On Monday, United Airlines announced that it will no longer accept bulk shipments of rechargeable batteries citing safety concerns.

Rechargeable batteries are also called lithium-ion batteries, which are used to power everything from smartphones to laptops to power tools.

These batteries dictate the global battery industry as they're cheap to make, lightweight and can hold a lot more energy than other types of batteries.

Delta Air Lines have stopped accepting bulk shipments of the rechargeable batteries on February 1, while a third major US carrier, American Airlines, have also banned bulk shipments on February 23.

However, the airline has not banned small packages shipments of batteries grouped together or 'overpacked' into a single cargo container. According to several aviation officials, France has also stopped accepting bulk shipments of the batteries.

All the three airlines said they will continue to accept bulk shipments in which batteries are placed inside equipment like laptops or in the same package as power tools.

This creates additional buffering and provides added protection. However, according to some experts, this theory hasn't been fully tested.

Over the past year, Federal Aviation Administration (FAA) tests haves showed that when a battery overheats, it causes other nearby batteries to short-circuit and overheat.

This results in a chain reaction and as the overheating spreads, the batteries emit explosive gases that build up inside the cargo container. As, a result, there fierce explosions can occur, that can blow the doors off containers followed by violent fires.

According to officials familiar with the tests, combination of gases released by the batteries varies according to the batteries' chemistry. However, the gas released in the greatest volume is hydrogen.

According to a slide presentation by Airbus engineer Paul Rohrbach, an airliner might be able to withstand fire generated by a small number of lithium-ion batteries, but a fire involving lots of them could destroy the plane.