Lithium batteries present a risk of both igniting and fueling fires in aircraft cargo/baggage compartments. To reduce the risk of lithium battery fires, the U.S. Department of Transportation’s Hazardous Materials Regulations (HMR), and equivalent International Civil Aviation Organization’s Technical Instructions for the Safe Transport of Dangerous Goods (ICAO TI), prohibit spare lithium batteries from checked baggage (including baggage checked at the gate or on-board the aircraft). The HMR and ICAO TI provide limited exceptions for passengers/crewmembers who carry-on spare lithium batteries intended for personal use.

For spare lithium batteries in carry-on baggage, ensure personnel understand the following:

• Each spare lithium battery must be individually protected so as to prevent short circuits (e.g., by placement in original retail packaging, by otherwise insulating terminals by taping over exposed terminals, or placing each battery in a separate plastic bag or protective pouch).
• Spare batteries must not come in contact with metal objects, such as coins, keys, or jewelry and take steps to prevent crushing, puncturing, or pressure on the battery.
• Batteries must not exceed the allowable quantity and size limitations

https://www.faa.gov/other_visit/aviation_industry/airline_operators/airline_safety/safo/all_safos/media/2015/SAFO15010.pdf