The powers that be at the U.S. Department of Transportation's Pipeline and Hazardous Materials Administration (PHSMA) have determined that batteries are a potential fire risk. As a result, you will no longer be allowed to bring spare batteries in your checked luggage.
Batteries actually installed inside devices are OK, and most spare batteries in your carry-on are fine, too. But carry-on batteries are now governed by a complicated new set of rules.
You can carry batteries with 8 grams of lithium or less in them in your carry on bags. They now, however, must be carried now in plastic bags. Cell phone, PDA and other gadget batteries, plus most laptop batteries, contain less than 8 grams of lithium.
You're now limited to a maximum of two batteries with between 8 and 25 grams of lithium in them. The most common batteries in this category are "extended life" laptop batteries, but also larger devices like projectors. If you carry on three such batteries, security will take one of them away.