"Yes, all external batteries with a capacity lower than 27027mAh (100 watt hours) can legally and safely be taken on board airplanes.
To calculate an external battery's capacity in watt hours: (Battery mAh Capacity X 3.7)/1000.
Here are the relevant requirements quoted from federal transportation security rules:
Effective January 1, 2008: Passengers may carry consumer-sized lithium ion batteries (a.k.a. rechargeable lithium, lithium polymer, LIPO, secondary lithium) with no more than 8 grams of equivalent lithium content or 100 watt hours (wh) per battery. This size covers AA, AAA, 9-volt, cell phone, PDA, camera, camcorder, Gameboy, and standard laptop computer batteries. Passengers can also bring two (2) larger lithium ion batteries (more than 8 grams, up to 25 grams of equivalent lithium content per battery) in their carry-on. This size covers larger extended-life laptop batteries. Most consumer lithium ion batteries are below this size."