The US Transportation Safety Administration (TSA), acting at the behest of the Federal Aviation Administration (FAA), is now restricting the amount and storage of lithium batteries brought onto an aircraft. The FAA fears that loose batteries in checked luggage could short circuit and start a fire, so they basically forbid people to check baggage containing batteries. But there's more to it, as you'll see.

In the past few years, airlines have seen a few cases of laptop bags smoking or catching fire because of "spare" lithium batteries. ("Spare" is TSA's new phrase for any battery that isn't currently installed in its intended device.) It's unlikely--but apparently possible--that an unlucky juxtaposition of lithium batteries and coins, paper clips, or other metallic objects will short circuit the battery, causing the battery to heat up enough to start a fire. Batteries sitting in their intended devices are, of course, less likely to short circuit. As a result, you can put almost (I'll explain the "almost" in a bit) any battery that you like in checked luggage, as long as it's in its device. (After all, all of those airline employees that handle your baggage behind closed doors need cameras, laptops, and cell phones too.)But there's more. In addition to banning spare lithium batteries from checked luggage, the FAA now imposes a maximum weight in lithium which all of your batteries together are allowed to contain--spare or otherwise.

As it turns out, there are two different kinds of lithium batteries: lithium ion and lithium metal . Most batteries in laptops, cell phones, cameras, and the like are lithium ion. Lithium metal batteries come in different sizes, but most of us are likely to run across only lithium metal "coin" batteries. The government cares about the type of battery because unlike lithium ion battery fires, lithium metal battery fires can't (according to the TSA) be extinguished with the fire-fighting equipment found on most commercial aircraft, so the government restricts lithium metal batteries more than lithium ion batteries.

The FAA and TSA won't let you bring a lithium metal battery containing more than 2 grams of lithium onboard any aircraft--even if the battery is installed in a device. However, you can carry aboard any number of lithium ion batteries, so long as the sum of their combined "lithium equivalent" (more TSA-speak) weight doesn't exceed 8 grams. But, for those who need considerably more stored power, the new rule adds that in addition to the 8-gram lithium equivalent, you're also allowed up to two extra "special" batteries. The one or two "special" batteries must: (1) each have more than 8 grams lithium and (2) collectively have less than 25 grams. So, for example, you couldn't carry on four 4-gram batteries, but you could carry on two 4-gram batteries and an 8.1-gram battery.

Wondering how many grams you're carrying around? At the time I'm writing this, most manufacturers don't report lithium grams, but you can calculate a rough estimate. Most rechargeable batteries are labeled with their capacity in a unit "mAh," or "milliamp-hour," as well as the number of volts that the battery provides. You can then estimate the grams of lithium equivalent in your battery like so:
Lithium grams = battery voltage x battery mAh / 12,500

For example, my cell phone's battery produces 3.7 volts and 740 mAh, leading to an estimate of about 0.2 grams of lithium. My laptop contains a bigger battery, offering 4800 mAh at 14 volts, so it weighs in in over 5 grams. My camera battery works out to about a gram, bringing my total to around 7 grams. My extended-life laptop battery, however, is just under 8 grams. So . . . anyone want to buy an HP battery?

Sigh. I feel so much safer now. Don't you?