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Actually, it looks like the TSA has created three size classes (i.e., Small, Medium, and Large) and set different limits on each class. The rules for the batteries that power most cell phone and laptops don't have carry-on limits. (Oddly, the TSA rules seem to be silent on the Large class. Also, the web page explaining the rules is a mess with inconsistencies in its tables -- TSA really needs to fix this to avoid confusing many people.)

The new rules for lithium-ion batteries are: 1) uninstalled batteries must be in your carry-on luggage (batteries installed in your devices may be in your checked or carry-on luggage) and must be switched off (i.e., have their contacts covered since most batteries don't have switches of their own), 2) for medium batteries (i.e., <b>over 100 WH</b>) no more than two with a total of 300 WH can be in your installed in devices in your checked bags, and 3) for medium batteries (i.e., <b>over 100 WH</b>) no more than two with a total of 300 WH can be in your carry-on bags.

So, most laptop and cell phone batteries are under 100 WH and thus there is no limit on the number that can be carried on board. My BlackJack II battery is 1,700 mWh at 3.7 V, or 6.29 WH. My Dell XPS m1330 has a 56 WH battery (a standard 6 cell lithium-ion notebook battery). Since both are under 100 WH, they are both classified as "Small" batteries.

Rule 1 above says that I can have "Small" batteries in my checked bags, as long as the batteries are installed in the devices that they power. (I would never check my phone or computer, so that's not an issue.) Rule 1 also says that I can bring any number of spare batteries on board, as long as they are turned off (i.e., have their contacts covered); so I can put each battery in its own zip-top bag to comply with the rule.

Some huge extended-duration laptop batteries may fall in to the "Medium" class. (Using industry-standard cell sizes, such a battery would have to be a 12-cell battery.) The rules would allow one in the checked luggage (installed in the device) and two (not installed) in the carry-on bags.

Thus the only change that this rules change really imposes on most of us is the individual wrapping of batteries to prevent electrical contacts in our carry-on bags. Between this and last year's liquids rule, it is clear that the zip-top bag industry is secretly in charge of the TSA.