An iPad 2 battery contains about 90 kilojoules of energy. This is equivalent to about 20g of TNT, which is only 1/3 of the TNT used in a WWII-era "pineapple" hand grenade.

However, the real reason one can bring a laptop battery on a plane is because no terrorist has tried to use it yet. They would probably need multiple batteries: after the "shoe bomber" attack, the government determined that as little as 50g of PETN could bring down a plane, which is equivalent to 80+g of TNT, which is four iPad 2 batteries.

Even then, it would be necessary to deliver that energy in a very small amount of time, which may not be possible rapidly enough with lithium-ion; they can burn, but can they explode quickly enough, with all of their energy? The TSA aays they have studied the issue, and the answer is no [1], they "cannot be used as an explosive and are not a security threat in personal carry-on quantities."

[1] http://usatoday30.usatoday.com/money/industries/travel/2010-...

> "Can they explode quickly enough...?"

Explosion? Starting a fire isn't enough?

Given that the TSA allows common lighters and matches to be taken on board, apparently not. I think aircraft cabins are designed to be "fire resistant" (for obvious reasons) though I'd be surprised if they were fireproof.

But then why don't they allow water bottles? Is a flammable liquid worse than a flammable solid like Lithium?

The entire plane interior is fire-retardant and there are lots of extinguishers around.

Given all the ways you could start a fire on a plane, it's probably better that they're engineered to survive a small fire being started anyways.

What if I filled an old iPod shell with PETN?

Because you can ban water bottles, and sell them in the shop that's after security checks, and that won't turn many people back. But banning laptop batteries would harm business travel really seriously - it would really stop very, very many business travellers.

Naw, the airlines would rent secure batteries at a huge markup.

People do want to have their batteries at the destination as well. The typical business traveler would be so severely inconvenienced by this that it would reduce air travel significantly.

Note that the water bottle scam isn't really profiting the airlines - it's benefiting the airport operators, in collusion with many national air regulators.