A new U.S. Department of Transportation air travel regulation that came into effect yesterday may require some to leave their laptop batteries at home. According to the DOT's Safe Travel sub-site, folks who travel by plane are no longer allowed to take spare lithium batteries in their checked luggage. Lithium batteries installed inside a device are okay with two exceptions: lithium-metal models that contain more than two grams of lithium are forbidden in all types of luggage, and lithium-ion models with more than eight grams of lithium-equivalent content are restricted in number.
The DOT says eight grams of lithium-equivalent content is enough for a 100 watt-hour battery, so this particular rule shouldn't affect most notebook users. Nonetheless, the new regulation allows travelers to take take up to two lithium-ion batteries that have an aggregate lithium-equivalent content of eight to 25 grams in carry-on luggage. Up to two such batteries can also be taken in checked luggage, but only if they're installed inside devices.
To help travelers wrap their head around the new rules, the DOT's Safe Travel page has a guide table with a list of allowed and forbidden batteries depending on luggage types.