So it seems that you can carry batteries, extra batteries, spare batteries, and other, presumably, Leyden jars on flights. The TSA warnings only mention banning “large, palletized shipments” of lithium-ion batteries on flights whereas other batteries are fine. They recommend covering the battery terminals, however.
On the other hand, the Department of Transportation says you are limited to bringing only two extended life batteries, which, according to the image associated with the press release, apparently only fit old Inspirons running Windows 95. The best way to avoid getting picked up for battery thoughtcrime, however, is to pack your batteries in plastic bags so they can’t accidentally brush up against that massive piece of metal in your carry-on luggage and short out. Welcome to 2008!