
The US Department of Transportation will allow only two of lithium batteries on aircraft if they weigh 8g-25g, and then only in cabin-baggage. Over 25g are not allowed at all.
It's to cut the fire risk. Anything with a lithium battery inside it and weighing under 8g is fine. So normal camera, mobile phones etc. are OK if they're installed. They're not allowed loose in checked baggage.