A growing number of airlines have announced restrictions on luggage with built-in batteries because of a potential fire hazard they may create in the cargo area of a plane.  Two of the world’s largest airlines, American and Delta, announced this month that bags with built-in batteries that cannot be removed will not be allowed as checked luggage starting Jan. 15. Since then, United, Southwest, Alaksa and Hawaiian Airlines have announced similar restrictions. The move comes amid growing worry that lithium batteries can ignite a fire in the cargo hold of a plane.  The Federal Aviation Administration has imposed restrictions on spare lithium batteries carried in the cargo area but has left it up to airlines to impose restrictions on larger batteries inside of electronic equipment, such as smart bags.  Smart bags have become increasingly popular with tech-loving travelers because the bags come with built-in devices that can weigh the bag, track its location using GPS and lock it remotely using a smartphone app. Some bags even include wheels and a motor so that travelers can ride the luggage like a scooter.