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Comment by bradleyland | Hacker News Reader

It matters only in so much that you needn't short a Li-Ion battery to have a violent discharge of heat energy.

Shorting any battery will cause it to heat up, but lithium is a potent alkali metal. It reacts violently (as in fire and possible explosion) with water. The moisture in your skin will do just fine. Contrast this with other battery chemistries such as nickel-cadmium, which are practically benign by comparison.

Here's a great video illustrating the results of dropping bits of various alkali metals in to a bathtub full of water:

http://www.youtube.com/watch?v=m55kgyApYrY

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That Brainiac video was faked. They didn't get the violent reaction they were looking for from caesium, so they just rigged up a bomb in the bathtub.

http://www.badscience.net/2006/07/brainiac-fake-experiments-...

Boy is that disappointing. Braniac had the feel of a show that would pull that kind of stunt though.

Isn't the low energy density of NiCd batteries the main reason it's so friendly? According to the all-knowing Wikipedia, the best Li-ion batteries are over 4x more energy dense than the best NiCds, so there's over 4x more bang (and presumably 4x more desire to go bang).

It does appear that Lithium's reactivity with water is has much more energy release potential than the battery itself does, but from a bit of reading around on the all-knowing internet, this reaction is supposedly fairly gentle. "Lithium's density is only about half that of water so it floats on the surface, gently fizzing and giving off hydrogen. It gradually reacts and disappears, forming a colourless solution of lithium hydroxide."

Seems to me that if you physically abuse a battery, it's going to release the energy that it has stored. The only way to really avoid that is to not store so much energy. Or are there battery chemistries that can take abuse without releasing their energy?

Don't discount the chemistry. Sugar is more energy dense than a Lithium Ion battery and it can obviously be made to release its energy in a very safe and controlled manner. Alkaline batteries are almost as energy dense (although not rechargeable) and they are substantially more stable. You can physically abuse a normal battery without it discharging it's energy right?

It's the runaway reaction that makes Lithium ion batteries less safe. They require pressure vents and special circuitry to make them safe as they short out if fully discharged after which attempting to charge them is unsafe.

But you have to remember that the battery producers aren't especially interested in the safety of the battery (beyond a certain level)-- they care about price, capacity and re-usability.

Sugar itself is not more energy dense than a battery, is it? You need an oxidizer to get energy out of something like sugar. In theory, sugar, gasoline, etc. are much more energy dense, but they need to be very carefully mixed with oxygen to release it all at once. That's why cars aren't generally bombs on wheels even though they theoretically contain sufficient energy to meet that description.

The thing with batteries isn't just the amount of energy that they contain, but that it comes entirely pre-mixed and ready for instantaneous release.

As far as normal batteries withstanding physical abuse, I don't know. I certainly wouldn't try puncturing or badly overheating a regular battery, and wouldn't be surprised if such treatment caused it to release what it has. I could, of course, be wrong.

Alkaline batteries can explode however it is quite different than the failure of Lithium batteries. If shorted out or exposed to current they can produce heat and hydrogen gas and because they are in a sealed metal canister they pop, sometimes violently. Note that this is different than Li-ion which can short out internally.

Here's an article about it happening:

http://www.wthr.com/story/15008952/13-investigates-exploding...

Considering how much more common standard batteries are, you can assume it is indeed pretty rare.

As for the energy density of sugar you're right that they're not really directly comparable but yeah I think it is more energy dense. Actually, wikipedia has a nice table for it:

http://en.wikipedia.org/wiki/Energy_density

I think we're actually in agreement here. My whole point was that the chemistry of how the energy is released is more important than the energy density.

Yeah, just some quibbling over details. My point is just that any battery that's as energy dense as a modern Li-ion battery will have similar potential to cause havoc if abused, and the problem will get worse as the tech improves because we'll be packing more and more energy into our pockets and backpacks. Maybe it is the chemistry more than the density, but I'm going to bet that any chemistry with comparable density would share the same problem. So, mildly different perspectives on the same ultimate effect, I think.

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