 0.3g of lithium metal or more than 2.7Wh of lithium-ion. Skymark's FAQ says a device left in sleep or standby cannot be accepted and asks for the main power to be turned off.

A power bank with no watt-hour figure

ANA's FAQ notes that many power banks on sale carry no watt-hour figure, and that an item whose rating is unknown is refused both in the cabin and at check-in. Its formula is rated capacity in Ah multiplied by rated voltage. ANA's worked example is a 2500mAh battery at 10.8V, which comes to 27Wh.

South Korea counts 5, Australia counts lithium metal spares

Counts change abroad. South Korea's list lets each passenger carry 5 spare lithium-ion batteries of 100Wh or less into the cabin and asks for airline approval from 6. Australia's CASA limits spare non-rechargeable lithium metal batteries of up to 2 grams of lithium content to no more than 2.

The carry-on answer for lithium batteries is the same in all 8 other places we checked. The limits themselves are not identical, so read the row for the country you fly from.

Each row comes from that authority's own page. Airlines can be stricter, and the screening officer has the final say.

What to do

Move lithium batteries to the bag you check in before you reach security, within the checked-baggage limits above. Narita Airport states that anything given up at the checkpoint is discarded and cannot be returned.