As if air travel isn't bad enough. . .
New rules for bringing spare batteries on planes with you:
Pack spare batteries in carry-on baggage. In the passenger compartment, flight crews can better monitor safety conditions to prevent an incident, and can access fire extinguishers, if an incident does happen.
Keep spare batteries in the original retail packaging, to prevent unintentional activation or short-circuiting.
For loose batteries, place tape across the battery's contacts to isolate terminals. Isolating terminals prevents short-circuiting.
If original packaging is not available, effectively insulate battery terminals by isolating spare batteries from contact with other batteries and metal. Place each battery in its own protective case, plastic bag, or package. Do not permit a loose battery to come in contact with metal objects, such as coins, keys, or jewelry.
Only charge batteries which you are sure are rechargeable! Non-rechargeable batteries are not designed for re-charging, and become hazards if they are placed in a battery charger. NEVER attempt to recharge a battery unless you know it is rechargeable.
If you have already charged a non-rechargeable battery, do NOT bring such a battery on board an aircraft.
Use only chargers designed for your type of batteries. If unsure about compatibility, contact the product manufacturer.
Take steps to prevent crushing, puncturing, or putting a high degree of pressure on the battery, as this can cause an internal short-circuit, resulting in overheating.
And see Airlines Impose New Restrictions on Batteries
The new rules are confusing and extensive (and are being reported incorrectly in numerous mainstream publications), so I'll try to boil it down for you here, accurately. Hit the link at the end of the story for the entire text of the new rules straight from the horse's mouth. The rules took effect on Jan. 1, 2008.
* Installed batteries (already in your phone, laptop, camera, etc.) and spare batteries (carried loose) are treated differently. Only lithium-based batteries are concerned here, not nickel-based rechargeables or alkaline batteries.
* You can't pack spare batteries in checked baggage...but you may check equipment with batteries installed.
* In your carry-on baggage, you can take as many batteries along as you want (installed or spare), as long as they contain less than 8 grams of lithium content each. How do you know how much lithium is in a battery? An 8-gram battery equals about 100 watt-hours of power. Now, your battery won't say how many watt-hours it provides, but it's easy to do the math. Look on the bottom and you'll find a voltage rating and a mAh (milliamp-hours) rating. Multiply these two together and divide by 1,000. That's your watt-hours. In the (big) battery I'm looking at as an example, it offers 11.1 volts and 7,800 mAh. Multiply and divide by 1,000 and you get 86.58 watt-hours, acceptable under the new rules.
* Now, you can also bring two spare batteries that break the above rule. These two batteries can have a total lithium content of 25 grams, or about 300 watt-hours. Where might you find such a giant battery? Namely in those third-party laptop battery slabs designed to give you a full day of computing. A product like this Electrovaya PowerPad 300 would just barely make it... but would probably earn you a delay at security.
* These rules mainly concern lithium-ion batteries. Lithium metal batteries (which are comparably rare) have more stringent rules. Check the link for full details if you use lithium metal batteries, but since lithium metal batteries are usually quite small, there's not that much cause for concern.