The U.S. Department of Transportation has a new rule about bringing spare lithium ion batteries onto an airplane. For it to be effective, though, both consumers and Transportation Security Administration agents at airport security checkpoints must be able to determine how much lithium is in a battery. But there's one problem.

As Xantrex's PowerSource Mobile 100 proves, there's no way to tell how much lithium is in anything. For example, according to Xantrex product marketing manager Grant Dunbar, even though the PowerSource Mobile 100 weighs one pound, it only contains 4 grams of lithium ion, which is well within the limits set by the U.S. DOT's new rule (by the way, it was this on-board fire that inspired the new rule). Unfortunately, most lithium ion-based batteries don't have any information on them regarding their total amount of lithium ion content.

Dunbar says that, down the road, we should expect to see battery manufacturers like Xantrex include that information right on the battery. But is that good enough? First, can it be trusted (and who oversees that labeling)? Second, will the TSA agents at airport security checkpoints have the time or expertise needed to examine the batteries and enforce the rules?

Meanwhile, even though Xantrex just proved the futility of the new rule, the product itself is very cool. It has one AC outlet and two USB ports and can really help to keep your devices powered up in a pinch.