If you fly soon, know the rules for lithium-ion battery chargers, also called power banks. These chargers are useful for phones and other devices. But airlines now have stricter rules because of smoke and fire cases.

You cannot put these chargers in checked luggage. They must go in your carry-on bag. Travelers can usually bring two power banks of 100 watt hours without airline approval.

If a battery is bigger, from 100 to 160 watt hours, you need airline approval. The FAA says newer batteries should show the watt hour number. If the battery shows mAH, you must do some math or use the FAA online calculator.

Airlines are worried because battery fires can be very dangerous. The FAA says there have been nine lithium battery air incidents this year, and six were about power banks. On the plane, keep the power bank where you can reach it fast, not in the overhead bin. Do not use it to charge other devices, and do not charge it with the plane’s power sockets.