Battery fires in personal electronic devices can be scary. But if a battery ignites on a plane, the risks are much greater.

With more people traveling with an assortment of portable electronics  sometimes a plane has more devices than passengers  fires are occurring on airliners with increasing frequency. More than half of the 22 battery fires in the cabin of passenger planes since 1999 have been in the last three years. One air safety expert suggested that these devices might be “the last unrestricted fire hazard” people can bring on airplanes.

This month, the Federal Aviation Administration along with the Pipeline and Hazardous Materials Safety Administration issued special advisories to airlines about yet another gadget: the credit card readers that many have begun to issue to flight attendants to ring up sales of food, drinks and other amenities.

While airlines have used portable credit card readers for several years, the F.A.A. said earlier this month that they needed approval from the agency’s hazardous materials division. Like the majority of hand-held consumer electronic devices, the readers are powered by rechargeable lithium batteries, which the government considers hazardous.