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. (See How Electricity Works and How Batteries Work for more details).

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:

­ 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.

Check out the next page for more information­, including how to determine if the recall affects your laptop.