At the centre of the biggest-ever recall, Sony has admitted that both Dell and Sony knew about problems with the laptop batteries as long as 10 months ago.

The two companies discussed manufacturing problems with the Sony-made Lithium-Ion batteries in October 2005 and again in February 2006 but held off on issuing a recall, a Sony representative said. It was only when those flaws caused the batteries to catch fire that the companies acted.

Discussions revolved around 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, 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, Sony said.

"We didn't have confirmation of incidents [involving fires] until relatively recently. We received reports, but didn't know if there were environmental situations not related to the systems themselves," said the spokesman.

"Different measures were taken in February and in October to further ensure that there were as few of these particles as possible and that they were as small as possible."

Last week, Dell announced it would recall 4.1 million Dell laptop battery packs, citing a fire hazard. The recall covers Dell-branded battery packs that use certain Sony Li-ion battery cells sold through July 2006. Those batteries were manufactured prior to changes made in February, Sony said.

A Dell spokeswoman refused to comment on the conversations with Sony in October and February, but said Dell was "confident that the manufacturing process at Sony has been changed to address this issue."

Lithium-Ion batteries are constructed with coated anode and cathode foils separated by thin layers of polymer material, said 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 said.

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 particals, 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 said.

Generally, the polymer separator is very thin - less than 25 micron (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 said.