Saturday, December 11, 2010

The bomb in your pocket

Lithium batteries are a cornerstone of modern portable technology.

They allow an incredible amount of energy to be stored in a very small space and thus enable our mobile phones, laptops, tablets, media-players and other devices to run for reasonable periods of time between recharges.

However, there is a problem with concentrating all that energy into such a tiny package.

Sometimes, they go bang!

The average mobile phone battery, for instance, may have a capacity of around 1 amp/hour at 3.7V which is a total of 3.7 watt-hours (13 KiloJoules for the physicists amongst us), or about
the same as a rifle bullet in flight.

When released over several hours, most of that energy is put to good use, running processors, illuminating LEDs and LCD backlights, even sending and receiving data via radio-frequencies.

Release the same amount of energy in just a fraction of a second though -- and "boom!" -- you have a problem.

Most of the time, these lithium batteries work as intended and simply release their energy in a trickle, as demanded by whatever appliance or device they're installed in. On occasion however, something goes wrong and the result is a fire or explosion.

But just what is it that makes a lithium battery so prone to the kind of unexpected and sudden energy release we're talking about?

Well this article gives an insight into what goes on when these batteries are charged. As you can see, massive physical changes take place within the tiny conductors that make up the plates of the batteries and, over repeated charge/discharge cycles, this can cause physical damage to the cells -- eventually resulting in catastrophic failure.

So what is being done to try and make lithium batteries safer?

Well there have been alternative chemistries developed such as lithium iron phosphate (LiFePO4). Batteries based on this chemistry have proven to be far safer and almost immune to the kind of fiery venting that the more common lithium-ion and lithium-polymer cells sometimes produce. Unfortunately, LiFePO4 are more expensive and don't have the same energy-density hence, are not favoured by gadget-makers.

In fact, it looks as if lithium batteries may get even more dangerous before they get safer because their energy density looks set to see a ten-fold increase, if this article is anything to go by.

Personally, I find it amusing that in this era of paranoia, when passengers are frisked before they're allowed on an aircraft and at one time it was even forbidden to take more than a few ml of any liquid onboard for fear it may be part of an incendiary device -- nobody cares that most of us are carrying a rather potent bomb everywhere we go -- in the form of our mobile phones, netbooks, ipads, ipods and other lithium-powered devices.

But fear-not, airlines are aware of the danger and train their staff to respond accordingly -- as in this FAA video.

I never had these problems with my old zinc-carbon Eveready AA cells. Ain't progress a wonderful thing?

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