Dell and Sony knew about — and had discussed — manufacturing problems with Sony-made lithium-ion batteries as long as ten months ago, according to a Sony spokesman Rick Clancy. But they held off issuing a recall until the flaws were clearly linked to catastrophic failures causing those batteries to catch fire, says Clancy.
Clancy says the companies had conversations in October 2005 and again in February 2006. Discussions were about the problem of small metal particles that had contaminated lithium-ion battery cells manufactured by Sony, causing batteries to fail and, in some cases, overheat.
As a result of those conversations, Sony made changes to its manufacturing process to minimise the presence and size of the particles in its batteries. However, the company did not recall batteries that it thought might contain the particles because it wasn’t clear that they were dangerous, Clancy says.
“We didn’t have confirmation of incidents [involving fires] until relatively recently. We received reports, but didn’t know if these were environmental situations not related to the systems themselves,” he says. “Different measures were taken in February and in October [2005] to further ensure that there were as few of these particles as possible and that they were as small as possible.”
Lithium-ion batteries are constructed with coated anode and cathode foils separated by thin layers of polymer material, says Dan Doughty, manager of the Advanced Power Sources Research and Development department at Sandia National Laboratory.
“It looks like a jelly roll. You get a high surface area with thin layers. The thinner they go with the separators, the more room there is for the active material,” Doughty says.
The coated layers are wound up on commercial machines to create the individual li-ion cell, and it’s at that stage that contaminants, such as metallic particles, can get embedded in the battery cell. The metallic particles mentioned by Sony and Dell may have been cast off by those commercial machines, he says.
Generally, the polymer separator is very thin — less than 25 microns (one millionth of a metre) thick. If that is punctured by an electrically conductive material, like a metal particle, the battery cell’s anode and cathode short circuit, Doughty says.
He says an internal short circuit is “the worst scenario in battery design, because there’s nothing you can do to control it”. In contrast, manufacturers have a variety of measures to guard the battery contents from external threats, like ambient heat.
Doughty says li-ion batteries usually fail “benignly”, in what researchers call a “soft short”, in which a low current path is created within the battery that causes it to discharge its energy into the cell.
However, if a large particle of conductive material penetrates the separator, connecting the anode and cathode, or a small particle of conductive material manages to find just the right position between the anode and cathode, a “hard short” can result, in which the battery cell releases all its energy at once, in what’s called “thermal runaway”.
But as manufacturers pack more and more power into the li-ion batteries, the risk of catastrophic failure grows, Doughty says. “The more energy put into it if things go bad, you lose the ability to have a graceful failure.”
Clancy says that if Sony’s batteries fail it is usually a benign short circuit that stops the battery from working. “System-related issues” that are unique to Dell came into play to cause the fire, he says.
“These lithium-ion battery packs are put together based on specifications from the manufacturer,” he says. The configuration of the cells in the pack, heat environment, battery charging specifications and proximity to other heat sources in the laptop all vary between manufacturers, says Clancy.
Fujitsu, Toshiba and Hewlett-Packard all say they use Sony li-ion batteries with their systems but that these batteries are different from those being recalled by Dell. The companies say they do not see a fire risk for customers and so do not plan a battery recall.
“We’ve worked with Sony to identify whether the cells we used were part of the contaminated batch,” says HP spokesman Mike Hockey. “We’ve not seen any issues whatsoever.”
But Doughty says that there’s no guarantee that the fire problem with metal particles in the Sony li-ion cells would be limited to Dell systems. “I’m an R&D guy but, the way I understand it, if you’ve got an automobile part that’s defective in one vehicle you replace it in other vehicles as well.”