Generally nothing happens. You can carry most electronic devices with installed lithium batteries in checked luggage. You cannot carry loose batteries or ones in external chargers. Refer to the FAA FAQ on batteries .

If you accidentally leave a small lithium battery in your checked luggage in 2026, the outcome depends on whether the battery is installed in a device or is a spare. According to FAA and TSA rules, small lithium-ion batteries (like those in cell phones, Kindles, or cameras) are actually allowed in checked bags if they are installed inside the device and the device is completely powered off. However, spare (uninstalled) lithium batteries and all power banks are strictly prohibited in checked luggage because they pose a fire risk known as "thermal runaway," which cannot be easily extinguished in a cargo hold. If a prohibited spare battery is detected by X-ray scanners, TSA will likely open your bag and remove it. They may leave a notice of inspection, and while you usually won't be fined for a first-time honest mistake, you will lose your battery or power bank, as they are typically confiscated and disposed of rather than held for your return.