Editors Note: This story has been changed to reflect Apples recall on Aug. 24 of Sony-manufactured batteries.
Dell announced last week that it was recalling 4.1 million notebook PC batteries, manufactured by Sony, following what Dell asserts to be a small number of fires (although at least one former Dell employee has said the problem is long-standing and more common than the company admits). A statement from the Consumer Product Safety Commission called this the largest computer-related recall to date.
eWEEK Labs uses two Dell Latitude D10s and a Latitude D20 as part of our testbed. It turns out that none of our laptop batteries was affected by the recall. Questions remain as to how many laptop computer batteries made by Sony for other brands of laptops may share the problem. CPSC spokesperson Scott Wolfson told eWEEK Labs on Aug. 17 that this possibility was under investigation; Aug. 24 announcements from Apple, Sony and the CPSC have since confirmed that batteries made by Sony for Apple are also at risk and are being recalled for exchange by Apple.
So why would a laptop battery be at risk of bursting into flame? When a battery powers a circuit, the battery itself is part of the circuit—meaning that the batterys internal resistance, multiplied by the square of the current, becomes an unavoidable source of waste heat. Low internal resistance is therefore considered a good thing, and laptop batteries are designed to minimize it.
What low resistance means, though, is that any short circuit across (or inside) the battery can result in a very large current flow (amps of current equal voltage divided by resistance). A battery thus can unleash a lot of energy (watts equals volts times amps; energy equals watts times seconds) in a very short amount of time.
Batteries are characterized by their "C" value—the number of amps of current that discharge the battery in 1 hour. Different battery types can handle differing peak currents, expressed as a multiple of C: A NiCad battery can readily deliver a peak current of 1C, while a NiMH battery is more typically rated at only 0.2C. Higher currents due to short circuits may stress a battery beyond design limits.
Laptop makers also strive for the highest possible value of watt-hours per pound, but thats just another way of saying theres a lot more energy in a much smaller package. All of this means that battery makers must practice care in design and manufacture and that laptop makers must be wary of battery-charger system interactions so that energy is released only at the proper rate and in the intended manner.