New Travel Restrictions on Batteries

Effective January 1, 2008, if you travelling to or within the US on airlines, you may not pack spare lithium batteries in your checked baggage – that is, the baggage you give to the airline for handling.

Spare batteries are the batteries you carry separately from the devices they power, such as a video camera. When batteries are installed in a device, they are not considered spare batteries. You may not pack a spare lithium battery in your checked baggage.

You may pack spare lithium batteries in carry-on baggage!

For personal use, there is generally no restriction on the number of spare batteries allowed in carry-on baggage. That includes cell phone batteries, “hearing aid” button cells, and AA/AAA batteries available in retail stores, as well as almost all standard laptop computer batteries. However, you may carry no more than two larger batteries, in the range of 100 – 300 watt hour rating. These include some extended life laptop computer batteries, such as the “universal” lithium ion battery pictured below.

Rules for larger batteries (these apply for all larger batteries, whether spare or installed, in either carry-on or checked baggage):
Larger lithium metal batteries have over 2 grams of lithium. You may not carry any of these. These larger lithium metal batteries are not used in consumer devices.

Those battery details in full http://safetravel.dot.gov/whats_new_batteries.html