Sony has provided more information about the problem with its laptop batteries that has caused the largest ever battery recall for 9.6 million packs.

When the problem was first acknowledged in August by Dell, it was explained that metallic particles had got into the battery during the manufacturing progress. Sony has now explained that the particles, believed to be nickel, likely got into the battery during two stages in production: when a groove was created in the battery case and when the electrolyte was poured into the cell.

But that alone wouldn't be enough to cause the fires that have been reported by laptop owners. For that to happen Sony believes that the particles would have to fall into a small triangular gap in the cell body right at the point where the cathode ends between two layers of spacer material. Then, depending on system configuration, the conditions could be right for a fire to start in the battery.

"The probability of this occurrence very much depends on system configuration," said Takashi Enami, senior general manager of the energy business group at Sony. He said size and shape of the battery pack and the charging configuration could all increase the risk but he wouldn't offer any specific information citing confidentiality agreements between Sony and its customers.

As a result of the problems 6.1 million batteries have been recalled by Dell and Apple. An additional 3.5 million batteries are covered by a Sony-led replacement program that offers new batteries to laptop users who are worried about the safety of their system. Last week, Sony said it anticipates costs of ¥51 billion (£228 million) as a result of the battery problems.