What causes laptop batteries to overheat?

In conjunction with the United States Consumer Product Safety
Commission (CPSC), Dell and Apple Computer announced large
recalls of laptop batteries in the summer of 2006, followed
by Toshiba and Lenovo. Sony manufactured all of the recalled
batteries, and in October 2006, the company announced its own
large-scale recall. Under the right circumstances, these
batteries could overheat, potentially causing burns,
an explosion or a fire.

To understand why that happened, it's helpful to know
a little bit about how batteries work. Batteries have
a negatively charged terminal and a positively charged
terminal. In a battery, energy from electrochemical reactions
causes electrons (negatively charged particles) to collect at
the battery's negatively charged pole. Charged particles are
attracted to opposite charge, so if you connect a battery to
a circuit, the electrons will flow from the negative pole,
through the circuit and to the battery's positively charged
pole. In other words, the battery generates a moving charge,
or electricity.

The exact reaction that generates the electrons varies,
depending on the type of battery. In a lithium-ion battery,
you'll find pressurized containers that house a coil of metal
and a flammable, lithium-containing liquid. The manufacturing
process creates tiny pieces of metal that float in the liquid.
Manufacturers can't completely prevent these metal fragments,
but good manufacturing techniques limit their size and number.
The cells of a lithium-ion battery also contain separators
that keep the anodes and cathodes, or positive and negative
poles, from touching each other.

If the battery gets hot through use or recharging, the pieces
of metal can move around, much like grains of rice in a pot
of water. If a piece of metal gets too close to the separator,
it can puncture the separator and cause a short circuit.
There are a few possible scenarios for what can go wrong in
the case of a short circuit:

* If it creates a spark, the flammable liquid can ignite,
causing a fire.
* If it causes the temperature inside the battery to rise
rapidly, the battery can explode due to the increased
pressure.
* If it causes the temperature to rise slowly, the
battery can melt, and the liquid inside can leak out.

There are several reasons why­ multiple laptop battery models
have been recalled in the past few years. People want small,
lightweight laptops that they can use for long periods. They
also want their laptops to have bright screens and lots of
processing power. For these reasons, laptop batteries have to
be relatively small, but they also have to hold a lot of
energy and last a long time.

Making lithium-ion batteries that can hold more power for
a longer period requires vital components, including the
separators, to be small and thin. The reduction in size makes
it more likely that the batteries can fail, break, leak or
short circuit.