The United States Postal Service (USPS) will be prohibiting international shipments of devices containing lithium ion batteries starting May 16th. After coming to grips with serious financial problems last year, USPS has now decided that lithium batteries pose too great of a risk to be shipped overseas. Devices like smartphones, laptops, and tablets will fall under this ban, but domestic shipment of these products will be unaffected.

USPS has revised its Domestic Mail Manual — an official document outlining shipment policies — after evaluating recent discussions by the International Civil Aviation Organiza­tion and the Universal Postal Union. Meanwhile, UPS and FedEx are not changing their stance on the matter, which isn't surprising considering how much money is involved. The international shipment of electronic goods is a multi-billion dollar industry, and the added risk associated with transporting devices with lithium batteries is a gamble these companies are willing to take. The only problem is that neither of these couriers will deliver to APO, FPO, or DPO addresses overseas, meaning that troops abroad will not be able to receive Kindles and iPads to fill the downtime between combat.

An outright ban on air-shipping lithium batteries may seem like an exaggerated response to this ambiguous problem, but there have been several plane crashes directly attributed to exploding lithium batteries in the last few years. There are two reasons why a lithium battery might experience a runaway thermal expansion, or explosion. The first reason lies with the battery's chemistry, and the second is contingent on physical or environmental stress.

Lithium batteries have a very wide variety of chemistries, some being far more volatile than others. As a general rule, higher capacity batteries will rely on a more unstable chemistry. This forces electronics manufacturers to choose riskier batteries to fuel their power-hungry gadgets with quad core processors and high resolution displays. For example, lithium iron phosphate batteries — used in electric vehicles — are considered far safer than their more common cobalt-based counterparts but have 25 percent less energy density. Lithium iron phosphate batteries also offer thousands of charge cycles, while lithium cobalt batteries are only rated for several hundred cycles. As both chemsitries decay, oxygen and other elements can accumulate, increasing the risk of explosion.

From a physical standpoint, lithium batteries have layers of insulation that are filled with electrolyte, and any pressure on these components can cause an internal short. Where things get a little ambiguous is deciding how much altitude affects lithium batteries. The Underwriters Laboratory specifically lists altitude, or rather the variation in atmospheric pressure, as a possible vulnerability to lithium batteries. Though pressure testing is common in some industries, USPS has not cited any studies on these effects on lithium batteries in relation to its decision to ban them from international shipment.

Even though USPS is pegging May 16th as the definitive cutoff for overseas shipment of lithium-powered electronic goods, the company says it expects to lift this restriction in January of 2013. After this date, customers will be allowed to mail "specific quantities" of lithium batteries, or devices that contain them. (A specific reason isn't cited.) In the meantime, if you've got family and friends overseas, you might want to schedule a trip to the post office sooner rather than later.