About a week ago, a Samsung Galaxy Note7 smartphone in Korea reportedly caught fire while charging. After a second report surfaced in Korea, Samsung was prompted to suspend shipments of the model to Korean cell providers, pending an investigation.

According to the Yonhap News Agency and other Korean news outlets, the problem was traced to the batteries in a small number of phones.

Samsung has not disclosed how it plans to address this issue. Phil Berne, a public relations manager, said in an email that the company is "conducting a thorough inspection. We will share the findings as soon as possible."

The phone is still available in U.S. stores. If your Note7 is unusually hot to the touch when plugged into a charger, disconnect it immediately and take it to the store for service or replacement.

Thanks to redundant safety technology routinely installed in batteries and charging systems, as well as manufacturing standards developed and shared by independent labs such as UL, situations like these are rare. But they're often frightening.  

“A battery, from a chemistry perspective, is a bomb, albeit a controlled bomb in that its energy is released in a controlled way,” says Qichao Hu, CEO of SolidEnergy, a developer of high-energy-density material used in mobile devices, high-altitude drones, and clean-energy vehicles.

Lithium ion batteries, commonly used in smartphones and laptops, have a higher energy density than other batteries, which means they pack more energy into a smaller form factor. This makes them more attractive for portable devices. But it also makes them more volatile when things go wrong.

“If the charger is damaged in any way as to affect the way it regulates voltage or current inside the battery," says Hu, "then you could have situation where you’re overcharging the battery."

Overcharging causes the cathode to become unstable, he explains, releasing oxygen and making the battery overheat. As the battery gets hotter and more unstable, it can suddenly discharge all of its energy.

"The resulting sparks," says Hu, "could cause that tightly packed bundle of oxygen, heat, and volatile materials, which are themselves trapped inside the phone, to explode.”

Consumer Reports testers have been conducting a routine evaluation of Note7s models from the major carriers for more than a week. To date, we've found no major issues, despite subjecting the phones to a series of battery-related tests involving heavy use of the camera, display, and wireless connections and repeated charging and discharging of the batteries.

Our complete test results will appear in our ratings soon.